Emission Mössbauer studies of some organocobalamins

Subtle changes in the Mössbauer parameters are observed while going from methyl- to ethyl- to adenosylcobalamin, and also when the "base" is detached from the cobalt. The observation of these changes demonstrates that the Co-C bond, among others, remains intact after the Auger event, accompanying the electron-capture decay of the cobalt-57. The differences between ethylcobalamin and the other two organocobalamins in the magnitude of the quadrupole splittings have been interpreted on the basis of the sigma-donating tendency of the organic moiety and the Co-C bond length. The latter is presumably determined by the steric hindrance offered to the group in approaching the cobalt atom. The ethyl- and adenosylcobalamins in their "base-off" form exhibit a larger quadrupole splitting than the corresponding "base-on" form. In the "bas-off" form, the cobalt atom is perhaps raised above the mean plane of the four equatorial nitrogen atoms of the corrin ring, which may result in the diminution of the delocalization of the 3dpi electron density. The higher population of dpi orbitals and the enhanced metallic character of the dz2, resulting from shrinkage of the Co-C bond length, enhances the magnitude of the quadrupole splitting.