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Environmentally Persistent Free Radicals (EPFRs). 3. Free versus Bound Hydroxyl Radicals in EPFR Aqueous Solutions

Environmentally Persistent Free Radicals (EPFRs). 3. Free versus Bound Hydroxyl Radicals in EPFR... Article pubs.acs.org/est Terms of Use Environmentally Persistent Free Radicals (EPFRs). 3. Free versus Bound Hydroxyl Radicals in EPFR Aqueous Solutions ,† ‡ †,‡ † Lavrent Khachatryan,* Cheri A. McFerrin, Randall W. Hall, and Barry Dellinger Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States Department of Natural Sciences and Mathematics, Dominican University of California, San Rafael, California 94901, United States * Supporting Information ABSTRACT: Additional experimental evidence is presented for in vitro generation of hydroxyl radicals because of redox cycling of environmentally persistent free radicals (EPFRs) produced after adsorption of 2-monochlorophenol at 230 °C (2-MCP-230) on copper oxide supported by silica, 5% Cu(II)O/silica (3.9% Cu). A chemical spin trapping agent, 5,5-dimethyl-1-pyrroline-N- oxide (DMPO), in conjunction with electron paramagnetic resonance (EPR) spectroscopy was employed. Experiments in spiked O water have shown that ∼15% of hydroxyl radicals formed as a result of redox cycling. This amount of hydroxyl radicals arises from an exogenous Fenton reaction and may stay either partially trapped on the surface of particulate matter (physisorbed or chemisorbed) or transferred into solution as free OH. Computational work confirms the highly stable nature of the DMPO−OH adduct, as an intermediate produced by interaction of DMPO with physisorbed/chemisorbed OH (at the interface of solid catalyst/solution). All reaction pathways have been supported by ab initio calculations. INTRODUCTION and the remainder may be released into solution as free OH without any significant effect on the scavengers (because of the Resonance-stabilized, environmentally persistent free radicals low concentration of hydroxyl radicals). On the other hand, the (EPFRs) (semiquinone, phenoxyl, cyclopentadienyl, etc.) can significance of the concerted reaction between a metal site, form on the surfaces of fine particles and persist almost 1−3 H O , and a target (here DMPO) without participation of OH 2 2 indefinitely in the environment. Redox cycling of adsorbed 7,8 in the general process cannot be excluded. EPFRs may be a source of reactive oxygen species (ROS), such • • − In other words, it is always challenging and in most cases as hydroxyl radicals ( OH), superoxide anion radicals (O ), 7,9 1 unclear to ascertain the origin of OH radicals. The large hydrogen peroxide (H O ), etc. These results were partially 2 2 2,4 problem is that the DMPO−OH adduct (as an indicator for supported by later works. Recently, a chemical spin trapping free OH) may also be formed by nucleophilic addition of water agent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) in conjunc- 10−12 to DMPO catalyzed by a transition-metal impurity (or tion with electron paramagnetic resonance (EPR) spectroscopy through intermediate DMPO radical cation ). The non-radical was employed to measure the production of ROS in an aqueous nucleophilic reaction of water has been proposed to be a suspension of particle-associated EPFRs derived from adsorp- significant pathway for the formation of DMPO−OH radical tion of 2-monochlorophenol (2-MCP) on 5% Cu(II)O/silica 14,15 5,6 adducts, even during a Fenton reaction; i.e., 80−90% of the (3.9% Cu) particles. It was established that hydroxyl radicals total DMPO−OH in O-enriched water was due to iron- are generated by a surface-mediated redox cycle, with the dependent nucleophilic addition of water. However, the same resulting hydroxyl radicals remaining completely or largely on authors also discuss a water-independent mechanism of the surface such that they cannot be readily scavenged to form DMPO−OH formation and how an Fe or Cu ion-induced secondary organic radicals in quantities detectable using 6 nucleophilic addition of water to DMPO may be significantly currently available methods. The surface-bound hydroxyl suppressed in experiments performed in most common radical as well the reduced metal in the immediate vicinity buffers. are responsible for the enhanced activity of the particles. The These arguments are the main reasons for performing the concentration of hydroxyl radicals was measured at ∼1 μM for 5 spin-trapping experiments using O-labeled water in the a 140 min incubation of EPFR-containing solution. presence of EPFRs associated with CuO/SiO nanoparticles. Failure to form secondary radicals using standard scavengers, We provide here additional evidence of in vitro generation of such as ethanol, dimethyl sulfoxide, sodium formate, and hydroxyl radicals by EPFRs produced from the adsorption of 2- sodium azide, suggests that caution must be used to interpret free hydroxyl radical generation in solution. There is the dilemma: first, hydroxyl radicals may form on the surface via a Received: March 13, 2014 non-homogeneous reaction of H O because of “site-specific Revised: July 8, 2014 2 2 OH production” known as “site-specific Fenton reaction”. A Accepted: July 18, 2014 fraction may react with the target (in our case, with DMPO), Published: July 18, 2014 © 2014 American Chemical Society 9220 dx.doi.org/10.1021/es501158r | Environ. Sci. Technol. 2014, 48, 9220−9226 Environmental Science & Technology Article monochlorophenol at 230 °C (2-MCP-230) on a copper oxide reduction, dimerization, nucleophilic addition of water, etc.), catalyst supported by silica nanoparticles, 5% Cu(II)O/silica if they occur, we believe have the same contribution for both 16,17 (3.9% Cu). the control and sample solutions. We use ab initio calculations to determine the thermody- EPR Measurements. EPR spectra were recorded using a namically favored physisorbtion/chemisorption of hydroxyl Bruker EMX-20/2.7 EPR spectrometer (X-band) with dual radicals on a particulate matter (PM) surface as well as illustrate cavities and modulation and microwave frequencies of 100 kHz the highly stable nature of the DMPO−OH adduct adsorbed at and 9.516 GHz, respectively. Typical parameters were sweep the interface of a solid catalyst in solution. width of 100 G, EPR microwave power of 10 mW, modulation amplitude of 0.8 G, time constant of 40.96 ms, and sweep time EXPERIMENTAL SECTION of 167.77 s. Simulation Procedure. Bruker Win-EPR SimFonia Materials. High-purity DMPO (99%+, GLC) was obtained spectral simulation program was used that runs on a personal from ENZO Life Sciences International and used without computer (PC) under Microsoft Windows. further purification. 2-MCP (99+%), copper nitrate hemi- The simulation of DMPO− OH gives EPR spectrum with pentahydrate (99.9+%), and 0.01 M phosphate-buffered saline 1:2:2:1 intensity distribution, while incorporation of O atom (PBS, 0.138 M NaCl/0.0027 M KCl) was all obtained from in DMPO−OH (DMPO− OH adduct) increases the number Sigma-Aldrich. Cab-O-Sil were obtained from Cabot (EH-5, 17 17 18 of EPR lines from 4 (for DMPO− OH) to 15 (for 99+%). O-Labeled water (40.7% O, 1.6% O, and 57.7% 17 17 17 DMPO− OH) because of the O coupling ( O has a O) was obtained from ICON Isotope (Summit, NJ). 25,26 nuclear spin of / ), vide infra. In the case of simulation for EPFR Surrogate Synthesis. The 5% CuO/silica (3.9% Cu) the mixture with different contents of DMPO− OH/ particles were prepared by impregnation of silica powder (Cab- DMPO− OH, the total number of lines will reach 19 (cf. O-Sil) with 0.1 M solution of copper nitrate hemipentahydrate panels a and b of Figure 2), with the relative intensity of each and calcinated at 450 °C for 12 h. The sample was then spin adduct spectra directly proportional to their percentage ground and sieved (mesh size of 230, 63 μm). Prior to content. exposure, the particles were heated in situ in air to 450 °C for 1 Computational Details. Ab initio calculations were h to pretreat the surface. They were then exposed to saturated performed with the Gaussian 09 suite of programs. The vapors of 2-MCP at 230 °C using a custom-made vacuum B3LYP hybrid functional was chosen because it has recently exposure chamber for 5 min. Once exposure was complete, the −2 been shown consistent with experimental spin-trapping results temperature of the system was cooled to 150 °C for 1 h at 10 involving DMPO and provides reliable ground-state structural Torr. EPR spectra were then acquired at ambient conditions to parameters for copper-containing structures. Homolytic bond confirm the existence of EPFRs. dissociation energies (BDEs) studied with a variety of density In Vitro Studies. Both control and sample solution functional theory (DFT) methods also indicate B3LYP usage suspensions, containing particles without and with EPFRs, 6,5 with a correlation-consistent basis set minimizing the deviation respectively, were prepared in a similar manner. The final from benchmark calculations. As a result, we used the composition of the suspension in most experiments was correlation-consistent, double-ζ polarized cc-pVDZ basis set in particles (50 μg/mL) + DMPO (150 mM) + reagent (200 μL). our calculations. Each stationary-point structure (B3LYP/cc- For experiments with ( O) H O, all reagents were dissolved pVDZ) yielded only real frequencies. Scaling factors for the in ( O) H O at the same concentration mentioned above frequencies were not applied. [only half of the amounts of components were used to save the ( O)-labeled water, i.e., balanced at 100 μL]. RESULTS AND DISCUSSION The solutions prepared in either 100% ( O) H O or 40.7% 17 16 The hypothesized Scheme 1 may be a source of ROS ( O) H O + 57.7% ( O) H O were kept in the dark and 2 2 5,6 generation. It involves (1) electron transfer from the EPFR shaken for 30 s using a Vortex Genie 2 (Scientific Industries) in touch mode. A total of 20 μL (10 μL in the case of O-labeled Scheme 1 water) of the solution was transferred to an EPR capillary tube (inner diameter of ∼1 mm and outer diameter of 1.55 mm) and sealed at one end with a sealant (Fisherbrand). The capillary was next inserted in a 4 mm EPR tube and placed into the EPR resonator. The intensities of the EPR spectra of DMPO−OH adducts were reported in arbitrary units, DI/N [double to molecular oxygen, forming superoxide radical ion, and (2) integrated (DI) intensity of the EPR spectrum normalized hydrogen peroxide and a hydroxyl radical are produced via (N) to account for the conversion time, receiver gain, number dismutation and Fenton reactions, respectively. The spin- of data points, and sweep width]. Each experiment was trapping experiments in O-spiked water may spread a light on performed at least twice, and the final intensity of the EPR the problem of whether DMPO−OH adducts are generated by spectrum of DMPO−OH represents an average of all spectra nucleophilic addition of water to DMPO or via Scheme 1. obtained for each experiment. Spin Trapping by DMPO in O-Enriched Water. The Because the chemistry of interaction of chelators with the results of spin-trapping experiments performed in O water 21−24 surface of the model particles is unclear, we abstained from and O-enriched water are represented in Figure 1. The the use of chelators, such as desferrioxamine (DFO) and intensity of DMPO− OH adducts is consistently higher in diethylenetriaminepentaacetic acid (DETAPAC), which mini- sample solutions: curve 1 in comparison to the control in 5,6 mize the iron content in solution. The comparative method regular H O water (not shown). The same trend is 16 17 (this work), a comparison of sample and control solutions observed for DMPO− OH adducts in O-enriched water: 5,6 exactly at the same conditions, is preferable. All secondary curve 2 represents the sample, and curve 2′ represents the processes (DMPO decay, oxidation by dissolved oxygen, control solutions. On the other hand, the isotopic effect on the 9221 dx.doi.org/10.1021/es501158r | Environ. Sci. Technol. 2014, 48, 9220−9226 Environmental Science & Technology Article H O + 57.7% ( O) H O (black and red lines in Figure 2a). 2 2 The extra nuclear hyperfine splitting observed in Figure 2a is due to the DMPO− OH adduct [15 lines with hyperfine splitting constant (hsc) = 4.66 G for O, which has a nuclear 5 16 spin, I = / ] along with DMPO− OH (4 lines with hsc = 15.0 G for H and N, where O has no nuclear spin, I = 0). The appearance of DMPO− OH splitting is only indicative for 15,32 nucleophilic addition of water on the DMPO. These extra lines are clearly seen in simulated spectra in Figure 2b at 17 16 composition of 80% ( O) H O + 20% ( O) H O mixture 2 2 with superposition of DMPO− OH (assigned by asterisks) and DMPO− OH (rest of the 15 lines) adducts. When simulated spectrum C (in Figure 2a) at composition 17 16 of 40.7% ( O) H O + 57.7% ( O) H O is subtracted from the 2 2 experimental spectrum B, a residue spectrum B−C is shown, which is typical for the DMPO− OH adduct EPR spectrum. The residue B−C spectrum shows that there is an additional Figure 1. Difference in the DMPO−OH adduct spectral intensity for source of formation of DMPO− OH, which is not due to the 16 17 the samples containing EPFRs in O water (line 1) and O-enriched nucleophilic addition of water to DMPO and may be likely due water (line 2). Line 2′ stands for control solution in O-enriched to Scheme 1. A simple examination for the amount of residue water. spectrum in overall spectral intensity of experimental spectrum B (Figure 2a) demonstrated that ∼85% of the oxygen atoms present in the DMPO−OH adduct originated through accumulation of spin adducts is clearly seen; i.e., the nucleophilic addition of H O to DMPO, while ∼15% DMPO− OH spectra intensity in water with composition of 17 16 16 DMPO− OH adduct was due to the trapping of the hydroxyl 40.7% ( O) H O + 57.7% ( O) H O is less than in 100% O 2 2 16 • − radical formed from the superoxide ( O ) dismutation water (lines 2 and 1 in Figure 1, respectively). The difference reaction (Scheme 1). between sample, curve 2, and control solutions, curve 2′ The idea that most contribution in spin-trapping experiments (currently ∼15−20% at high incubation time), can be markedly is produced by the addition of water to DMPO, as mentioned increased after centrifuging the sample by removing large 25,33 above, is not without literature precedence. Ultimately, the clusters in the particle solution. The smaller the size of the pathway of the water-independent mechanism for DMPO−OH nanocluster, the higher the activity to generate ROS. As a adduct formation must always be checked. result, a 40−50% difference can be seen between sample and The next question of interest is whether hydroxyl radicals control solutions, unambiguously showing the fact of produced from the exogenous Fenton reaction (site-specific generation of hydroxyl radicals during redox cycling. Finally, while the characteristic four lines of the DMPO−OH Fenton reaction ) stay on the surface or leave it? This problem spectrum were typical for the EPFR solution prepared in 100% (free versus bound OH radical) was partly addressed in our 16 6 O water, a modified EPR spectrum was detected in EPFR previous publication. It is also a dispute theme in the 7,34−41 solution prepared in water with composition of 40.7% ( O) literature. 17 16 Figure 2. (a) EPR spectra of DMPO− OH/DMPO− OH adducts at an incubation time of 300 min for a solution of EPFRs (50 μg/mL) + 17 17 DMPO (150 mM) + PBS (total 100 μL) with content of 17.3% ( O) H O (black line A) and ∼41% ( O) H O (red line B). Line C is a computer 2 2 17 16 17 16 simulation of DMPO− OH/DMPO− OH adducts at a concentration of 41% O and 59% O based on the parameters from panel b. B−C is the residue spectrum where the 3 lines assigned by squares represent the EPR spectrum for N(∼0.37% isotopic abundance in nature). (b) Computer 17 16 17 16 simulation of the DMPO− OH/DMPO− OH adduct EPR spectrum at a concentration of 80% O and 20% O in water (the spectrum assigned 16 17 by an asterisk corresponds to DMPO− OH). The hsc values for N and H are ∼15.01 and 4.66 G for O. g, 2.0061; ΔH , 1.15 G; and the EPR p−p line shape, Gaussian. 9222 dx.doi.org/10.1021/es501158r | Environ. Sci. Technol. 2014, 48, 9220−9226 Environmental Science & Technology Article Figure 3. Illustration of the (a) chained-shaped cluster with trigonal planar form of Cu, (b) adsorption (trapping) of OH because of hydrogen bonding shown by the arrow on the CuO/SiO surface, and (c and d) further interaction of the cluster with DMPO (C, brown; N, blue; O, red; and H, white). Dark gray, Cu; light gray, Si; red, O; white, H. Figure 4. (a) Adsorption of OH on cluster (tetrahedral Cu) and (b) interaction of DMPO (black, C; blue, N; red, O; white, H) with adsorbed OH by (c) formation of stabilized DMPO−OH on cluster surfaces. The hydrogen bonding is shown by dashed lines. Dark gray, Cu; light gray, Si; red, O; white, H. One of the plausible experimental facts of surface site bound experimental fact is surprising. For comparison note, the half- OH is deduced from the high stability of the DMPO−OH life time of DMPO−OH in homogeneous media depends upon adduct (days) at the interface of solid catalyst/solution. This the environment and may be changed from 2 to 20 min 9223 dx.doi.org/10.1021/es501158r | Environ. Sci. Technol. 2014, 48, 9220−9226 Environmental Science & Technology Article 12,42 43 (aqueous solution) or 55 min in phosphate buffer. A long been experimentally found to exhibit both trigonal and 54−58 lifetime is only reported in ref 44: the DMPO−OH spin tetragonal coordination around the Cu atom. As a result, adducts in water solution last for hours depending upon the we also added a hydroxyl moiety to the 3-coordinate Cu cluster temperature. (directly to Cu atom) shown in Figure 3a to produce a 4- In fact, it may be emphasized that the portion of DMPO− coordinate Cu cluster (in Figure 4a). The addition of OH to OH-formed in an independent way (from the addition of OH the 3-coordinate Cu clusters yields reaction energy of −33.2 to DMPO) is stable likely on the silica surfaces or the catalyst kcal/mol. site. There is literature experimental data about stabilization of The hydrogen bonds arranged in both head-to-tail and DMPO adducts on secondary organic aerosol particles such as intramolecular fashion shown in both Figures 3b and 4b allow DMPO−HO , DMPO−RO, DMPO−RO , and DMPO−OH for the physisorption of OH (Figure 3b; bond distance of 1.62 2 2 detected by electrospray ionization−tandem mass spectrometry Å) with an exoergic reaction energy of ΔE = −21.95 kcal/mol (ESI−MS/MS) and DMPO−glutathionyl in an intracellular or chemisorption of reactive hydroxyl groups (Figure 4b; bond environment using high-performance liquid chromatography distance of 1.49 Å) with an exoergic reaction energy of −29.5 (HPLC). kcal/mol. To address the existence of surface site bound OH as well as Further addition of DMPO to the physisorbed/chemisorbed hydroxyl radicals in Figures 3b and 4b is also exoergic, leading high stability of DMPO−OH in an environment of CuO/SiO , to the stabilization and formation of the DMPO−OH adduct, ab initio calculations were initiated. The calculations were used for instance in Figure 3d, with an exoergic reaction energy of to assess the thermodynamic basis for the current interpretation −86.7 kcal/mol. The addition of DMPO to the chemisorbed of experimental results by hypothesizing the following: (1) OH in Figure 4b also yields an exoergic reaction energy of Because of EPFRs cycling mechanism the H O is formed at 2 2 5,6 −73.2 kcal/mol. Stabilization of DMPO−OH because of the interface of nanoparticle/water solution. The hydroxyl hydrogen bonding on the cluster surface is shown in Figures radicals may be generated by either an exogenous Fenton 5,6,47 3d and 4c. It has been shown recently that DMPO and reaction or by direct decomposition of H O on the surface 2 2 inorganic radicals favor radical addition over nucleophilic sites, defects (see the Supporting Information). (2) The addition in the presence of hydrogen bonding, both hydroxyl radicals are stabilized by surface-active centers. (3) experimentally and computationally. DMPO attacks stabilized (physisorbed/chemisorbed) OH These theoretical calculations complement experimental radicals, forming a DMPO−OH adduct, which stays on the evidence for the highly stable nature of DMPO−OH adducts surface for a long time because of energetic stabilization. in CuO/SiO aqueous solutions. CuO/SiO Model Systems with Both Trigonal- and Therefore, the integrated intensity of DMPO−OH adducts Tetrahedral-Coordinated Cu Sites. We have performed ab may be considered a sum of DMPO−OH formed from the initio calculations to investigate the stabilization (physisorption addition of free OH (because of the exogenous Fenton reaction and chemisorption) of OH radicals on model CuO/SiO generated by the cycle) to DMPO and a portion of DMPO− surfaces, followed by further interaction of the adsorbed OH OH stabilized on a particle surface (as a result of the attack of with DMPO. Note that physisorption of the OH radical is DMPO to OH trapped on the surface). Currently, the primarily characterized by the hydrogen bonding taking place DMPO−OH adducts formed in a solution or on the surfaces (the bonding distance of ≤2 Å), whereas chemisorption is of particles are not distinguishable. We may hypothesize that characterized by the absence of hydrogen bonding (the the rate of accumulation of DMPO−OH adducts on particle bonding distance close to the covalent bond value of, for surfaces decreases during incubation because of sluggish instance, in HO−OH, ∼1.45−1.47 Å). generation of OH; i.e., the initial EPFRs as well as reductants Experimental synthesis of copper-containing silicates reveals 48,48,49 are consumed in secondary reactions not generating additional a mixture of copper in each of its valence states. X-ray amounts of OH in the cycle. Because DMPO−OH decays photoelectron spectroscopy (XPS) reveals the presence of faster in solution, we conclude that the surface-stabilized copper hydroxide, copper oxide, and Si−O−Cu bonds in these DMPO−OH adducts are responsible for the longer incubation clusters. While Chang et al. argue that the stable Si−O−Cu times. In addition, the surface-stabilized DMPO−OH adducts bonds are primarily electrostatic, Parameswaran et al. suggest are not expected to return to solution because the exoergicity of that their stable nature is covalent. the reactions for both a trigonal planar Cu (Figure 3) and a Our model reactant surface is a copper-containing silica-like tetrahedral Cu (Figure 4) are sufficiently high at −86.7 and structure derived from the addition of a −O−Cu−(OH) −73.2 kcal/mol, respectively. moiety to the previously optimized tetrahedrally-coordinated, Our calculations also show that DMPO may interact with radical hydroxide cluster found by Kubicki et al. [Figures 3a physisorbed or chemisorbed hydroxyl groups on the CuO/SiO (3-coordinate Cu) and 4a (vide infra tetrahedral Cu cluster)]. 2 cyclic cluster surfaces with the release of DMPO−OH into Radical-ended, as opposed to ionic, silica-like structures have solution (see the Supporting Information). As a consequence, been computationally shown to add water favorably via a radical there may be multiple hydroxyl-radical-generating pathways: (i) silicate−water mechanism, as opposed to a cationic or anionic The mayor channel of OH generation is the cycling scheme of silicate mechanism, with both radical pathways (H O+ SiO or 5,47 EPFR proposed earlier. OH forms through the exogenous SiO ) resulting in a hydroxylated silica surface site. We have Fenton reaction as follows: considered a limited number of atoms around an active site to make the calculations tractable, as small models have been used 50,51,41 H O + Cu(I) (organic ligand) successfully by other researchers. − • The optimized Cu atoms in structures a and b of Figure 3 are →+ Cu(II) (organic ligand) OH+ OH (1) both incorporated into a trigonal planar geometry, with Cu− OH and Cu−O bond distances similar to experimental Hydroxyl radicals formed in reaction 1 are either partially 52,53 values. Small inorganic Cu(I) and Cu(II) complexes have transferred into solution and form homogeneously DMPO− 9224 dx.doi.org/10.1021/es501158r | Environ. Sci. Technol. 2014, 48, 9220−9226 Environmental Science & Technology Article (8) Zhang, G. Y.; Long, J. L.; Wang, X. X.; Zhang, Z. Z.; Dai, W. X.; OH adducts or partially stabilized on the particle surfaces, Liu, P.; Li, Z. H.; Wu, L.; Fu, X. Z. Catalytic role of Cu sites of Cu/ forming DMPO−OH stable adducts (Figures 3 and 4). (ii) Ab MCM-41 in phenol hydroxylation. Langmuir 2010, 26 (2), 1362− initio calculations show that a partial decomposition of H O on 2 2 the silica surface active sites is also possible. For instance, (9) Bataineh, H.; Pestovsky, O.; Bakac, A. pH-Induced mechanistic because of homolytic cleavage of H O on the silica active sites 2 2 changeover from hydroxyl radicals to iron(IV) in the Fenton reaction. (defects, dangling bonds, etc.), one hydroxyl group hydrox- Chem. Sci. 2012, 3 (5), 1594−1599. ylates the surface site (chemisorption) and the second hydroxyl (10) Burkitt, M. J.; Tsang, S. Y.; Tam, S. C.; Bremner, I. Generation radical is trapped between neighboring Si−OH groups on the of 5,5-dimethyl-1-pyrroline N-oxide hydroxyl and scavenger radical surface (by hydrogen bonds) (see Figure S1B of the Supporting adducts from copper/H O mixturesEffects of metal ion chelation 2 2 Information). Further experimental addition of DMPO leads to and the search for high-valent metal−oxygen intermediates. Arch. stabilization and formation of DMPO−OH on the surfaces Biochem. Biophys. 1995, 323 (1), 63−70. (Figure S2 of the Supporting Information). (11) Finkelstein, E.; Rosen, G. M.; Rauckman, E. J. Spin trapping of superoxide and hydroxyl radicalPractical aspects. Arch. Biochem. Biophys. 1980, 200 (1), 1−16. ASSOCIATED CONTENT (12) Makino, K.; Hagiwara, T.; Murakami, A. Fundamental aspects of * Supporting Information spin trapping with DMPO. Radiat. Phys. Chem. 1991, 37 (5−6), 657− Data about the addition of OH and H O to the tetrahedral 2 2 form of Cu in CuO/SiO model systems (cyclic structure) and (13) Singh, R. J.; Karoui, H.; Gunther, M. R.; Beckman, J. S.; Mason, computational details about direct decomposition of H O on 2 2 R. P.; Kalyanaraman, B. Reexamination of the mechanism of hydroxyl silica active sites. This material is available free of charge via the radical adducts formed from the reaction between familial amyotrophic Internet at http://pubs.acs.org. lateral sclerosis-associated Cu,Zn superoxide dismutase mutants and H O . Proc. Natl. Acad. Sci. U. S. A. 1998, 95 (12), 6675−6680. 2 2 AUTHOR INFORMATION (14) Hanna, P. M.; Chamulitrat, W.; Mason, R. P. When are metal ion-dependent hydroxyl and alkoxyl radical adducts of 5,5-dimethyl-1- Corresponding Author pyrroline N-oxide artifacts. Arch. Biochem. Biophys. 1992, 296 (2), *Telephone: 225-578-4417. E-mail: lkhach1@lsu.edu. 640−644. Notes (15) Chamulitrat, W.; Iwahashi, H.; Kelman, D. J.; Mason, R. P. The authors declare no competing financial interest. Evidence against the 1−2−2−1 Quartet DMPO spectrum as the radical adduct of the lipid alkoxyl radical. Arch. Biochem. Biophys. 1992, ACKNOWLEDGMENTS 296 (2), 645−649. (16) Truong, H. Copper(II) oxide mediated formation and The authors gratefully acknowledge the partial support of this stabilization of combustion generated persistent free radicals. Ph.D. research under the National Institute of Environmental Health Dissertation, Department of Chemistry, Louisiana State University, Sciences (NIEHS) Grant (Superfund Research and Training Baton Rouge, LA, 2007. Program) PES013648Z, the LA-SIGMA NSF Grant EPS- (17) Lomnicki, S.; Truong, H.; Vejerano, E.; Dellinger, B. Copper 1003897, the Lillian L. Y. Wang Yin, Ph.D. Endowed Professor oxide-based model of persistent free radical formation on combustion- Chair held by Randall W. Hall, and the Patrick F. Taylor Chair derived particulate matter. Environ. Sci. Technol. 2008, 42 (13), 4982− held by Barry Dellinger. The authors also acknowledge the high-performance computing services provided by the Louisi- (18) Truong, H.; Lomnicki, S.; Dellinger, B. Potential for ana State University Center for Computational Technology and misidentification of environmentally persistent free radicals as molecular pollutants in particulate matter. Environ. Sci. Technol. the Louisiana Optical Network Initiative. 2010, 44 (6), 1933−1939. (19) Nakagawa, K. Is quartz flat cell useful for the detection of REFERENCES superoxide radicals? J. Act. Oxygens Free Radicals 1994, 5,81−85. (1) Squadrito, G. L.; Cueto, R.; Dellinger, B.; Pryor, W. A. Quinoid (20) Eaton, G. R.; Eaton, S. S.; Barr, D. P.; Weber, R. T. Quantitative redox cycling as a mechanism for sustained free radical generation by EPR; Springer-Verlag: Berlin, Germany, 2010; pp 185. inhaled airborne particulate matter. Free Radical Biol. 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Environmentally Persistent Free Radicals (EPFRs). 3. Free versus Bound Hydroxyl Radicals in EPFR Aqueous Solutions

Environmental Science & Technology , Volume 48 (16) – Jul 18, 2014

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Article pubs.acs.org/est Terms of Use Environmentally Persistent Free Radicals (EPFRs). 3. Free versus Bound Hydroxyl Radicals in EPFR Aqueous Solutions ,† ‡ †,‡ † Lavrent Khachatryan,* Cheri A. McFerrin, Randall W. Hall, and Barry Dellinger Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States Department of Natural Sciences and Mathematics, Dominican University of California, San Rafael, California 94901, United States * Supporting Information ABSTRACT: Additional experimental evidence is presented for in vitro generation of hydroxyl radicals because of redox cycling of environmentally persistent free radicals (EPFRs) produced after adsorption of 2-monochlorophenol at 230 °C (2-MCP-230) on copper oxide supported by silica, 5% Cu(II)O/silica (3.9% Cu). A chemical spin trapping agent, 5,5-dimethyl-1-pyrroline-N- oxide (DMPO), in conjunction with electron paramagnetic resonance (EPR) spectroscopy was employed. Experiments in spiked O water have shown that ∼15% of hydroxyl radicals formed as a result of redox cycling. This amount of hydroxyl radicals arises from an exogenous Fenton reaction and may stay either partially trapped on the surface of particulate matter (physisorbed or chemisorbed) or transferred into solution as free OH. Computational work confirms the highly stable nature of the DMPO−OH adduct, as an intermediate produced by interaction of DMPO with physisorbed/chemisorbed OH (at the interface of solid catalyst/solution). All reaction pathways have been supported by ab initio calculations. INTRODUCTION and the remainder may be released into solution as free OH without any significant effect on the scavengers (because of the Resonance-stabilized, environmentally persistent free radicals low concentration of hydroxyl radicals). On the other hand, the (EPFRs) (semiquinone, phenoxyl, cyclopentadienyl, etc.) can significance of the concerted reaction between a metal site, form on the surfaces of fine particles and persist almost 1−3 H O , and a target (here DMPO) without participation of OH 2 2 indefinitely in the environment. Redox cycling of adsorbed 7,8 in the general process cannot be excluded. EPFRs may be a source of reactive oxygen species (ROS), such • • − In other words, it is always challenging and in most cases as hydroxyl radicals ( OH), superoxide anion radicals (O ), 7,9 1 unclear to ascertain the origin of OH radicals. The large hydrogen peroxide (H O ), etc. These results were partially 2 2 2,4 problem is that the DMPO−OH adduct (as an indicator for supported by later works. Recently, a chemical spin trapping free OH) may also be formed by nucleophilic addition of water agent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) in conjunc- 10−12 to DMPO catalyzed by a transition-metal impurity (or tion with electron paramagnetic resonance (EPR) spectroscopy through intermediate DMPO radical cation ). The non-radical was employed to measure the production of ROS in an aqueous nucleophilic reaction of water has been proposed to be a suspension of particle-associated EPFRs derived from adsorp- significant pathway for the formation of DMPO−OH radical tion of 2-monochlorophenol (2-MCP) on 5% Cu(II)O/silica 14,15 5,6 adducts, even during a Fenton reaction; i.e., 80−90% of the (3.9% Cu) particles. It was established that hydroxyl radicals total DMPO−OH in O-enriched water was due to iron- are generated by a surface-mediated redox cycle, with the dependent nucleophilic addition of water. However, the same resulting hydroxyl radicals remaining completely or largely on authors also discuss a water-independent mechanism of the surface such that they cannot be readily scavenged to form DMPO−OH formation and how an Fe or Cu ion-induced secondary organic radicals in quantities detectable using 6 nucleophilic addition of water to DMPO may be significantly currently available methods. The surface-bound hydroxyl suppressed in experiments performed in most common radical as well the reduced metal in the immediate vicinity buffers. are responsible for the enhanced activity of the particles. The These arguments are the main reasons for performing the concentration of hydroxyl radicals was measured at ∼1 μM for 5 spin-trapping experiments using O-labeled water in the a 140 min incubation of EPFR-containing solution. presence of EPFRs associated with CuO/SiO nanoparticles. Failure to form secondary radicals using standard scavengers, We provide here additional evidence of in vitro generation of such as ethanol, dimethyl sulfoxide, sodium formate, and hydroxyl radicals by EPFRs produced from the adsorption of 2- sodium azide, suggests that caution must be used to interpret free hydroxyl radical generation in solution. There is the dilemma: first, hydroxyl radicals may form on the surface via a Received: March 13, 2014 non-homogeneous reaction of H O because of “site-specific Revised: July 8, 2014 2 2 OH production” known as “site-specific Fenton reaction”. A Accepted: July 18, 2014 fraction may react with the target (in our case, with DMPO), Published: July 18, 2014 © 2014 American Chemical Society 9220 dx.doi.org/10.1021/es501158r | Environ. Sci. Technol. 2014, 48, 9220−9226 Environmental Science & Technology Article monochlorophenol at 230 °C (2-MCP-230) on a copper oxide reduction, dimerization, nucleophilic addition of water, etc.), catalyst supported by silica nanoparticles, 5% Cu(II)O/silica if they occur, we believe have the same contribution for both 16,17 (3.9% Cu). the control and sample solutions. We use ab initio calculations to determine the thermody- EPR Measurements. EPR spectra were recorded using a namically favored physisorbtion/chemisorption of hydroxyl Bruker EMX-20/2.7 EPR spectrometer (X-band) with dual radicals on a particulate matter (PM) surface as well as illustrate cavities and modulation and microwave frequencies of 100 kHz the highly stable nature of the DMPO−OH adduct adsorbed at and 9.516 GHz, respectively. Typical parameters were sweep the interface of a solid catalyst in solution. width of 100 G, EPR microwave power of 10 mW, modulation amplitude of 0.8 G, time constant of 40.96 ms, and sweep time EXPERIMENTAL SECTION of 167.77 s. Simulation Procedure. Bruker Win-EPR SimFonia Materials. High-purity DMPO (99%+, GLC) was obtained spectral simulation program was used that runs on a personal from ENZO Life Sciences International and used without computer (PC) under Microsoft Windows. further purification. 2-MCP (99+%), copper nitrate hemi- The simulation of DMPO− OH gives EPR spectrum with pentahydrate (99.9+%), and 0.01 M phosphate-buffered saline 1:2:2:1 intensity distribution, while incorporation of O atom (PBS, 0.138 M NaCl/0.0027 M KCl) was all obtained from in DMPO−OH (DMPO− OH adduct) increases the number Sigma-Aldrich. Cab-O-Sil were obtained from Cabot (EH-5, 17 17 18 of EPR lines from 4 (for DMPO− OH) to 15 (for 99+%). O-Labeled water (40.7% O, 1.6% O, and 57.7% 17 17 17 DMPO− OH) because of the O coupling ( O has a O) was obtained from ICON Isotope (Summit, NJ). 25,26 nuclear spin of / ), vide infra. In the case of simulation for EPFR Surrogate Synthesis. The 5% CuO/silica (3.9% Cu) the mixture with different contents of DMPO− OH/ particles were prepared by impregnation of silica powder (Cab- DMPO− OH, the total number of lines will reach 19 (cf. O-Sil) with 0.1 M solution of copper nitrate hemipentahydrate panels a and b of Figure 2), with the relative intensity of each and calcinated at 450 °C for 12 h. The sample was then spin adduct spectra directly proportional to their percentage ground and sieved (mesh size of 230, 63 μm). Prior to content. exposure, the particles were heated in situ in air to 450 °C for 1 Computational Details. Ab initio calculations were h to pretreat the surface. They were then exposed to saturated performed with the Gaussian 09 suite of programs. The vapors of 2-MCP at 230 °C using a custom-made vacuum B3LYP hybrid functional was chosen because it has recently exposure chamber for 5 min. Once exposure was complete, the −2 been shown consistent with experimental spin-trapping results temperature of the system was cooled to 150 °C for 1 h at 10 involving DMPO and provides reliable ground-state structural Torr. EPR spectra were then acquired at ambient conditions to parameters for copper-containing structures. Homolytic bond confirm the existence of EPFRs. dissociation energies (BDEs) studied with a variety of density In Vitro Studies. Both control and sample solution functional theory (DFT) methods also indicate B3LYP usage suspensions, containing particles without and with EPFRs, 6,5 with a correlation-consistent basis set minimizing the deviation respectively, were prepared in a similar manner. The final from benchmark calculations. As a result, we used the composition of the suspension in most experiments was correlation-consistent, double-ζ polarized cc-pVDZ basis set in particles (50 μg/mL) + DMPO (150 mM) + reagent (200 μL). our calculations. Each stationary-point structure (B3LYP/cc- For experiments with ( O) H O, all reagents were dissolved pVDZ) yielded only real frequencies. Scaling factors for the in ( O) H O at the same concentration mentioned above frequencies were not applied. [only half of the amounts of components were used to save the ( O)-labeled water, i.e., balanced at 100 μL]. RESULTS AND DISCUSSION The solutions prepared in either 100% ( O) H O or 40.7% 17 16 The hypothesized Scheme 1 may be a source of ROS ( O) H O + 57.7% ( O) H O were kept in the dark and 2 2 5,6 generation. It involves (1) electron transfer from the EPFR shaken for 30 s using a Vortex Genie 2 (Scientific Industries) in touch mode. A total of 20 μL (10 μL in the case of O-labeled Scheme 1 water) of the solution was transferred to an EPR capillary tube (inner diameter of ∼1 mm and outer diameter of 1.55 mm) and sealed at one end with a sealant (Fisherbrand). The capillary was next inserted in a 4 mm EPR tube and placed into the EPR resonator. The intensities of the EPR spectra of DMPO−OH adducts were reported in arbitrary units, DI/N [double to molecular oxygen, forming superoxide radical ion, and (2) integrated (DI) intensity of the EPR spectrum normalized hydrogen peroxide and a hydroxyl radical are produced via (N) to account for the conversion time, receiver gain, number dismutation and Fenton reactions, respectively. The spin- of data points, and sweep width]. Each experiment was trapping experiments in O-spiked water may spread a light on performed at least twice, and the final intensity of the EPR the problem of whether DMPO−OH adducts are generated by spectrum of DMPO−OH represents an average of all spectra nucleophilic addition of water to DMPO or via Scheme 1. obtained for each experiment. Spin Trapping by DMPO in O-Enriched Water. The Because the chemistry of interaction of chelators with the results of spin-trapping experiments performed in O water 21−24 surface of the model particles is unclear, we abstained from and O-enriched water are represented in Figure 1. The the use of chelators, such as desferrioxamine (DFO) and intensity of DMPO− OH adducts is consistently higher in diethylenetriaminepentaacetic acid (DETAPAC), which mini- sample solutions: curve 1 in comparison to the control in 5,6 mize the iron content in solution. The comparative method regular H O water (not shown). The same trend is 16 17 (this work), a comparison of sample and control solutions observed for DMPO− OH adducts in O-enriched water: 5,6 exactly at the same conditions, is preferable. All secondary curve 2 represents the sample, and curve 2′ represents the processes (DMPO decay, oxidation by dissolved oxygen, control solutions. On the other hand, the isotopic effect on the 9221 dx.doi.org/10.1021/es501158r | Environ. Sci. Technol. 2014, 48, 9220−9226 Environmental Science & Technology Article H O + 57.7% ( O) H O (black and red lines in Figure 2a). 2 2 The extra nuclear hyperfine splitting observed in Figure 2a is due to the DMPO− OH adduct [15 lines with hyperfine splitting constant (hsc) = 4.66 G for O, which has a nuclear 5 16 spin, I = / ] along with DMPO− OH (4 lines with hsc = 15.0 G for H and N, where O has no nuclear spin, I = 0). The appearance of DMPO− OH splitting is only indicative for 15,32 nucleophilic addition of water on the DMPO. These extra lines are clearly seen in simulated spectra in Figure 2b at 17 16 composition of 80% ( O) H O + 20% ( O) H O mixture 2 2 with superposition of DMPO− OH (assigned by asterisks) and DMPO− OH (rest of the 15 lines) adducts. When simulated spectrum C (in Figure 2a) at composition 17 16 of 40.7% ( O) H O + 57.7% ( O) H O is subtracted from the 2 2 experimental spectrum B, a residue spectrum B−C is shown, which is typical for the DMPO− OH adduct EPR spectrum. The residue B−C spectrum shows that there is an additional Figure 1. Difference in the DMPO−OH adduct spectral intensity for source of formation of DMPO− OH, which is not due to the 16 17 the samples containing EPFRs in O water (line 1) and O-enriched nucleophilic addition of water to DMPO and may be likely due water (line 2). Line 2′ stands for control solution in O-enriched to Scheme 1. A simple examination for the amount of residue water. spectrum in overall spectral intensity of experimental spectrum B (Figure 2a) demonstrated that ∼85% of the oxygen atoms present in the DMPO−OH adduct originated through accumulation of spin adducts is clearly seen; i.e., the nucleophilic addition of H O to DMPO, while ∼15% DMPO− OH spectra intensity in water with composition of 17 16 16 DMPO− OH adduct was due to the trapping of the hydroxyl 40.7% ( O) H O + 57.7% ( O) H O is less than in 100% O 2 2 16 • − radical formed from the superoxide ( O ) dismutation water (lines 2 and 1 in Figure 1, respectively). The difference reaction (Scheme 1). between sample, curve 2, and control solutions, curve 2′ The idea that most contribution in spin-trapping experiments (currently ∼15−20% at high incubation time), can be markedly is produced by the addition of water to DMPO, as mentioned increased after centrifuging the sample by removing large 25,33 above, is not without literature precedence. Ultimately, the clusters in the particle solution. The smaller the size of the pathway of the water-independent mechanism for DMPO−OH nanocluster, the higher the activity to generate ROS. As a adduct formation must always be checked. result, a 40−50% difference can be seen between sample and The next question of interest is whether hydroxyl radicals control solutions, unambiguously showing the fact of produced from the exogenous Fenton reaction (site-specific generation of hydroxyl radicals during redox cycling. Finally, while the characteristic four lines of the DMPO−OH Fenton reaction ) stay on the surface or leave it? This problem spectrum were typical for the EPFR solution prepared in 100% (free versus bound OH radical) was partly addressed in our 16 6 O water, a modified EPR spectrum was detected in EPFR previous publication. It is also a dispute theme in the 7,34−41 solution prepared in water with composition of 40.7% ( O) literature. 17 16 Figure 2. (a) EPR spectra of DMPO− OH/DMPO− OH adducts at an incubation time of 300 min for a solution of EPFRs (50 μg/mL) + 17 17 DMPO (150 mM) + PBS (total 100 μL) with content of 17.3% ( O) H O (black line A) and ∼41% ( O) H O (red line B). Line C is a computer 2 2 17 16 17 16 simulation of DMPO− OH/DMPO− OH adducts at a concentration of 41% O and 59% O based on the parameters from panel b. B−C is the residue spectrum where the 3 lines assigned by squares represent the EPR spectrum for N(∼0.37% isotopic abundance in nature). (b) Computer 17 16 17 16 simulation of the DMPO− OH/DMPO− OH adduct EPR spectrum at a concentration of 80% O and 20% O in water (the spectrum assigned 16 17 by an asterisk corresponds to DMPO− OH). The hsc values for N and H are ∼15.01 and 4.66 G for O. g, 2.0061; ΔH , 1.15 G; and the EPR p−p line shape, Gaussian. 9222 dx.doi.org/10.1021/es501158r | Environ. Sci. Technol. 2014, 48, 9220−9226 Environmental Science & Technology Article Figure 3. Illustration of the (a) chained-shaped cluster with trigonal planar form of Cu, (b) adsorption (trapping) of OH because of hydrogen bonding shown by the arrow on the CuO/SiO surface, and (c and d) further interaction of the cluster with DMPO (C, brown; N, blue; O, red; and H, white). Dark gray, Cu; light gray, Si; red, O; white, H. Figure 4. (a) Adsorption of OH on cluster (tetrahedral Cu) and (b) interaction of DMPO (black, C; blue, N; red, O; white, H) with adsorbed OH by (c) formation of stabilized DMPO−OH on cluster surfaces. The hydrogen bonding is shown by dashed lines. Dark gray, Cu; light gray, Si; red, O; white, H. One of the plausible experimental facts of surface site bound experimental fact is surprising. For comparison note, the half- OH is deduced from the high stability of the DMPO−OH life time of DMPO−OH in homogeneous media depends upon adduct (days) at the interface of solid catalyst/solution. This the environment and may be changed from 2 to 20 min 9223 dx.doi.org/10.1021/es501158r | Environ. Sci. Technol. 2014, 48, 9220−9226 Environmental Science & Technology Article 12,42 43 (aqueous solution) or 55 min in phosphate buffer. A long been experimentally found to exhibit both trigonal and 54−58 lifetime is only reported in ref 44: the DMPO−OH spin tetragonal coordination around the Cu atom. As a result, adducts in water solution last for hours depending upon the we also added a hydroxyl moiety to the 3-coordinate Cu cluster temperature. (directly to Cu atom) shown in Figure 3a to produce a 4- In fact, it may be emphasized that the portion of DMPO− coordinate Cu cluster (in Figure 4a). The addition of OH to OH-formed in an independent way (from the addition of OH the 3-coordinate Cu clusters yields reaction energy of −33.2 to DMPO) is stable likely on the silica surfaces or the catalyst kcal/mol. site. There is literature experimental data about stabilization of The hydrogen bonds arranged in both head-to-tail and DMPO adducts on secondary organic aerosol particles such as intramolecular fashion shown in both Figures 3b and 4b allow DMPO−HO , DMPO−RO, DMPO−RO , and DMPO−OH for the physisorption of OH (Figure 3b; bond distance of 1.62 2 2 detected by electrospray ionization−tandem mass spectrometry Å) with an exoergic reaction energy of ΔE = −21.95 kcal/mol (ESI−MS/MS) and DMPO−glutathionyl in an intracellular or chemisorption of reactive hydroxyl groups (Figure 4b; bond environment using high-performance liquid chromatography distance of 1.49 Å) with an exoergic reaction energy of −29.5 (HPLC). kcal/mol. To address the existence of surface site bound OH as well as Further addition of DMPO to the physisorbed/chemisorbed hydroxyl radicals in Figures 3b and 4b is also exoergic, leading high stability of DMPO−OH in an environment of CuO/SiO , to the stabilization and formation of the DMPO−OH adduct, ab initio calculations were initiated. The calculations were used for instance in Figure 3d, with an exoergic reaction energy of to assess the thermodynamic basis for the current interpretation −86.7 kcal/mol. The addition of DMPO to the chemisorbed of experimental results by hypothesizing the following: (1) OH in Figure 4b also yields an exoergic reaction energy of Because of EPFRs cycling mechanism the H O is formed at 2 2 5,6 −73.2 kcal/mol. Stabilization of DMPO−OH because of the interface of nanoparticle/water solution. The hydroxyl hydrogen bonding on the cluster surface is shown in Figures radicals may be generated by either an exogenous Fenton 5,6,47 3d and 4c. It has been shown recently that DMPO and reaction or by direct decomposition of H O on the surface 2 2 inorganic radicals favor radical addition over nucleophilic sites, defects (see the Supporting Information). (2) The addition in the presence of hydrogen bonding, both hydroxyl radicals are stabilized by surface-active centers. (3) experimentally and computationally. DMPO attacks stabilized (physisorbed/chemisorbed) OH These theoretical calculations complement experimental radicals, forming a DMPO−OH adduct, which stays on the evidence for the highly stable nature of DMPO−OH adducts surface for a long time because of energetic stabilization. in CuO/SiO aqueous solutions. CuO/SiO Model Systems with Both Trigonal- and Therefore, the integrated intensity of DMPO−OH adducts Tetrahedral-Coordinated Cu Sites. We have performed ab may be considered a sum of DMPO−OH formed from the initio calculations to investigate the stabilization (physisorption addition of free OH (because of the exogenous Fenton reaction and chemisorption) of OH radicals on model CuO/SiO generated by the cycle) to DMPO and a portion of DMPO− surfaces, followed by further interaction of the adsorbed OH OH stabilized on a particle surface (as a result of the attack of with DMPO. Note that physisorption of the OH radical is DMPO to OH trapped on the surface). Currently, the primarily characterized by the hydrogen bonding taking place DMPO−OH adducts formed in a solution or on the surfaces (the bonding distance of ≤2 Å), whereas chemisorption is of particles are not distinguishable. We may hypothesize that characterized by the absence of hydrogen bonding (the the rate of accumulation of DMPO−OH adducts on particle bonding distance close to the covalent bond value of, for surfaces decreases during incubation because of sluggish instance, in HO−OH, ∼1.45−1.47 Å). generation of OH; i.e., the initial EPFRs as well as reductants Experimental synthesis of copper-containing silicates reveals 48,48,49 are consumed in secondary reactions not generating additional a mixture of copper in each of its valence states. X-ray amounts of OH in the cycle. Because DMPO−OH decays photoelectron spectroscopy (XPS) reveals the presence of faster in solution, we conclude that the surface-stabilized copper hydroxide, copper oxide, and Si−O−Cu bonds in these DMPO−OH adducts are responsible for the longer incubation clusters. While Chang et al. argue that the stable Si−O−Cu times. In addition, the surface-stabilized DMPO−OH adducts bonds are primarily electrostatic, Parameswaran et al. suggest are not expected to return to solution because the exoergicity of that their stable nature is covalent. the reactions for both a trigonal planar Cu (Figure 3) and a Our model reactant surface is a copper-containing silica-like tetrahedral Cu (Figure 4) are sufficiently high at −86.7 and structure derived from the addition of a −O−Cu−(OH) −73.2 kcal/mol, respectively. moiety to the previously optimized tetrahedrally-coordinated, Our calculations also show that DMPO may interact with radical hydroxide cluster found by Kubicki et al. [Figures 3a physisorbed or chemisorbed hydroxyl groups on the CuO/SiO (3-coordinate Cu) and 4a (vide infra tetrahedral Cu cluster)]. 2 cyclic cluster surfaces with the release of DMPO−OH into Radical-ended, as opposed to ionic, silica-like structures have solution (see the Supporting Information). As a consequence, been computationally shown to add water favorably via a radical there may be multiple hydroxyl-radical-generating pathways: (i) silicate−water mechanism, as opposed to a cationic or anionic The mayor channel of OH generation is the cycling scheme of silicate mechanism, with both radical pathways (H O+ SiO or 5,47 EPFR proposed earlier. OH forms through the exogenous SiO ) resulting in a hydroxylated silica surface site. We have Fenton reaction as follows: considered a limited number of atoms around an active site to make the calculations tractable, as small models have been used 50,51,41 H O + Cu(I) (organic ligand) successfully by other researchers. − • The optimized Cu atoms in structures a and b of Figure 3 are →+ Cu(II) (organic ligand) OH+ OH (1) both incorporated into a trigonal planar geometry, with Cu− OH and Cu−O bond distances similar to experimental Hydroxyl radicals formed in reaction 1 are either partially 52,53 values. Small inorganic Cu(I) and Cu(II) complexes have transferred into solution and form homogeneously DMPO− 9224 dx.doi.org/10.1021/es501158r | Environ. Sci. Technol. 2014, 48, 9220−9226 Environmental Science & Technology Article (8) Zhang, G. Y.; Long, J. L.; Wang, X. X.; Zhang, Z. Z.; Dai, W. X.; OH adducts or partially stabilized on the particle surfaces, Liu, P.; Li, Z. H.; Wu, L.; Fu, X. Z. Catalytic role of Cu sites of Cu/ forming DMPO−OH stable adducts (Figures 3 and 4). (ii) Ab MCM-41 in phenol hydroxylation. Langmuir 2010, 26 (2), 1362− initio calculations show that a partial decomposition of H O on 2 2 the silica surface active sites is also possible. For instance, (9) Bataineh, H.; Pestovsky, O.; Bakac, A. pH-Induced mechanistic because of homolytic cleavage of H O on the silica active sites 2 2 changeover from hydroxyl radicals to iron(IV) in the Fenton reaction. (defects, dangling bonds, etc.), one hydroxyl group hydrox- Chem. Sci. 2012, 3 (5), 1594−1599. ylates the surface site (chemisorption) and the second hydroxyl (10) Burkitt, M. J.; Tsang, S. Y.; Tam, S. C.; Bremner, I. Generation radical is trapped between neighboring Si−OH groups on the of 5,5-dimethyl-1-pyrroline N-oxide hydroxyl and scavenger radical surface (by hydrogen bonds) (see Figure S1B of the Supporting adducts from copper/H O mixturesEffects of metal ion chelation 2 2 Information). Further experimental addition of DMPO leads to and the search for high-valent metal−oxygen intermediates. Arch. stabilization and formation of DMPO−OH on the surfaces Biochem. Biophys. 1995, 323 (1), 63−70. (Figure S2 of the Supporting Information). (11) Finkelstein, E.; Rosen, G. M.; Rauckman, E. J. Spin trapping of superoxide and hydroxyl radicalPractical aspects. Arch. Biochem. Biophys. 1980, 200 (1), 1−16. ASSOCIATED CONTENT (12) Makino, K.; Hagiwara, T.; Murakami, A. Fundamental aspects of * Supporting Information spin trapping with DMPO. Radiat. Phys. Chem. 1991, 37 (5−6), 657− Data about the addition of OH and H O to the tetrahedral 2 2 form of Cu in CuO/SiO model systems (cyclic structure) and (13) Singh, R. J.; Karoui, H.; Gunther, M. R.; Beckman, J. S.; Mason, computational details about direct decomposition of H O on 2 2 R. P.; Kalyanaraman, B. Reexamination of the mechanism of hydroxyl silica active sites. This material is available free of charge via the radical adducts formed from the reaction between familial amyotrophic Internet at http://pubs.acs.org. lateral sclerosis-associated Cu,Zn superoxide dismutase mutants and H O . Proc. Natl. Acad. Sci. U. S. A. 1998, 95 (12), 6675−6680. 2 2 AUTHOR INFORMATION (14) Hanna, P. M.; Chamulitrat, W.; Mason, R. P. When are metal ion-dependent hydroxyl and alkoxyl radical adducts of 5,5-dimethyl-1- Corresponding Author pyrroline N-oxide artifacts. Arch. Biochem. Biophys. 1992, 296 (2), *Telephone: 225-578-4417. E-mail: lkhach1@lsu.edu. 640−644. Notes (15) Chamulitrat, W.; Iwahashi, H.; Kelman, D. J.; Mason, R. P. The authors declare no competing financial interest. Evidence against the 1−2−2−1 Quartet DMPO spectrum as the radical adduct of the lipid alkoxyl radical. Arch. Biochem. Biophys. 1992, ACKNOWLEDGMENTS 296 (2), 645−649. (16) Truong, H. Copper(II) oxide mediated formation and The authors gratefully acknowledge the partial support of this stabilization of combustion generated persistent free radicals. Ph.D. research under the National Institute of Environmental Health Dissertation, Department of Chemistry, Louisiana State University, Sciences (NIEHS) Grant (Superfund Research and Training Baton Rouge, LA, 2007. Program) PES013648Z, the LA-SIGMA NSF Grant EPS- (17) Lomnicki, S.; Truong, H.; Vejerano, E.; Dellinger, B. Copper 1003897, the Lillian L. Y. Wang Yin, Ph.D. Endowed Professor oxide-based model of persistent free radical formation on combustion- Chair held by Randall W. Hall, and the Patrick F. Taylor Chair derived particulate matter. Environ. Sci. Technol. 2008, 42 (13), 4982− held by Barry Dellinger. The authors also acknowledge the high-performance computing services provided by the Louisi- (18) Truong, H.; Lomnicki, S.; Dellinger, B. Potential for ana State University Center for Computational Technology and misidentification of environmentally persistent free radicals as molecular pollutants in particulate matter. Environ. Sci. Technol. the Louisiana Optical Network Initiative. 2010, 44 (6), 1933−1939. (19) Nakagawa, K. Is quartz flat cell useful for the detection of REFERENCES superoxide radicals? J. Act. Oxygens Free Radicals 1994, 5,81−85. (1) Squadrito, G. L.; Cueto, R.; Dellinger, B.; Pryor, W. A. Quinoid (20) Eaton, G. R.; Eaton, S. S.; Barr, D. P.; Weber, R. T. Quantitative redox cycling as a mechanism for sustained free radical generation by EPR; Springer-Verlag: Berlin, Germany, 2010; pp 185. inhaled airborne particulate matter. Free Radical Biol. 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