Reactions of hypobromous acid with dimethyl selenide, dimethyl diselenide and other organic selenium compounds: kinetics and product formation

Abstract

Selenium (Se) is an essential micronutrient for many living organisms particularly due to its unique redox properties. We recently found that the sulfur (S) analog for dimethyl selenide (DMSe), i.e. dimethyl sulfide (DMS), reacts fast with the marine oxidant hypobromous acid (HOBr) which likely serves as a sink of marine DMS. Here we investigated the reactivity of HOBr with dimethyl selenide and dimethyl diselenide (DMDSe), which are the main volatile Se compounds biogenically produced in marine waters. In addition, the reactivity of HOBr with further organic Se compounds was tested, i.e., SeMet (as N-acetylated-SeMet), and selenocystine (SeCys₂ as N-acetylated-SeCys₂), as well as the phenyl-analogs of DMSe and DMDSe, respectively, diphenyl selenide (DPSe) and diphenyl diselenide (DPDSe). Apparent second-order rate constants at pH 8 for the reactions of HOBr with the studied Se compounds were (7.1 ± 0.7) × 10⁷ M⁻¹ s⁻¹ for DMSe, (4.3 ± 0.4) × 10⁷ M⁻¹ s⁻¹ for DMDSe, (2.8 ± 0.3) × 10⁸ M⁻¹ s⁻¹ for SeMet, (3.8 ± 0.2) × 10⁷ M⁻¹ s⁻¹ for SeCys₂, (3.5 ± 0.1) × 10⁷ M⁻¹ s⁻¹ for DPSe, and (8.0 ± 0.4) × 10⁶ M⁻¹ s⁻¹ for DPDSe, indicating a very high reactivity of all selected Se compounds with HOBr. The reactivity between HOBr and DMSe is lower than for DMS and therefore this reaction is likely not relevant for marine DMSe abatement. However, the high reactivity of SeMet with HOBr suggests that SeMet may act as a relevant quencher of HOBr.