Refinement of triclinic lysozyme: II. The method of stereochemically restrained least squares

Abstract

Refinement of triclinic lysozyme by restrained least squares against the 2 .ANG. resolution X-ray data is described, beginning with the model for cycle 17 of the preceding paper [Hodsdon, Brown, Sieker and Jensen (1990). Acta Cryst. B46, 54-62]. After 20 refinement cycles, R stood at 0.172. Nevertheless, serious errors involving both main-chain and side-chain atoms still remained, requiring numerous model rebuilding sessions interleaved with refinement cycles. After 63 cycles R = 0.124 for the model which includes all protein atoms, 249 water oxygen sites and five nitrate ions. Although the overall B is relatively low, 10.5 .ANG.2, B''s for atoms in the region of residues 101-103, toward the termini of some of the longer side chains, and in the region of the C terminus of the main chain exceed 20 .ANG.2, indicating relatively high atomic mobilities, disorder, or remaining errors in the model.