Identification of Structural Markers for Vitamin b₁₂ and Other Corrinoid Derivatives in Solution Using Ftir Spectroscopy

The identification of structural markers for B₁₂/protein interactions is crucial to a complete understanding of vitamin B₁₂ transport and metabolic reaction mechanisms of B₁₂ coenzymes. Fourier transform infrared spectroscopy can provide direct measurements of changes in the side chains and corrin ring resulting from B₁₂/protein interactions. Using FTIR spectroscopy in various solvent systems, we have identified structural markers for corrinoids in the physiological state. We assign the major band (denoted B), which occurs at ca. 1630 cm^-1 in D₂O and ca. 1675 cm^-1 in ethanol, to the amide I C=O stretching mode of the propionamide side chains of the corrin ring. The lower frequency of band B in D₂O versus ethanol is due to the greater hydrogen-bonding properties of D₂O that stabilize the charged amide resonance form. Since the propionamides are known to be important in protein binding, band B is a suitable marker for monitoring the interaction of these side chains with proteins. We assign bands at ca. 1575 and 1545 cm^-1 (denoted C and D) as breathing modes of the corrin ring on the basis of the bands’ solvent independence and their sensitivity to changes in axial ligation. As the σ-donating strength of the axial ligands increases, the frequencies of bands C and D decrease, possibly indicating a lengthening of the corrin conjugated system. Band A, the known cyanide stretching frequency at ca. 2130 cm^-1, probes the cobalt-carbon distance in cyanocorrinoids. As the frequency of band A increases, the cobalt-carbon bond strength should decrease.