Flavin-sensitized photooxidations of tryptophan and tyrosine

Abstract

The rates of flavin-sensitized photooxidations of tryptophan and tyrosine with natural and synthetic flavins under aerobic conditions were determined colorimetrically. The effect that the tight intramolecular interactions observed within synthetic flavinyl peptides have on the ability of the light-excited flavin portions to oxidize aromatic amino acids intra- and intermolecularly has been examined. The photochemical reactivities of the parent flavin acids, 10-ω-carboxyalkylflavins, were also compared with those of the peptides. In general, similar results were obtained for both the tryptophan and the tyrosine series. Overall, the former reacts faster than the latter. The rates for the photooxidation of amino acid by intermolecularly supplied flavins follow FMN > 10- ω-carboxyalkylflavins > FAD > flavinyl peptides. All flavinyl peptides alone, especially the short one, lost but little of the amino acid portion when exposed to light. The rates of the reactions with 1:1 flavinyl peptide plus amino acid or plus FMN as compared to those for the peptide alone are increased by small and large percents, respectively. Effects of pH, temperature, and solvent on the rates for both series are generally similar. Up to the optima, higher pH values, higher temperatures, and a dipolar, organic solvent such as dimethyl sulfoxide in buffer favor the reactions. All of the foregoing results indicate that the flavin acids behave similar to FMN but in contrast to the flavinyl peptides. The tight association of flavin with just the aromatic amino acid residues of tryptophan and tyrosine, as can occur in some flavoproteins, may prevent flavin from being light-excited to photosensitize the oxidation of amino acids. However, the intramolecularly-bound amino acid can be relatively easily photooxidized by intermolecular flavin which can be activated.