The effect of temperature and light on the carbon-13 nuclear magnetic resonance spectra of alkylcorrinoids, selectively enriched with carbon-13

Abstract

¹³ C N.m.r. spectra of aqueous (D₂O) solutions of methyl-, ethyl-, and adenoxyl-cobalamin, and methylaquo-cobinamide, selectively enriched with carbon-13 in the alkyl ligands attached to cobalt have been recorded at 25.2 MHz and over the temperature range 5–90 °C. The methyl resonances of [¹³C]methylcobalamin and [¹³C]methylaquocobinamide, as well as the methylene resonances of [1,2-^l3C₂]ethylcobalamin and [5′-¹³C]adenosylcobalamin, exhibit small upfield shifts with increasing temperature. These small differences in chemical shift can best be accounted for by invoking a temperature-dependent conformational change involving substituents on the corrin ring, rather than a change in the co-ordination number of cobalt. ¹³C N.m.r. spectroscopy has also been used to study homolytic cleavage of the carbon–cobalt bond of the alkylcobalamins. Ethylcobalamin undergoes thermal decomposition even at 60 °C, while methyl- and adenosyl-cobalamin are stable in aqueous solution at 94 °C for periods of up to 5 h. Methylcobalamin is very resistant to photolytic cleavage in the absence of oxygen, whereas ethyl- and adenosyl-cobalamin are photolyzed to cob(II)alamin under the same conditions.