Coherent anti‐stokes raman spectra of isoalloxazines

Coherent anti‐Stokes Raman spectra (1700–1100 cm −1 ) of 7,8‐dimethyl‐10‐tetraacetylisoalloxazine (tetracetylriboflavin) in chloroform and several other solvents of higher polarity and proton availability are reported. Solvent polarity has no influence on either the intensities or frequencies of the Raman modes, whereas the influence of protic solvents is more marked. As well as altering the effective mass of the particular atoms in the isoalloxazine involved, hydrogen bonding is known to change the bond hybridization throughout the entire ring system. A similarly mild electronic perturbation can be produced by changing the methyl substitution on the benzene subnucleus of the isoalloxazine, and this is shown to produce significant changes in the Raman spectrum. Isotopic substitution with 13 C and 15 N is used to mimic the effective mass changes that would occur on hydrogen bonding. For the model compounds in chloroform, the effect of isotopic substitution on some modes is different from that reported for the same substitution in riboflavin bound to riboflavin binding protein. Frequencies of the prominent bands in all spectra were determined by lineshape analysis. On the basis of these results a tentative interpretation is made of the changes that occur in the Raman spectra of flavins when they bind to flavoproteins.