The structure of vitamin B 12 . VIII. The crystal structure of vitamin B 12 -5'-phosphate

The crystal structure of vitamin B 12 -5' -phosphate, a biosynthetic precursor of vitamin B 12 , has been determined by X-ray diffraction methods. The air-dried crystals are orthorhombic, space group P 2 1 2 1 2 1 , with a = 23.72 Å, b = 21.74 Å and c = 16.07 Å. The observed density of 1.362 g cm -3 indicates four vitamin B 12 -5' -phosphate and about sixty water molecules in the unit cell. The X-ray diffraction data to 1.2 Å resolution were collected with a PAILRED linear diffractometer using Cu K α radiation from a silicon monochromator. The structure was solved by using Patterson methods in conjunction with the tangent formula and was refined by Fourier and least-squares methods. The final R value for the 2112 observed reflexions is 16.2 %. The structure is very similar to that found by Hodgkin and co-workers for air-dried vitamin B 12 crystals. The difference Fourier synthesis between this compound and vitamin B 12 shows that two water molecules move into phosphate oxygen positions when the phosphate in the precursor is removed, and one acetamide in contact with these water molecules in the vitamin is rotated out of the way in the phosphate. A second acetamide is also differently oriented in the two crystals. The α -glycosidic bond between the ribose and the dimethylbenzimidazole is 16° out of the plane of the base. The ribose conformation is C2'>- exo , and the phosphate ester conformations are similar to those found in other B 12 crystals.