Acuracy of refined protein structures. II. Comparison of four independently refined models of human interleukin 1β

To assess the accuracy of refined structures, a comparison was made using independently determined structures of the same protein in the same crystal form. The models were re-refined against a common data set to minimize the effects of different data and different refinement protocols. The process did not converge to a single model. Rather the structures differed from each other by 0.84 A which was roughly three times that predicted by a Luzzati analysis [Luzzati (1952). Acta Cryst. 5, 802-810]. The individual structures are equally valid and at least partially independent as evidenced by a reduction of the R factor by 0.013 when a simple linear combination is used. Only 29 solvent molecules were common to all four models.