Reactivity of oxidants towards phenyl and benzyl substituted 5-selanylpentanoic acids: radiolytic and theoretical insights

Abstract

Two selanyl compounds, 5-(phenylselanyl)pentanoic acid (1) and 5-(benzylselanyl)pentanoic acid (2), were investigated for their reactivity towards one-electron oxidants, particularly the radiolytically generated hydroxyl radical (˙OH). The reactions of compounds 1 and 2 with ˙OH led to the formation of transient absorptions, characterized by peak wavelengths (λ_max) at 630 nm and 560 nm, respectively. Concentration-dependent absorption studies revealed that the transient from 1 with λ_max of 630 nm corresponds to a dimer species featuring a two-centre–three-electron (2c–3e) bond. Conversely, the species exhibiting λ_max of 560 nm for 2 was identified as a monomer radical cation. To validate the identities of these transients, compounds 1 and 2 underwent reactions with specific one-electron oxidants, namely the dibromide radical (Br₂˙⁻) and carbonate radical anion (CO₃˙⁻). The differences in the reactivities of the transients derived from compounds 1 and 2 were distinguished by measuring their ability to oxidize glutathione, a biologically important thiol. Interestingly, the transient from compound 2 exhibited a relatively lower oxidizing ability than the transient from compound 1. Compound 2 also demonstrated higher scavenging rate constants in reactions with Br₂˙⁻, CO₃˙⁻, and peroxyl radicals (derived from thermal degradation of 2,2′-azobis(2-amidinopropane) hydrochloride, AAPH). Computational calculations were carried out employing the M05-2X/6-311+G(d,p)/SMD method to offer additional proof in favor of dimer radical species during the interactions of compound 1 with ˙OH. The calculations revealed a competitive formation of both σ- and π-type dimer radicals from compound 1. However, the experimental spectrum primarily corresponded to the σ-type dimer, as indicated by the predicted λ_max (630 nm) using the TDDFT method. This study shows that the benzyl-containing selanyl system is a better free radical scavenger than the phenyl-containing selanyl system, highlighting its potential as a potent free radical scavenger.