Criado S, Marioli J M, Allegretti P E, Furlong J, Rodríguez Nieto F J, Mártire D O, García N A
Departamento de Química y Física, Universidad Nacional de Río Cuarto, 5800 Río Cuarto, Argentina.
J Photochem Photobiol B. 2001 Dec 1;65(1):74-84. doi: 10.1016/s1011-1344(01)00239-1.
Kinetics and mechanism of the oxidation of tyrosine (Tyr) and valine (Val) di- and tripeptides (Tyr-Val, Val-Tyr and Val-Tyr-Val) mediated by singlet molecular oxygen [O(2)((1)Delta(g))], phosphate (HPO(4)(-) and PO(4)(2-)) and sulfate (SO(4)(-)) radicals was studied, employing time-resolved O(2)((1)Delta(g)) phosphorescence detection, polarographic determination of dissolved oxygen and flash photolysis. All the substrates were highly photooxidizable through a O(2)((1)Delta(g))-mediated mechanism. Calculated quotients between the overall and reactive rate constants for the quenching of O(2)((1)Delta(g)) by Tyr-derivatives (k(t)/k(r) values, accounting for the efficiency of the effective photooxidation) were 1.3 for Tyr, 1 for Tyr-Val, 2.8 for Val-Tyr and 1.5 for Val-Tyr-Val. The effect of pH on the kinetics of the photooxidative process confirms that the presence of the dissociated phenolate group of Tyr clearly dominates the O(2)((1)Delta(g)) quenching process. Products analysis by LC-MS indicates that the photooxidation of Tyr di- and tripeptides proceeds with the breakage of peptide bonds. The information obtained from the evolution of primary amino groups upon photosensitized irradiation is in concordance with these results. Absolute rate constants for the reactions of phosphate radicals (HPO(4)(-) and PO(4)(2-), generated by photolysis of the P(2)O(8)(4-) at different pH) and sulfate radicals (SO(4)(-), produced by photolysis of the S(2)O(8)(2-)) with Tyr peptides indicate that for all the substrates, the observed tendency in the rate constants is: SO(4)(-) > or = HPO(4)(-) > or = PO(4)(2-). Formation of the phenoxyl radical of tyrosine was detected as an intermediate involved in the oxidation of tyrosine by HPO(4)(-).
采用时间分辨的单线态分子氧[O₂(¹Δg)]磷光检测、溶解氧的极谱测定和闪光光解技术，研究了单线态分子氧[O₂(¹Δg)]、磷酸根（HPO₄⁻和PO₄²⁻）及硫酸根（SO₄⁻）自由基介导的酪氨酸（Tyr）和缬氨酸（Val）二肽及三肽（Tyr-Val、Val-Tyr和Val-Tyr-Val）的氧化动力学及机理。所有底物均可通过单线态分子氧介导的机理发生高效光氧化。计算得出的酪氨酸衍生物猝灭单线态分子氧的总速率常数与反应速率常数之比（k(t)/k(r)值，代表有效光氧化效率），酪氨酸为1.3，Tyr-Val为1，Val-Tyr为2.8，Val-Tyr-Val为1.5。pH对光氧化过程动力学的影响证实，酪氨酸离解的酚氧基团的存在明显主导了单线态分子氧的猝灭过程。通过液相色谱-质谱联用（LC-MS）进行的产物分析表明，酪氨酸二肽和三肽的光氧化过程伴随着肽键的断裂。从敏化辐照后伯氨基的变化获得的信息与这些结果一致。不同pH下P₂O₈⁴⁻光解产生的磷酸根自由基（HPO₄⁻和PO₄²⁻）以及S₂O₈²⁻光解产生的硫酸根自由基（SO₄⁻）与酪氨酸肽反应的绝对速率常数表明，对于所有底物，观察到的速率常数趋势为：SO₄⁻≥HPO₄⁻≥PO₄²⁻。检测到酪氨酸苯氧自由基是HPO₄⁻氧化酪氨酸过程中的中间体。
