HIGH-RESOLUTION NEUTRON STUDY OF VITAMIN-B-12 COENZYME AT 15-K - SOLVENT STRUCTURE

The solvent structure of crystalline vitamin B-12 coenzyme, determined from a high-resolution (0.9 Angstrom) neutron data set collected at 1 5 K, is presented. The study involved the identification of solvent pe aks and the formulation of possible solvent networks. The solvent dist ribution within the crystal can be described in two regions, namely (a ) a channel, comprizing statically disordered water molecules and (b) leading into this channel, a region of highly ordered water molecules. The identification of the disordered solvent peaks has enabled the fo rmulation of two main solvent networks per asymmetric unit, with the a ssistance of criteria used in the analyses of solvent structures in cr ystal hydrates of small molecules. The two networks comprise 17 water molecules each. A comparison of these solvent networks is made with th ose identified in a previous study of a crystal of the coenzyme at 279 K. The covalent and hydrogen-bond geometries involving the water mole cules of these networks have been analysed and agree well with those f ound in small molecular crystal hydrates. Furthermore, the analysis of the water structure around apolar groups of the B-12 coenzyme indicat es the presence of clathrate-like water structures, as well as short d istances which others have identified as C-H...O hydrogen bonds. Short range O...O non-hydrogen-bonded contacts obey known repulsive restric tion rules formulated from small-molecule hydrate crystals.