Low-temperature water reconstruction in concanavalin A, with implications for controlled protein crystal annealing

Flash-cooled crystals of the I222 form of concanavalin A undergo a sharp non-destructive non-reversible phase transition upon warming to between 160 and 165 K, characterized by an anomalous increase in unit-cell volume. The expansion is anisotropic and primarily affects the b and c axes. Three sets of 1.7 Å X-ray diffraction data were collected from one crystal: immediately after flash-cooling (130 K), after slow warming to 170 K and after re-cooling to 130 K. Measurable changes in reflection width after the transition were apparent. Each data set was collected under very similar conditions (aside from temperature). Structures refined against each data set using a powerful phase-bias reduction algorithm indicate negligible rearrangement of the protein molecule and its first hydration shell. Further away from the protein surface, on the edge of the fully disordered solvent regions, a small number of more dramatic changes were apparent. Little is known about the behaviour of water confined within dimensionally restricted spaces, but solvent cavities and channels in protein crystals provide a rich source of reproducible nanoscale water assemblies. This paper presents information on the behaviour of water confined within protein cavities in relation to the physics of protein crystal annealing.