Spontaneous resonance Raman or resonance-enhanced AC-coupled inverse Raman spectra were obtained for 8-mercaptoriboflavin, 8-(dimethylamino)riboflavin, 8- hydroxyriboflavin, and 8-aminoriboflavin all free in solution. These Raman spectra were all similar to one another and markedly different from that of riboflavin. In addition, the Raman spectra of 8-mercaptoflavin bound to the apoproteins of old yellow enzyme, glucose oxidase, and L-lactate oxidase were determined. The Raman spectra of these protein-bound flavins were distinctly different from both those of the above 8-substituted riboflavins and that of riboflavin. An argument is presented in favor of assigning a quinonoid electronic structure to these protein-bound flavins.
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