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

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• 作者：Natalya N. Saprygina ; Olga B. Morozova ; Günter Grampp ; Alexandra V. Yurkovskaya
• 刊名：Journal of Physical Chemistry A
• 出版年：2014
• 出版时间：January 16, 2014
• 年：2014
• 卷：118
• 期：2
• 页码：339-349
• 全文大小：406K
• ISSN：1520-5215

文摘

The kinetics of the photooxidation of aromatic amino acids histidine (His), tyrosine (Tyr), and tryptophan (Trp) by 3,3鈥?4,4鈥?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 k<sub>qsub> = 2.2 脳 10<sup>9sup> M<sup>鈥?sup> s<sup>鈥?sup> (4.9 < pH < 9.4), k<sub>qsub> = 1.6 脳 10<sup>9sup> M<sup>鈥?sup> s<sup>鈥?sup> (6.0 < pH < 9.2), and k<sub>qsub> = 1.5 脳 10<sup>9sup> M<sup>鈥?sup> s<sup>鈥?sup> (4.9 < pH < 9.0), respectively. Tryptophan, histidine, and tyrosine with a neutral amino group quench the TCBP triplets with the corresponding rate constants k<sub>qsub> = 8.0 脳 10<sup>8sup> M<sup>鈥?sup> s<sup>鈥?sup> (pH > 9.4), k<sub>qsub> = 3.0 脳 10<sup>8sup> M<sup>鈥?sup> s<sup>鈥?sup> (pH > 9.2), and k<sub>qsub> = (4.0鈥?0.0) 脳 10<sup>8sup> M<sup>鈥?sup> s<sup>鈥?sup> (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.