The Biosynthesis of Vitamin B12: A Study by 13C Magnetic Resonance Spectroscopy

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The origin of the methyl group on C-1 of Ring A of the corrin ring of vitamin B12 was investigated by 13C magnetic resonance spectroscopy. The proton-decoupled 13C spectra of vitamin B12 synthesized from [5-13C]δ-aminolevulinic acid by Propionibacteria were obtained by Fourier-transform nuclear magnetic resonance of high resolution, and spectra of high-resolution proton magnetic resonance of the 13C-labeled B12 were also taken. The δ-carbon atom of δ-aminolevulinic acid is the source of seven or eight known positions of vitamin B12, depending on whether the C-1 methyl group is also derived from the labeled substrate. We have found seven resonances whose chemical shifts enable us to identify the position of the 13C atoms in the molecule from the assignment of Dodderell and Allerhand. We observed no 13C resonance corresponding to the C-1 methyl group of Ring A. Furthermore, the proton magnetic resonance spectrum showed no spin-spin splitting of the proton peak at the τ values assigned for the H atoms in this methyl group. It would thus appear that the methyl group at C-1, which has been considered to have the same origin as the δ-methine bridge of porphyrins, does not originate from the δ-carbon atom of δ-aminolevulinic acid. This finding may aid in elucidation of the mechanism of synthesis of the functional isomer of vitamin B12 and porphyrins. The 13C-labeled vitamin B12 enabled us to make further assignments to the 13C magnetic resonance of the vitamin.

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