Abstract

A procedure is described whereby crude atomic coordinates obtained from a medium-resolution electron density map (~3-4 A) with the aid of Watson-Kendrew models may be refined to native data of near-atomic resolution using only a limited subset of the data and non-interactive computer graphics. This refinement procedure for the Bence-Jones protein Rhe included phase extension from 3.0 to 1.9 A and led to an improved crystallographic sequence for the protein. The structure was refined, by a stereochemically restrained least-squares technique in reciprocal space, to a residual value RF = 0.284 for a model consisting of 795 non-hydrogen protein atoms with an overall thermal factor using data with d ranging from 5.0 to 1.9 A. Relative weights for the structure factor and stereochemical-restraint observations were determined empirically and the optimum weights were found to be those which yield values of W|Fo - Fc|2 which are typically 4 to 6 times the value of W|do - dI|2, where do and dI are the current and ideal values for the stereochemically restrained parameters. It was found that reasonable refinement may be obtained with only 32% of the observed data.