Interaction kinetics of selenium-containing compounds with oxidants.

Selenium compounds have been identified as potential oxidant scavengers for biological applications due to the nucleophilicity of Se, and the ease of oxidation of the selenium centre. Previous studies have reported apparent second order rate constants for a number of oxidants (e.g. HOCl, ONOOH) with some selenium species, but these data are limited. Here we provide apparent second order rate constants for reaction of selenols (RSeH), selenides (RSeR') and diselenides (RSeSeR') with biologically-relevant oxidants (HOCl, H2O2, other peroxides) as well as overall consumption data for the excited state species singlet oxygen (1O2). Selenols show very high reactivity with HOCl and 1O2, with rate constants > 108 M-1 s-1, whilst selenides and diselenides typically react with rate constants one- (selenides) or two- (diselenides) orders of magnitude slower. Rate constants for reaction of diselenides with H2O2 and other hydroperoxides are much slower, with k for H2O2 being <1 M-1 s-1, and for amino acid and peptide hydroperoxides ~102 M-1 s-1. The rate constants determined for HOCl and 1O2 with these selenium species are greater than, or similar to, rate constants for amino acid side chains on proteins, including the corresponding sulfur-centered species (Cys and Met), suggesting that selenium containing compounds may be effective oxidant scavengers. Some of these reactions may be catalytic in nature due to ready recycling of the oxidized selenium species. These data may aid the development of highly efficacious, and catalytic, oxidant scavengers.