Electroreduction of a Series of Alkylcobalamins:  Mechanism of Stepwise Reductive Cleavage of the Co−C Bond

Abstract

The electrochemical (EC) reduction mechanism of methylcobalamin (Me-Cbl) in a mixed DMF/MeOH solvent in 0.2 M tetrabutylammonium fluoroborate electrolyte was studied as a function of temperature and solvent ratio vs a nonaqueous Ag/AgCl/Cl(-) reference electrode. Double-potential-step chronoamperometry allowed the rate constant of the subsequent homogeneous reaction to be measured over the temperature range from 0 to -80 degrees C in 40:60 and 50:50 DMF:MeOH ratios. Activation enthalpies are 5.8 +/- 0.5 and 7.6 +/- 0.3 kcal/mol in the 40:60 and 50:50 mixtures of DMF/MeOH, respectively. Digital simulation and curve-fitting for an EC mechanism using a predetermined homogeneous rate constant of 5.5 x 10(3) s(-1) give E degrees' = -1.466 V, k degrees = 0.016 cm/s, and alpha = 0.77 at 20 degrees C for a quasi-reversible electrode process. Digital simulation of the results of Lexa and Savéant (J. Am. Chem. Soc. 1978, 100, 3220-3222) shows that the mechanism is a series of stepwise homogeneous equilibrium processes with an irreversible step following the initial electron transfer (ET) and allows estimation of the equilibrium and rate constants of these reactions. An electron coupling matrix element of H(kA) = (4.7 +/- 1.1) x 10(-4) eV ( approximately 46 J/mol) is calculated for the nonadiabatic ET step for reduction to the radical anion. A reversible bond dissociation enthalpy for homolytic cleavage of Me-Cbl is calculated as 31 +/- 2 kcal/mol. The voltammetry of the ethyl-, n-propyl-, n-butyl-, isobutyl-, and adenosyl-substituted cobalamin was studied, and estimated reversible redox potentials were correlated with Co-C bond distances as determined by DFT (B3LYP/ LANL2DZ) calculations.

The Nature of the Co-C Bond Cleavage Processes in Methylcob(II)Alamin and Adenosylcob(III)Alamin

“…However, even with this barrier, a kinetic calculation shows that fast bond cleavage is improbable. Furthermore, the energy barrier of the Co-C bond breaking is much higher for base-on compared to the base-off species [39,68,106]. Our estimations show that the energy barrier of the geometry optimized 1e-reduced base-on specie is higher by about 0.5 eV compared to the energy barrier of the geometry optimized 1e-reduced base-off specie (see Figure 7 for single points calculations).…”

“…Despite the progress made by the research community on the theoretical explanation of vitamin B 12 cofactor reaction, there are still signifi cant disagreements of theoretical results with experimental data. Thus, our calculation of the total energy with geometry optimization at each constrained C-Co bond length for the base-off CH3-Co(II) model gives the lowest energy barrier for C-Co bond breaking (value of 0.7 eV, Figure 6) [39,106]. However, even with this barrier, a kinetic calculation shows that fast bond cleavage is improbable.…”

“…The methylcobalamide compound shows a single irreversible cathodic wave at low sweep rates corresponding to the reductive cleavage of the cobalt-carbon bond. At the slow scan rate of 0.3 V/s t he cyclic voltammetry (CV) of methylcobalamin showed no return wave; however, as the scan rate increased to 10 V/s and above at −20 °C, a return wave appeared which allowed Lexa and Savéant [38] Unfortunately, the above mechanism is not fully supported by theory when considering DFT calculation data [39,40]. Additionally, the generally accepted methylcobalamin reductive mechanism (Figures 2 and 3), based on various experimental methods including X-ray diffraction, does not fi t exactly with the electrochemistry based methylcobalamin reductive mechanism (Figure 4).…”

“…As it is depicted in Figure 3: (a) the methylcobalamin-dependent methionine synthase generally includes as a primary turnover the cycling between Co(III) methylcobalamin and cob(I)alamin, which allows the heterolytic Co-C cleavage process, while in the adenosyl-cobalamin-dependent mutases the Co-C bond of adenosylcobalamin is cleaved homolytically, giving rise to a fi ve-coordinate cob(II)alamin and an adenosyl radical; (b) the association of the substrate and adenosyl-ligand during the Co-C cleavage process in adenosylcobalamin dependent mutases has been proved by various experimental data including X-ray diffraction results [35][36][37], while such effect is totally absent in studies regarding methylcobalamindependent mutases. The Co-C cleavage and electron transfer processes have been studied by different methods such as electrochemistry [38][39][40], thermo- [41] or photochemistry [42][43][44][45][46] of methylcobalamin Co(III) by a signifi cant number of researchers. As a result, many triggering factors of Co-C bond breaking have been considered and various mechanistic mechanisms of the Co-C cleavage process have been proposed.…”

“…Consequently, various properties have been studied, including electron densities and reduction potentials determination [69,76,88,91,96], the comparison of DFT and CASSCF electronic structure data of truncated models of methylcobalamin species [70,71,74,81,85], the infl uence of various factors on the Co-C bond forming and cleavage in methyl-and 5-adenosylcobalamin models [68,[72][73][74][75][76]79,80,83,84,89,90,92,94,97,99,100], and excited states, spectroscopy and photo-dissociation analysis of vitamin B 12 species [78,82,86,87,93,95,98]. We used DFT calculations on Me-Cbl models to calculate total energy as a function of Co-C bond distance as the bond is stretched [39]. The generally used base-on model of R-Cbl(III)(R=CH 3 ) includes the full corrin ring with all side-chain groups replaced by hydrogen atoms and with the dimethylbenzimidazole base ligand replaced by benzimidazole (or imidazole), as it is shown in Figure 5.…”