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Emission mössbauer studies of some cobalamins with with and without coordination linkage to the 5,6-dimethyl-benzimidazole base
Journal article

Emission mössbauer studies of some cobalamins with with and without coordination linkage to the 5,6-dimethyl-benzimidazole base

Kimio Inoue and Amar Nath
Bioinorganic chemistry, v 7(2)
1977
DOI: https://doi.org/10.1016/S0006-3061(00)80066-6

Abstract

The emission Mössbauer spectra of “base on” and “base off” forms of the cyanocob(III)alamin (vitamin B 12) and the cob(II)alamin (B 12r) and the cob(I)alamin (B 12s) are studied. The s-electron density on the daughter iron nucleus as reflected by the isomer shift does not change very much while going from B 12 → B 12r → B 12s especially for the latter, despite the fact that the increasing population of the d subshell is expected to enhance the shielding of the s-electron cloud resulting in diminution of s-electron density on the iron nucleus. These observations can be rationalized if one invokes considerable delocalization of d π electrons from the daughter iron atom onto the corrin ring and also the appreciable hybrid character of d z 2 which permits increase of the 4 s-electron density concomitantly with an increase in the population of d z 2 orbital. The quadrupole splitting is perhaps predominantly determined by the anisotropy of covalent bonding to the iron atom by the corrin ring and the axial ligands. In the “base off” forms, a water molecule replaces the coordinated benzimidazole base, whereby the α-bonding of the axial ligand (s) is further diminished. The enhanced anisotropy of covalent bonding results in an increase in the magnitude of the quadrupole splittings for the “base off” forms. The finger prints of the “base on” and the “base off” B 12 and B 12r and B 12s would be valuable in identifying the intermediates in enzymatic reactions involving B 12.

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