HIGH-RESOLUTION NEUTRON STUDY OF VITAMIN-B12 COENZYME AT 15-K - STRUCTURE-ANALYSIS AND COMPARISON WITH THE STRUCTURE AT 279-K

C72H100N18O17PCo.nH2O, where n = 17-18, M(r) = 1597 dalton (assuming that 19 coenzyme H atoms have been exchanged for D atoms), P2(1)2(1)2(1), a = 27.550 (7), b = 21.568 (5), c = 15.343 (3) angstrom, V = 9117 (4) angstrom3, Z = 4, D(x) = 1.38 (1) Mg m-3; Cu(220) monochromator, lambda1 = 1.5446 (5) angstrom, 5505 unique reflections to nominal resolution 0.98 angstrom, mu = 2.20 cm-1; Ge(115) monochromator, lambda2 = 1.3169 (3) angstrom, 2361 unique reflections from nominal resolution 1.02 to 0.9 angstrom, mu = 1.6 cm-1; T = 15 K, final R = 0.085 for 7378 reflections. A high-resolution and low-temperature structural study of the vitamin B-12 coenzyme has been undertaken using neutrons. Details of the data collection and refinement of the low-temperature structure are described, and a comparison of the coenzyme molecule and solvent structures at 15 and 279 K made. A shrinkage of around 2% is observed in the volume of the unit cell, and the orientation of the coenzyme molecule is rotated within the unit cell by approximately 2-degrees, about an axis almost parallel to c and close to the Co atom. Incomplete or reverse exchange between D and H atoms led to some difficulties assigning certain exchangeable H positions in the coenzyme. 56 solvent atoms have been identified from neutron difference Fourier maps.