Chemistry Department, Universidade Federal de São Carlos, Rod. Washington Luís km 235 - SP-310, São Carlos CEP 13565-905, Brazil.
Environ Sci Process Impacts. 2018 Aug 16;20(8):1148-1157. doi: 10.1039/c8em00119g.
The degradation in water of the most widespread herbicide, glyphosate, is still under debate. Experimental disagreements on this process exist and there are only a few theoretical studies to support any conclusions. Moreover, the relationship between glyphosate and glycine is underestimated. Besides the structural similarity, glycine is a product of glyphosate degradation; hence, their studies are complementary. In this study, two mechanisms for the decomposition of the glyphosate molecule and glycine molecule in water are proposed. These mechanisms were explored by using quantum mechanical calculations. A combined microsolvation/PCM approach was employed to find and characterize their transition states, by which the reaction pathways were determined via the IRC method. The results have shown that the degradation processes might occur via a C-C bond cleavage, through a concerted mechanism, whereby the proton transfers and the CO2 detachments occur simultaneously. The second mechanism had two consecutive steps, a decarboxylation followed by the proton transfers. The water molecules served as a conduit for the proton transfers, away from the amine group (or the phosphonate, glyphosate case). Their function was to assist the reactions in a water-mediated decarboxylation. In these particular cases, the free energy of activation was 42.68 and 42.28 kcal mol-1 for the glycine structure and the glyphosate structure, respectively. These results agreed with the photodegradation and thermodegradation of glyphosate, as well as with the spontaneous decarboxylation of glycine. A concerted mechanism might be expected to yield C-P and C-N bond cleavages in the glyphosate molecule.
目前，关于最广泛使用的除草剂草甘膦在水中的降解仍存在争议。关于这一过程，实验结果存在分歧，仅有少数理论研究可以支持某些结论。此外，人们对草甘膦与甘氨酸之间的关系还存在低估。除了结构相似性，甘氨酸还是草甘膦降解的产物，因此，对它们的研究是相辅相成的。在本研究中，提出了两种草甘膦分子和甘氨酸分子在水中分解的机制。这些机制是通过量子力学计算来探索的。采用了一种组合的微溶剂/PCM 方法来寻找和描述它们的过渡态，通过 IRC 方法确定了反应途径。结果表明，降解过程可能通过 C-C 键的断裂，通过协同机制发生，其中质子转移和 CO2 的脱离同时发生。第二种机制有两个连续的步骤，即脱羧和质子转移。水分子作为质子转移的通道，远离胺基（或膦酸，草甘膦的情况）。它们的作用是通过水介导的脱羧来协助反应。在这些特定情况下，甘氨酸结构和草甘膦结构的活化自由能分别为 42.68 和 42.28 kcal/mol。这些结果与草甘膦的光降解和热降解以及甘氨酸的自发脱羧一致。协同机制可能导致草甘膦分子中的 C-P 和 C-N 键断裂。
