Muñoz-Rugeles Leonardo, Galano Annia, Alvarez-Idaboy Juan Raúl
Facultad de Química, Departamento de Física y Química Teórica, Universidad Nacional Autónoma de México, México, DF 04510, Mexico.
Phys Chem Chem Phys. 2017 Jun 14;19(23):15296-15309. doi: 10.1039/c7cp01557g.
The results presented in this work demonstrate the high complexity of chemical reactions involving species with multiple acid-base equilibria. For the case study investigated here, it was necessary to consider two radical species for tryptophan (Trp˙ and Trp˙) and three fractions for uric acid (HUr, HUr and HUr) in order to properly reproduce the experimental results. At pH = 7.4, two main reaction mechanisms were identified: proton-electron sequential transfer (PEST) and sequential proton gain-electron transfer (SPGET). Combined, they account for more than 99% of the overall reaction, despite the fact that they involve minor species, i.e., HUr and Trp˙, respectively. The excellent agreement between the calculated overall rate constant and the experimental value seems to support this proposal. In addition, if only the dominant species at pH = 7.4 (HUr and Trp˙) were considered, there would be a large discrepancy with the experimental value (about 4 orders of magnitude), which also supports the finding that the key species in this case are not the most abundant ones. The influence of the pH on the kinetics of the investigated reaction was explored. It was found that the maximum repairing ability of uric acid does not occur at physiological pH, but at a more acidic pH (pH = 5.0).
本研究呈现的结果表明，涉及具有多个酸碱平衡物种的化学反应具有高度复杂性。对于此处所研究的案例，有必要考虑色氨酸的两种自由基物种（Trp˙和Trp˙）以及尿酸的三种组分（HUr、HUr和HUr），以便恰当地重现实验结果。在pH = 7.4时，确定了两种主要反应机制：质子 - 电子顺序转移（PEST）和顺序质子获得 - 电子转移（SPGET）。尽管它们分别涉及次要物种，即HUr和Trp˙，但综合起来它们占总反应的比例超过99%。计算得到的总速率常数与实验值之间的出色一致性似乎支持了这一推测。此外，如果仅考虑pH = 7.4时的主要物种（HUr和Trp˙），则与实验值会存在很大差异（约4个数量级），这也支持了在此情况下关键物种并非最丰富物种的这一发现。研究了pH对所研究反应动力学的影响。结果发现，尿酸的最大修复能力并非出现在生理pH值时，而是在更酸性的pH值（pH = 5.0）时。
