Thin layer spectroelectrochemical study of vitamin B12 and related cobalamin compounds in aqueous media

Interaction of C-Si, C-Sn, and Si-Si sigma-bonds with chalcogen lone pairs

The ability of neighboring C-Si, C-Sn, and Si-Si groups in conformationally constrained cyclic molecules to reduce the lowest ionization energies of sulfur, selenium, and tellurium compounds has been determined by charge-transfer spectroscopy of complexes with tetracyanoethylene. For selected compounds, ionization energies were determined by gas-phase photoelectron spectroscopy. The lowest ionization energies measured by photoelectron spectroscopy, with one exception, correlate with the charge-transfer spectroscopic data. In addition, theoretical analysis has provided insight into the photoelectron spectra and the geometry-dependent interaction between C-Si or C-Sn bonds and chalcogen lone pairs. Substantial lowering of ionization energies is found which is anticipated to have important consequences in the chemistry of these and related species.

A novel form of vitamin B-12 and its derivatives

A new isomeric form of cobalamins is reported. The conversion of cobalamin to cobalamin (the new form) is achieved by substituting the benzimidazole base by a less bulky group like H2O or CN- and modest thermal treatment. The back conversion of adenosylcobalamin to the corresponding regular form occurs in the "base-off" form at room temperature. It seems that the corrin ring becomes quite flexible in the "base-off" form and the freer axial movement of the cobalt atom flips the corrin ring into a different conformation. The change in conformation is borne out by subtle changes in the proton magnetic resonances on the corrin ring and the base, and very marked variation in the emission Mössbauer spectra. The latter is indicative of appreciable changes in the spatial conformation in the immediate vicinity of the central metal atom. The ultraviolet-visible and infrared spectra of cobalamin are indistinguishable from those of its corresponding regular form. The new conformational isomeric species is present as an impurity in all commercially available cobalamins (including pharmaceutical preparations). It raises the question whether the cobalamins' constitute the real biologically active anti-anemic factor in humans.