Effect of Amino Group Charge on the Photooxidation Kinetics of Aromatic Amino Acids

The kinetics of the photooxidation of aromatic amino acids histidine (His), tyrosine (Tyr), and tryptophan (Trp) by 3,3′,4,4′-benzophenonetetracarboxylic acid (TCBP) has been investigated in aqueous solutions using time-resolved laser flash photolysis and time-resolved chemically induced dynamic nuclear polarization. The pH dependence of quenching rate constants is measured within a large pH range. The chemical reactivities of free His, Trp, and Tyr and of their acetylated derivatives, <i>N</i>-AcHis, <i>N</i>-AcTyr, and <i>N</i>-AcTrp, toward TCBP triplets are compared to reveal the influence of amino group charge on the oxidation of aromatic amino acids. The bimolecular rate constants of quenching reactions between the triplet-excited TCBP in the fully deprotonated state and tryptophan, histidine, and tyrosine with a positively charged amino group are <i>k</i>_q = 2.2 × 10⁹ M^–1 s^–1 (4.9 < pH < 9.4), <i>k</i>_q = 1.6 × 10⁹ M^–1 s^–1 (6.0 < pH < 9.2), and <i>k</i>_q = 1.5 × 10⁹ M^–1 s^–1 (4.9 < pH < 9.0), respectively. Tryptophan, histidine, and tyrosine with a neutral amino group quench the TCBP triplets with the corresponding rate constants <i>k</i>_q = 8.0 × 10⁸ M^–1 s^–1 (pH > 9.4), <i>k</i>_q = 3.0 × 10⁸ M^–1 s^–1 (pH > 9.2), and <i>k</i>_q = (4.0–10.0) × 10⁸ M^–1 s^–1 (9.0 < pH < 10.1) that are close to those for the N-acetylated derivatives. Thus, it has been established that the presence of charged amino group changes oxidation rates by a significant factor; i.e., His with a positively charged amino group quenches the TCBP triplets 5 times more effectively than <i>N</i>-AcHis and His with a neutral amino group. The efficiency of quenching reaction between the TCBP triplets and Tyr and Trp with a positively charged amino group is about 3 times as high as that of both Tyr and Trp with a neutral amino group, <i>N</i>-AcTyr and <i>N</i>-AcTrp