Laser Photolysis Study on Photo-Oxidation Reactions of Aromatic Amino Acids with Triplet 2-Methylanthraquinone

The transient photo-sensitive oxidation between 2-methylanthraquinone (MAQ) and aromatic amino acids (namely tryptophan, tyrosine and phenylalanine) in acetonitrile/water (1 1, V/V) solution was investigated by laser photolysis techniques with the excitation wavelength of 355 nm. The triplet state absorption of 2-methylanthraquinone was attributed to 390, 450 and 590 nm and the anion radical absorption of MAQ was due to 390 and 490 nm identified by the electron transfer reactions from 4-diazabicyclo [2. 2. 2] octane (DABCO) or diphenylamine (DPA) to (3)MAQ*. With the decay of (3)MAQ*, the MAQ(center dot-) at 490 nm appeared, which indicated that the electron transfer reactions between 3MAQ* and amino acids occurred. Moreover, the rate constants were deduced to be 3. 0 X 10(9), 1.1X 10(9) andl. 8X10(8) L.mol(-1) s(-1) for TrpH, TyrOH and PHE, respectively. On the other hand, the free energy changes (Delta G) of the reactions showed that the proposed electron transfer steps are thermodynamically feasible.