Chu Chiheng, Stamatelatos Dimitrios, McNeill Kristopher
Institute of Biogeochemistry and Pollutant Dynamics (IBP), Department of Environmental Systems Science, ETH Zurich, 8092 Zurich, Switzerland.
Environ Sci Process Impacts. 2017 Dec 13;19(12):1518-1527. doi: 10.1039/c7em00324b.
Natural peptidic thiols play numerous important roles in aquatic systems. While thiols are known to be susceptible to sensitized photoreaction, the photochemical transformation of thiols in surface waters remains largely unknown. This study systematically assessed the photochemical transformation of naturally occurring thiols, including arginylcysteine (RC), γ-glutamylcysteine (γEC), glutathione (GSH), and phytochelatin (PC) in solutions containing dissolved organic matter (DOM). The results show that all thiols underwent rapid indirect photochemical transformation. The transformation rates of thiols were highly pH-dependent and increased with increasing solution pH. γEC and GSH show lower transformation rates than free Cys, which was ascribed to their higher thiol pK values. In comparison, PC and RC show much higher transformation rates than γEC and GSH, due to more reactive thiol groups contained in the PC molecule and sorption of RC to DOM macromolecules, respectively. While all investigated pathways contributed to thiol transformation, hydroxyl radical-mediated oxidation dominated at low solution pH and singlet oxygen-mediated oxidation dominated at high solution pH in the DOM-sensitized phototransformations of γEC, GSH, and PC. Furthermore, the effects of metal complexation and solution salinity on thiol transformation rates were examined. Thiol reactivity was not affected by Fe and Ag, slightly enhanced in the presence of Zn, Cd and Hg, and significantly enhanced by Cu. Additionally, enhanced thiol transformation rates were observed in solutions with high salinity.
天然肽硫醇在水生系统中发挥着众多重要作用。虽然已知硫醇易受敏化光反应影响，但硫醇在地表水中的光化学转化仍 largely 未知。本研究系统评估了天然存在的硫醇在含有溶解有机物（DOM）的溶液中的光化学转化，包括精氨酰半胱氨酸（RC）、γ-谷氨酰半胱氨酸（γEC）、谷胱甘肽（GSH）和植物螯合肽（PC）。结果表明，所有硫醇都经历了快速的间接光化学转化。硫醇的转化速率高度依赖于 pH，且随溶液 pH 的升高而增加。γEC 和 GSH 的转化速率低于游离半胱氨酸（Cys），这归因于它们较高的硫醇 pK 值。相比之下，PC 和 RC 的转化速率分别比 γEC 和 GSH 高得多，这分别是由于 PC 分子中含有更多反应性硫醇基团以及 RC 吸附到 DOM 大分子上。虽然所有研究的途径都对硫醇转化有贡献，但在 DOM 敏化的 γEC、GSH 和 PC 的光转化中，羟基自由基介导的氧化在低溶液 pH 下占主导，单线态氧介导的氧化在高溶液 pH 下占主导。此外，还研究了金属络合和溶液盐度对硫醇转化速率的影响。硫醇反应性不受 Fe 和 Ag 的影响，在 Zn、Cd 和 Hg 存在下略有增强，而在 Cu 存在下显著增强。此外，在高盐度溶液中观察到硫醇转化速率增加。
