The structure of vitamin B 12 III. The crystal structure analysis of the air-dried B 12 crystals

Abstract

The X-ray structure analysis of vitamin B 12 in the air-dried crystals is described in detail up to the point at which the molecular structure was considered established apart from certain double bond and hydrogen atom positions. The heavy-atom technique was used, based on the cobalt atom which is an intrinsic part of the B 12 molecule. While this atom was much more effective in phase determination than a formal calculation of its scattering power as against that of the rest of the crystal asymmetric unit would indicate, it was not a sufficiently dominating influence, and consequently the electron density maps based on cobalt phases alone contained too many spurious peaks for a complete structure to be postulated at the initial stage. Some form of trial was therefore necessary; this took the form of comparing the densities generated by several rather different postulated scattering nuclei included in the phase calculations. The techniques by which the very strong bias towards atoms inserted in the calculations in the non-centrosymmetric case could be circumvented are described, as are other aspects of the work which have a bearing on future structure analyses of very complex organic molecules.