⁷⁷Se NMR studies of bis(alkylthio)selenides of biological thiols

Abstract

Abstract To determine the feasibility of using 77 Se NMR spectroscopy to characterize bis(alkylthio)selenides formed by reaction of selenite with thiol groups of biological molecules, 77 Se NMR spectra have been measured for bis(alkylthio)selenides formed by reaction of selenite with cysteine, homocysteine, penicillamine, N ‐acetylpenicillamine, 2‐mercaptopropanoic acid, mercaptosuccinic acid, cysteamine and glutathione. The 77 Se chemical shift is extremely sensitive to the nature of the alkylthiol group, covering a range greater than 150 ppm for this series of thiols. The chemical shift is also sensitive to the stereochemistry of the alkylthiol group, and resolved resonances are observed for meso and D,L ‐stereoisomers, e.g. the meso and D,L ‐stereoisomers of bis(cysteine)selenide. 77 Se NMR spectra were also measured for mixed bis(alkylthio)selenides (RSSeSŔ) formed by reaction of selenite with two‐component mixtures of the various thiols. The effect of the two alkylthiol groups on the 77 Se chemical shift is additive, with 77 Se chemical shifts of the mixed bis(alkylthio)selenides located mid‐way between those of the two symmetric compounds. The 77 Se chemical shift is also sensitive to the protonation state of carboxylic acid and ammonium groups of the alkylthiol, and p K A values were determined from 77 Se chemical shift vs pH titration curves for selected symmetrical and mixed bis(alkylthio)selenides. The results demonstrate that 77 Se NMR is a sensitive method for the characterization and study of bis(alkylthio)selenides.