ONE-ELECTRON REDUCTION POTENTIALS OF COENZYME-B12 AND ALKYLCOBALAMINS

The one-electron reduction potentials for the alkylcobalamins, R = CH3, CH2CH3, n-propyl, isobutyl, neopentyl and deoxyadenosyl, were examined by differential pulse polarography in 1:1 DMF:H2O (μ = 0.10 LiClO4) at 24.6 °C. The E 1 2 values for the couple RCo + e- ⇄ RCo· (RCo = alkylcobalamin) were found to be -1.60 V, R = CH3; -1.54 V, R = CH2CH3; 1.55 V, R = n-propyl; -1.48 V, R = isobutyl; -1.38 V, R = neopentyl; -1.35 V, R = deoxyadenosyl versus SCE. E 1 2 correlates linearly with the Taft steric parameter, Es; a new Es value for the deoxyadenosyl functional unit is estimated to be -2.03 from this relationship. A moderate solvent influence was observed by replacing H2O with D2O for methylcobalamin (E 1 2 = -1.68 V versus SCE) and for deoxyadenosylcobalamin (coenzyme B12) (-1.43 V). This suggests that solvation effects are about the same for alkylcobalamins compared to methylcobalamin and therefore do not account for the 0.22 V more negative reduction potential of methylcobalamin. The gradation in E 1 2 starting with methylcobalamin and continuing through the alkylcobalamin series may reflect changes in the axial ligand distances which modulate the energy of the lowest σ type MO (LUMO) of these complexes. © 1990.