Abstract
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 pKa 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 Fe3+ and Ag+, slightly enhanced in the presence of Zn2+, Cd2+ and Hg2+, and significantly enhanced by Cu2+. Additionally, enhanced thiol transformation rates were observed in solutions with high salinity.
Highlights
Thiols are group of biogenic sul ydryl-containing compounds with multiple essential functions in biogeochemical systems.[1,2,3,4] Marine microbes such as phytoplankton contain high concentrations of thiols (Table 1) such as cysteine (Cys), gglutamylcysteine, arginylcysteine (RC), glutathione (GSH), and phytochelatins (PC)
The results show that the reactivity of thiols was greatly affected by their intrinsic physicochemical properties such as number of reactive thiol groups, pKa values of thiol groups, and sorption affinity of thiols to chromophoric fraction of DOM (CDOM) macromolecules
The transformation rates of thiols were highly dependent on surrounding aquatic environments such as solution pH, solution salinity, and complexation with metals
Summary
Thiols are group of biogenic sul ydryl-containing compounds with multiple essential functions in biogeochemical systems.[1,2,3,4] Marine microbes such as phytoplankton contain high concentrations (mM to mM) of thiols (Table 1) such as cysteine (Cys), gglutamylcysteine (gEC), arginylcysteine (RC), glutathione (GSH), and phytochelatins (PC). The tripeptide GSH has high intracellular concentrations (780–3300 mM)[5] and 1518 | Environ.
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