Biosynthesis of corrins. II. Incorporation of 13C-labeled substrates in vitamins B12

Carbon-13 nuclear magnetic resonance studies of adenosylcobalamin and alkylcorrinoids, selectively enriched with carbon-13

The carbon-13 nuclear magnetic resonance spectra of a series of alkylcorrinoids, selectively enriched with 13C in the alkyl ligand, were recorded at 25.2 MHz and 25 degrees. The nature of the axial ligands markedly affects the chemical shift of the labeled alkyl moiety (trans effect) as well as the 13C resonances of selected carbon atoms of the corrin ring (cis effect). Although a number of factors appear to influence the trans effect on the chemical shift of the alkyl ligand (important among them being electric field effects), the cis effect appears to be dominated by changes in charge density (at the methine bridge carbon atoms, C-5, C-10, C-15) and by steric effects (at the methyl groups at C-1, C-5, and C-15) accompanying axial ligation. Spin-latice relaxation times of several organocorrinoids, selectively labeled with 13C in the ligands attached to cobalt, were also measured. The T1 values of the methylene carbons of [5'-13C]adenosylcobalamin and [2-13C]carboxymethylcobalamin are very similar to that of the methine bridge carbon atom C-10 of the corrin ring, indicating that rotation about the carbon-cobalt bond of these two corrinoids is severely restricted. On the other hand, internal rotation about the carbon-cobalt bond of methylcobalamin is rapid.