DIRECT OBSERVATION OF PROTEIN SOLVATION AND DISCRETE DISORDER WITH EXPERIMENTAL CRYSTALLOGRAPHIC PHASES

A complete and accurate set of experimental crystallographic phases to a resolution of 1.8 angstroms was obtained for a 230-residue dimeric fragment of rat mannose-binding protein A with the use of multiwavelen gth anomalous dispersion (MAD) phasing. An accurate image of the cryst al structure could thus be obtained without resort to phases calculate d from a model. Partially reduced disulfide bonds, local disorder, and differences in the mobility of chemically equivalent molecules are ap parent in the experimental electron density map. A solvation layer is visible that includes well-ordered sites of hydration around polar and charged protein atoms, as well as diffuse, partially disordered solve nt shells around exposed hydrophobic groups. Because the experimental phases and the resulting electron density map are free from the influe nce of a model, they provide a stringent test of theoretical models of macromolecular solvation, motion, and conformational heterogeneity.