Determination of disorder using the three-dimensional pair distribution function
Arkadiy Simonov, Thomas M. Weber, W. Steurer · Acta Crystallographica Section A Foundations of Crystallography · 2010
The seminal work by Dorothy C. Hodgkin and her group on vitamin B12 (cyanocobalamin) [1-3] sparked a wealth of subsequent structural investigations on the vitamin and its derivatives.[4] The crystals are prone to solvent loss and disorder, in particular that of the solvent region,[5] making data collection and structural refinement not straightforward.We have determined the hydrated, "wet" structure of the title compound by a) compressing a single crystal to 1.0 GPa and by b) growing a single crystal in situ in a capillary at ambient conditions.Diffraction data were collected at roomtemperature using X-ray synchrotron radiation.A comparison of the two structures reveals: a) dramatic increase in ordering of the water solvent region, b) smaller displacement parameters of the B12 moiety and c) substantial conformational change in the side chains of the vitamin induced by high pressure.Possible mechanisms that account for these observations are discussed and related to observations made on protein crystals.[6]The use of high pressure and low temperature as tools to probe conformational flexibility and increase order in crystals containing a large amount of water are also discussed.