Spectroscopic and computational studies of Co1+ cobalamin:: Spectral and electronic properties of the "superreduced" B12 cofactor

The 4-coordinate, low-spin cob(I) alamin (Co(1+)Cbl) species, which can be obtained by heterolytic cleavage of the Co-C bond in methylcobalamin or the two-electron reduction of vitamin B-12, is one of the most powerful nucleophiles known to date. The supernucleophilicity of Co(1+)Cbl has been harnessed by a number of cobalamin-dependent enzymes, such as the B-12-dependent methionine synthase, and by enzymes involved in the biosynthesis of B-12, including the human adenosyltransferase. The nontoxic nature of the Co(1+)Cbl supernucleophile also makes it an attractive target for the in situ bioremediation of halogenated waste. To gain insight into the geometric, electronic, and vibrational properties of this highly reactive species, electronic absorption, circular dichroism (CD), magnetic CD, and resonance Raman (rR) spectroscopies have been employed in conjunction with density functional theory (DFT), time-dependent DFT, and combined quantum mechanics/molecular mechanics computations. Collectively, our results indicate that the supernucleophilicity of Co(1+)Cbl can be attributed to the large destabilization of the Co 3d(z2)-based HOMO and its favorable orientation with respect to the corrin macrocycle, which minimizes steric repulsion during nucleophilic attack. An intense feature in the CD spectrum and a prominent peak in the rR spectra of Co(1+)Cbl have been identified that may serve as excellent probes of the nucleophilic character, and thus the reactivity, of Co(1+)Cbl in altered environments, including enzyme active sites. The implications of our results with respect to the enzymatic formation and reactivity of Co(1+)Cbl are discussed, and spectroscopic trends along the series from Co(3+)Cbls to Co(2+)Cbl and Co(1+)Cbl are explored.