MECHANISM OF THE OH. RADICAL INDUCED OXIDATION OF METHIONINE IN AQUEOUS-SOLUTION
MECHANISM OF THE OH. RADICAL INDUCED OXIDATION OF METHIONINE IN AQUEOUS-SOLUTION
复制标题
DOI：
10.1021/ja00400a042
复制
发表时间：
1981-01-01
影响因子：
15
通讯作者：
ASMUS, KD
中科院分区：
文献类型：
--
作者：
HILLER, KO;MASLOCH, B;ASMUS, KD
用辐射化学方法，主要是ns-ms脉冲辐解，研究了水溶液中OH·自由基诱导蛋氨酸氧化的反应机理。反应的初始步骤是OH·自由基与硫原子的加成和氢的夺取之间的竞争过程，二者的比例为4：1，前者更有利。在强酸溶液（pH S3）中，随后的机理实际上与脂肪族硫醚的氧化机理相同。在这种情况下，最感兴趣的中间体是在480 nm处具有最大吸收的以硫为中心的自由基阳离子络合物。在较高的pH值下，在弱酸性、中性和碱性溶液中，该机制更为复杂，基本上涉及通过主要氧化的硫官能团对氨基进行的快速（k> 4 × 106 s-111）分子内和可能的辅助氧化。这个过程包括氨基酸的不可逆脱羧作用。已经鉴定了CO2和取代氨基自由基，并测定了它们的产率。N-氧化取决于氨基的还原电位，并且发生在碱性和两性离子形式的甲硫氨酸中-在后一种情况下，由于NH3+和COO™基团之间的快速质子交换。羧基在非常酸性的溶液中完全质子化，或通过加入强质子供体，例如H2 PO 4，或引入吸电子取代基，例如在VV-乙酰甲硫氨酸中，将还原电位降低到这样的程度，即氧化位点可以有效地保留在硫上。详细的机制与相关的反应动力学和一些中间体的物理化学性质的介绍和讨论。
The reaction mechanism of the OH· radical induced oxidation of methionine in aqueous solutions has been investigated by radiation chemical methods, mainly ns-ms pulse radiolysis. The initial step is a competitive process between addition of the OH· radical to the sulfur atom and hydrogen abstraction at a 4: 1 ratio in favor of the former. In strong acid solutions (pH S3) the subsequent mechanism is practically identical with the oxidation mechanism of aliphatic thioethers. The intermediate of most interest in this case is a sulfur-centered radical cation complex with an absorption maximum at 480 nm. At higher pH in slightly acid, neutral, and basic solutions the mechanism is more complex, essentially involving a fast (k> 4 X 106 s'1 11) intramolecular and probablysterically assisted oxidation of the amino group by the primarily oxidized sulfur function. This process includesirreversible decarboxylation of the amino acid. Both the C02 and the remaining-amino radicals have been identified and their yields determined. The N-oxidation depends on the reduction potential of the amino group and takes place with both thebasic and the zwitterionic form of methionine—in the latter case on account of the rapid proton exchange between the NH3+ and COO™ groups. Complete protonationof the carboxyl group in very acid solutions, or by the addition of powerful proton donors, eg, H2P04~, or the introduction of an electron-withdrawing substituent such as in VV-acetylmethionine lowers the reduction potential to such an extent that the oxidation site may effectively remain at the sulfur. A detailed mechanism with the associated reaction kinetics and some physicochemical properties of the intermediates are presented and discussed.