Perez-Benito Joaquin F, Ferrando Jordi
Departamento de Quimica Fisica, Facultad de Quimica, Universidad de Barcelona , Marti i Franques, 1, 08028 Barcelona, Spain.
J Phys Chem B. 2014 Dec 26;118(51):14949-60. doi: 10.1021/jp5089564. Epub 2014 Dec 12.
The reduction of permanganate ion to MnO(2)-Mn(2)O(3) soluble colloidal mixed oxide by l-phenylalanine in aqueous phosphate-buffered neutral solutions has been followed by a spectrophotometric method, monitoring the decay of permanganate ion at 525 nm and the formation of the colloidal oxide at 420 nm. The reaction is autocatalyzed by the manganese product, and three rate constants have been required to fit the experimental absorbance-time kinetic data. The reaction shows base catalysis, and the values of the activation parameters at different pHs have been determined. A mechanism including both the nonautocatalytic and the autocatalytic reaction pathways, and in agreement with the available experimental data, has been proposed. Some key features of this mechanism are the following: (i) of the two predominant forms of the amino acid, the anionic form exhibits a stronger reducing power than the zwitterionic form; (ii) the nonautocatalytic reaction pathway starts with the transfer of the hydrogen atom in the α position of the amino acid to permanganate ion; and (iii) the autocatalytic reaction pathway involves the reduction of Mn(IV) to Mn(II) by the amino acid and the posterior reoxidation of Mn(II) to Mn(IV) by permanganate ion.
在磷酸盐缓冲中性水溶液中，用L-苯丙氨酸将高锰酸根离子还原为MnO(2)-Mn(2)O(3)可溶性胶体混合氧化物的过程，采用分光光度法进行跟踪，监测525nm处高锰酸根离子的衰减以及420nm处胶体氧化物的形成。该反应由锰产物自催化，需要三个速率常数来拟合实验吸光度-时间动力学数据。该反应表现出碱催化作用，并测定了不同pH值下的活化参数。提出了一种包括非自催化和自催化反应途径且与现有实验数据相符的机理。该机理的一些关键特征如下：(i) 在氨基酸的两种主要形式中，阴离子形式比两性离子形式具有更强的还原能力；(ii) 非自催化反应途径始于氨基酸α位的氢原子向高锰酸根离子的转移；(iii) 自催化反应途径涉及氨基酸将Mn(IV)还原为Mn(II)，以及随后高锰酸根离子将Mn(II)再氧化为Mn(IV)。
