Department of Chemical Engineering and Technology, School of Chemistry, Biology and Material of Science, East China University of Technology, Nanchang 330013, Jiangxi Province People's Republic of China.
Department of Inorganic Chemistry, Faculty of Sciences, University of Pécs, Ifjúság u. 6, Pécs, Hungary, H-7624.
J Phys Chem A. 2020 Jul 23;124(29):6029-6038. doi: 10.1021/acs.jpca.0c04271. Epub 2020 Jul 9.
The methionine-iodine reaction was reinvestigated spectrophotometrically in detail monitoring the absorbance belonging to the isosbestic point of iodine at 468 nm, at = 25.0 ± 0.1 °C, and at 0.5 M ionic strength in buffered acidic medium. The stoichiometric ratio of the reactants was determined to be 1:1 producing methionine sulfoxide as the lone sulfur-containing product. The direct reaction between methionine and iodine was found to be relatively rapid in the absence of initially added iodide ion, and it can conveniently be followed by the stopped-flow technique. Reduction of iodine eventually leads to the formation of iodide ion that inhibits the reaction making the whole system autoinhibitory with respect to the halide ion. We have also shown that this inhibitory effect appears quite prominently, and addition of iodide ion in the millimole concentration range may result in a rate law where the formal kinetic order of this species becomes -2. In contrast to this, hydrogen ion has just a mildly inhibitory effect giving rise to the fact that iodine is the kinetically active species in the system but not hypoiodous acid. The surprisingly complex kinetics of this simple reaction may readily be interpreted via the initiating rapidly established iodonium-transfer process between the reactants followed by the subsequent hydrolytic decomposition of the short-lived iodinated methionine. A seven-step kinetic model to be able to describe the most important characteristics of the measured kinetic curves is established and discussed in detail.
甲硫氨酸-碘反应在详细的分光光度法中重新进行了研究，详细监测了在 468nm 处属于碘的等色点的吸光度，在 = 25.0 ± 0.1°C 和 0.5M 离子强度下，在缓冲酸性介质中。反应物的化学计量比确定为 1:1，生成甲硫氨酸亚砜作为唯一含硫的产物。在没有最初添加碘化物离子的情况下，发现甲硫氨酸和碘之间的直接反应相对较快，并且可以通过停流技术方便地进行跟踪。碘的还原最终导致形成抑制反应的碘离子，使整个系统对卤化物离子具有自动抑制作用。我们还表明，这种抑制作用非常明显，在毫摩尔浓度范围内添加碘化物离子可能导致该物种的形式动力学顺序变为-2 的速率定律。与此相反，氢离子仅具有轻微的抑制作用，导致碘是该体系中动力学活性物质而不是次碘酸。这个简单反应令人惊讶的复杂动力学可以通过反应物之间迅速建立的碘翁转移过程来解释，随后是短暂存在的碘化甲硫氨酸的水解分解。建立并详细讨论了一个能够描述测量动力学曲线的最重要特征的七步动力学模型。
