ESR-study of the proton exchange with aliphatic amino acids in toluene

This paper presents the results of an ESR spectroscopic study of the intermolecular proton exchange kinetics with some isomers of aminocaproic acid, such as 2-aminohexanoic and 6-aminohexanoic acids in the toluene indifferent medium. A stable semiquinone radical, namely 3,6-di-tert-butyl-2-hydroxyphenoxyl, was used as a spin probe. The ESR spectra were recorded on a RE-1306 spectrometer. The obtained dynamic spectra of the intermolecular proton exchange process are given in this article. Modeling of the proton exchange ESR spectra was carried out using a previously developed program, which is based on a four-jump model of this reaction and modified Bloch equations. The kinetic parameters of the process of acid-base interaction of the spin probe with amino acids and the activation barrier of the reaction were estimated with a minimum error based on a comparison of the model spectra with the experimental ones. The obtained data were analyzed and compared with previously known information. It was found that the reaction rate of the intermolecular proton exchange between 3,6-di-tert-butyl-2-hydroxyphenoxy and studied aliphatic amino acids is comparable to the same with aliphatic carboxylic acids. But in our case, there is an increase in the activation barrier, which apparently is associated with a negative effect on the intramolecular hydrogen bonding process between the amino and the carboxyl groups in the amino acid molecule