Effect of alkyl chain length on one-electron oxidation of bis(alkyl carboxylic acid) selenides: implication on their antioxidant ability

Abstract

One-electron redox reactions of three bis(alkyl carboxylic acid) selenide (SeC) derivatives viz., seleno bis(butanoic acid) (SeBA), seleno bis(propanoic acid) (SePA) and seleno bis(ethanoic acid) (SeEA), were carried out in aqueous solutions using nanosecond pulse radiolysis and the resultant transients were detected by absorption spectroscopy. SeC reacted with ˙OH radicals to form hydroxyl selenouranyl radicals (Se∴OH) which subsequently gave different transient species, like the selenium centered radical cation (SeC˙⁺), dimer radical cation (Se∴Se)⁺, or a selenium-carboxyl oxygen stabilized monomer radical (Se∴O). The relative yield of these transient species depended on pH, concentration of SeC and position of the carboxylate functional group. SeEA and SeBA gave exclusively (Se∴Se)⁺ absorbing at 490 nm, while in the case of SePA, along with (Se∴Se)⁺, (Se∴O) was also formed. The stability of (Se∴Se)⁺ was estimated in terms of the equilibrium constant and was in the order SeBA > SeEA > SePA. Secondary electron transfer reactions of the transients were performed with 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid), thionine and methyl viologen. The study showed that SeC˙⁺ undergoes decarboxylation with the formation of the corresponding α-(alkylseleno)alkyl radical and the yield of CO₂ formed was in the order SeEA < SeBA < SePA. The formation of a stabilised monomer radical cation in SePA is responsible for its lower yield of CO₂ and the same is reflected in its higher free radical scavenging antioxidant activity, established by comparing the rate constants for scavenging of peroxyl radicals.