Dipartimento di Chimica, Università degli Studi di Torino, Via Pietro Giuria 5, 10125 Torino, Italy.
Molecules. 2022 Jul 29;27(15):4855. doi: 10.3390/molecules27154855.
Nitrogen dioxide (NO) is produced in sunlit natural surface waters by the direct photolysis of nitrate, together with OH, and upon the oxidation of nitrite by OH itself. NO is mainly scavenged by dissolved organic matter, and here, it is shown that NO levels in sunlit surface waters are enhanced by high concentrations of nitrate and nitrite, and depressed by high values of the dissolved organic carbon. The dimer of nitrogen dioxide (NO) is also formed in the pathway of NO hydrolysis, but with a very low concentration, i.e., several orders of magnitude below NO, and even below OH. Therefore, at most, NO would only be involved in the transformation (nitration/nitrosation) of electron-poor compounds, which would not react with NO. Although it is known that nitrite oxidation by CO in high-alkalinity surface waters gives a minor-to-negligible contribution to NO formation, it is shown here that NO oxidation by Br can be a significant source of NO in saline waters (saltwater, brackish waters, seawater, and brines), which offsets the scavenging of OH by bromide. As an example, the anti-oxidant tripeptide glutathione undergoes nitrosation by NO preferentially in saltwater, thanks to the inhibition of the degradation of glutathione itself by OH, which is scavenged by bromide in saltwater. The enhancement of NO reactions in saltwater could explain the literature findings, that several phenolic nitroderivatives are formed in shallow (i.e., thoroughly sunlit) and brackish lagoons in the Rhône river delta (S. France), and that the laboratory irradiation of phenol-spiked seawater yields nitrophenols in a significant amount.
二氧化氮（NO）在阳光照射的自然地表水中通过硝酸盐的直接光解与 OH 一起产生，并通过 OH 本身氧化亚硝酸盐产生。NO 主要被溶解的有机物清除，这里表明，在阳光照射的地表水中，NO 水平会受到硝酸盐和亚硝酸盐浓度的升高以及溶解的有机碳含量的升高的影响。二氧化氮（NO）的二聚体也在 NO 水解的途径中形成，但浓度非常低，即比 NO 低几个数量级，甚至比 OH 还低。因此，NO 最多只会参与缺电子化合物的转化（硝化/亚硝化），这些化合物不会与 NO 反应。虽然已知在高碱性地表水中，CO 氧化亚硝酸盐对 NO 形成的贡献很小，但这里表明，Br 氧化 NO 可以成为盐水（咸水、半咸水、海水和卤水）中 NO 的重要来源，这抵消了溴化物对 OH 的清除作用。例如，抗氧化三肽谷胱甘肽在盐水中优先通过 NO 发生亚硝化，这要归功于 OH 对谷胱甘肽自身降解的抑制作用，而 OH 在盐水中被溴化物清除。在盐水中增强了 NO 的反应可以解释文献中的发现，即在罗纳河三角洲（法国南部）的浅（即充分阳光照射）和半咸水泻湖中形成了几种酚类硝基衍生物，并且在实验室辐照含苯酚的海水中会产生大量的硝基苯酚。
