Criado S, Soltermann A T, Marioli J M, García N A
Departamento de Química y Física, Universidad Nacional de Río Cuarto, Argentina.
Photochem Photobiol. 1998 Oct;68(4):453-8.
This paper studies the dye-sensitized photooxidation of tyrosine (tyr) and tyr di- and tripeptides (tyr-tyr and tyr-tyr-tyr) mediated by singlet molecular oxygen (O2[1 delta g]) in alkaline media. Photooxidation quantum efficiencies (phi r) were obtained by determining the overall and reactive rate constants of interaction with the oxidative species, employing the time-resolved O2(1 delta g) phosphorescence detection method and static-photolysis actinometric method, respectively. The interaction of O2(1 delta g)-tyr derivatives occurs through an intermediate encounter complex with polar character. Ionization of the phenolic OH group of tyr derivatives and the polarity of the solvent favors the overall interaction. Nevertheless, phi r values decrease when changing from water to MeCN-water medium. This indicates that the reactive deactivation of the encounter complex, probably an entropy-controlled step, may be affected by solvent polarity in the same way as those processes in which charges are neutralized along the reaction pathway. Photooxidation quantum efficiencies indicate that the contribution to O2(1 delta g) physical quenching (a second alternative deactivation route for the encountered complex [O2(1 delta g)-tyr derivatives]) increases with the complexity of the peptide. As a result, the selfprotection of the peptidic entity against physical quenching also increases. The information obtained from the fractional consumption mol O2/mol tyr derivative (in tyr, the di- and tripeptides and the respective methyl ester of tyr and the tripeptide), together with the evolution (either consumption and/or generation) of primary amino groups upon photosensitized irradiation of the same compounds clearly indicates that the photooxidation of di- and tri-tyr peptides proceeds with the breakage of peptidic bonds. As a consequence, in the final balance each tyr unity behaves as an independent photooxidizable target.
本文研究了在碱性介质中，单线态分子氧（O₂[¹Δg]）介导的酪氨酸（tyr）以及酪氨酸二肽和三肽（tyr-tyr和tyr-tyr-tyr）的染料敏化光氧化反应。分别采用时间分辨O₂(¹Δg)磷光检测法和静态光解光度法，通过测定与氧化物种相互作用的总反应速率常数和反应速率常数，获得了光氧化量子产率（ϕr）。O₂(¹Δg)与tyr衍生物的相互作用通过具有极性特征的中间遭遇络合物发生。tyr衍生物酚羟基的电离以及溶剂的极性有利于整体相互作用。然而，当从水介质变为MeCN-水介质时，ϕr值降低。这表明遭遇络合物的反应性失活，可能是一个熵控制步骤，可能会受到溶剂极性的影响，其方式与反应途径中电荷被中和的过程相同。光氧化量子产率表明，对O₂(¹Δg)物理猝灭（遭遇络合物[O₂(¹Δg)-tyr衍生物]的另一种失活途径）的贡献随着肽的复杂性增加而增加。因此，肽实体对物理猝灭的自我保护也增加。从tyr衍生物（tyr、二肽和三肽以及tyr和三肽各自的甲酯）的O₂消耗分数（mol O₂/mol tyr衍生物）获得的信息，以及相同化合物在光敏辐照下伯氨基的演变（消耗和/或生成）清楚地表明，二肽和三肽tyr的光氧化伴随着肽键的断裂。因此，在最终平衡中，每个tyr单元都表现为一个独立的可光氧化靶标。
