Structure solution of a cubic crystal of concanavalin A complexed with methyl alpha-D-glucopyranoside.

The solution of the cubic crystal form (a = 167.8 A) of concanavalin A complexed with the monosaccharide methyl α-d-glucopyranoside is described. The space group has been determined as I213 rather than I23. The use of cadmium to replace cobalt at the transition metal-ion binding site and to replace calcium at its binding site proved to be crucial to the successful solution of the crystal structure. The relatively small isomorphous signals of 21 e− for the replacement of cobalt and 28 e− for the replacement of calcium, yielded interpretable difference Patterson maps. The electron-density map calculated in space group I213 at 5.4 A resolution, based on phases derived from single- and double-substituted cadmium differences, revealed a classical concanavalin A tetramer of 222 point symmetry, as seen in all the known crystal structures of concanavalin A. Rigid-body refinement at 3.6 A using the refined coordinates of saccharide-free concanavalin A converged to an R factor of 27.4%. A molecular-replacement analysis, consistent with this crystal structure, and initial experiences in the incorrect space group I23 are described as these also prove to be instructive.