A Raman Study on the C(4)=O Stretching Mode of Flavins in Flavoenzymes: Hydrogen Bonding at the C(4)=O Moiety

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Abstract
Raman spectroscopy was used to investigate the hydrogen bonding at the C(4)=O moiety of the isoalloxazine nucleus in a series of flavins and flavoproteins. Isotope effects of Raman bands confirmed that the band observed around 1,710 cm-1 is mainly derived from C(4)=O stretching vibrational mode. A linear correlation was observed between the frequency of C(4)=O stretching and the chemical shift of 13C(4), suggesting that the data from both Raman and NMR spectroscopies reflect a common perturbation, i.e., hydrogen bonding. The maximum difference of C(4)=O frequency among flavins and flavoproteins examined is 36 cm-1 [1,723 cm-1 for riboflavin-binding protein (Kim, M. and Carey, P.C. (1993) J. Am. Chem. Soc. 115, 7015-7016) and 1,687 cm-1 for the complex of medium-chain acyl-CoA dehydrogenase with acetoacetyl-CoA]; the maximum difference of 40–70 kJ/mol in the hydrogen bonding strength at the C(4)=O exists among flavoproteins. By use of an empirical linear correlation between the frequency of C=O stretching and the bond length of the C=O, it is estimated that the maximum difference in the bond length among flavoproteins treated here is ca. 0.017 Å. The hydrogen bonding at the C(4)=O in medium-chain and short-chain acyl-CoA dehydrogenases becomes stronger upon complexation with substrate analogs. Since the hydrogen bonding at the C(4)=O is expected to enhance the electron-accepting capacity of the N(5) position, substrate-binding itself probably raises the reactivity of flavin, through enhancing the hydrogen bonding.

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