title: Effect of amino group charge on the photooxidation kinetics of aromatic amino acids.
abstract: 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, N-AcHis, N-AcTyr, and N-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 kq = 2.2 × 10(9) M(-1) s(-1) (4.9 < pH < 9.4), kq = 1.6 × 10(9) M(-1) s(-1) (6.0 < pH < 9.2), and kq = 1.5 × 10(9) 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 kq = 8.0 × 10(8) M(-1) s(-1) (pH > 9.4), kq = 3.0 × 10(8) M(-1) s(-1) (pH > 9.2), and kq = (4.0-10.0) × 10(8) 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 N-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, N-AcTyr and N-AcTrp.
authors.affiliation: International Tomography Center , Institutskaya 3a, 630090 Novosibirsk, Russia.
authors.forename: Natalya N
authors.initials: NN
authors.lastname: Saprygina
authors.forename: Olga B
authors.initials: OB
authors.lastname: Morozova
authors.forename: Günter
authors.initials: G
authors.lastname: Grampp
authors.forename: Alexandra V
authors.initials: AV
authors.lastname: Yurkovskaya
mesh_terms: D024322:Amino Acids, Aromatic / Q000737:chemistry*; D007700:Kinetics; D015394:Molecular Structure; D010084:Oxidation-Reduction; D055668:Photochemical Processes
chemical_list: D024322:Amino Acids, Aromatic