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 15 K, is presented. The study involved the identification of solvent peaks and the formulation of possible solvent networks. The solvent distribution 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 formulation of two main solvent networks per asymmetric unit, with the assistance of criteria used in the analyses of solvent structures in crystal hydrates of small molecules. The two networks comprise 17 water molecules each. A comparison of these solvent networks is made with those identified in a previous study of a crystal of the coenzyme at 279 K. The covalent and hydrogen-bond geometries involving the water molecules of these networks have been analysed and agree well with those found in small molecular crystal hydrates. Furthermore, the analysis of the water structure around apolar groups of the B-12 coenzyme indicates the presence of clathrate-like water structures, as well as short distances which others have identified as C-H...O hydrogen bonds. Short range O...O non-hydrogen-bonded contacts obey known repulsive restriction rules formulated from small-molecule hydrate crystals.