Near-IR FT-Raman Spectroscopy of Methyl-B12 and Other Cobalamins and of Imidazole and Imidazolate Methylcobinamide Derivatives in Aqueous Solution

Abstract

Near-IR FT-Raman spectra of B12 imidazole derivatives and cobalamins in aqueous solution were compared since there is now strong evidence that, in human B12-dependent enzymes, the 5,6-dimethylbenzimidazole (DMBz) is replaced by imidazole from a histidine in the protein. Derivatives studied include methylcobalamin [MeCbl (DMBz base-on) and MeCbl+ (base-off by acidification to protonate the DMBz)], methylaquacobinamide (MeCbi+) [Cbi's have the DMBz-bearing nucleotide loop removed by hydrolysis], and Me(N-acetylhistidine)Cbi [coordinated through imidazole in Me(N-AcHis)Cbi at pH 10 and imidazolate in Me(N-AcHis)Cbi- in 1 M NaOH]. Several marker bands changed with changes in the axial ligand trans to the methyl group. The frequency of the Co−CH3 stretching mode at ∼505 cm-1 (assigned by isotopic shift using −CD3) was similar for all MeCbl and MeCbi species; thus, the trans ligand, including the very powerful electron-donating imidazolate species, has little effect on Co−C bond strength. In contrast, the peak ...