CO-C FORCE-CONSTANTS FROM RESONANCE RAMAN-SPECTRA OF ALKYLCOBALAMINS - INSENSITIVITY TO DIMETHYLBENZYLIMIDAZOLE COORDINATION

The Co-C stretching vibration has been identified in resonance Raman s pectra of alkyl-cobalamins, via isotope substitution, permitting estim ation of the Co-C force constants, f=1.85, 1.77 and 1.50 mdyn Angstrom (-1) for methyl-, ethyl- and deoxyadenosyl-cobalamin, respectively (up silon(Co-C) = 506, 471 and 442/429 cm(-1)). These values scale with th e reported band dissociation energies, and support the view that the C o-C bond weakens with increasing bulk of the alkyl group due to steric interaction with the corrin ring. However, the force constants are un affected by dissociation of the dimethylbenzimidazole ligand at low pH , even though the bond dissociation energy rises significantly upon DM B dissociation in AdoCbl. This increase must therefore: reflect destab ilization of the Co-II product, rather than Co-C bond strengthening in the AdoCbl ground state. The insensitivity of the force constants to dimethylbenzimidazole dissociation implies that the steric effect of D MB coordination is not transmitted to the Co-C bond by the corrin ring . Consistent with this interpretation, the RR frequencies of the corri n ring modes are minimally perturbed by DMB dissociation, supporting e arlier NMR results that indicated little change in the corrin conforma tion. (C) 1998 Elsevier Science S.A.