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 cOH radicals to form hydroxyl selenouranyl radicals (pSerOH) which subsequently gave different transient species, like the selenium centered radical cation (SeCc + ), dimer radical cation (pSerSe) + , or a selenium-carboxyl oxygen stabilized monomer radical (pSerO). 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 (pSerSe) + absorbing at 490 nm, while in the case of SePA, along with (pSerSe) + , (pSerO) was also formed. The stability of (pSerSe) + 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 0 -azinobis(3-ethylbenzthiazoline-6-sulphonic acid), thionine and methyl viologen. The study showed that SeCc + undergoes decarboxylation with the formation of the corresponding a-(alkylseleno)alkyl radical and the yield of CO 2 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 CO 2 and the same is reflected in its higher free radical scavenging antioxidant activity, established by comparing the rate constants for scavenging of peroxyl radicals.

Effect of Molecular Interactions on Electron‐Transfer and Antioxidant Activity of Bis(alkanol)selenides: A Radiation Chemical Study

“…Previous studies on the antioxidant activities of both aliphatic as well as aromatic selenium compounds showed that the free-radical scavenging ability and the GPx activity do not correlate as the former proceeds through oneelectron transfer while the latter takes place by two-electron or oxygen transfer. [9,15] In the presenti nvestigation, we propose for the first time that one-electron-transfer processes in simple aliphatic bis(alkanol)selenide (SeROH;S cheme1)c an generate products that are redox cycled and participate in GPx antioxi-dant activity.F or this, detailed pulse radiolysis studies on the reactiono fCOH radical with SeROH were carried out, the products were estimated and the results were complimented by quantum chemical calculations.…”

“…The concentrationdependenta bsorbance at 490 nm is attributed to the dimer radicalo ft he type (> Se\Se <) + formed by the reaction of se-lenium-centered radical cation (> SeC + )w ith parent SeEOH, as observed (path IIc and IIIa in Scheme 2) with most sulfur and seleniumr adical cations,t hrough two-centered-three-electron (2cÀ3e) hemibond formation involving the p-orbital of the (> SeC + )w ith the p-orbital containing the lone pair of electrons of the other selenium atom of SeEOH. [9][10][11] The absorption band at 320 nm may be either due to the formation of (> Se\OH) (path Ii nS cheme 2) or selenium-centered radical cation (> SeC + )( path II a/b in Scheme 2).…”

“…The radicals generated by different reactions of SeROH with COH radicals can be both oxidizing and reducing in nature. [9,10] The yield of the oxidizing radicals was estimatedb yt heir ability to oxidize ABTS 2À to produce ABTSC À radical, absorbing at 645 nm. By following the rate of formation of ABTSC À radicala s af unctiono fA BTS 2À concentration, the bimolecular rate constant for the reaction of (> Se\Se <) + with ABTS 2À was estimated.F or this, N 2 O-saturated aqueous solutionsa tp H7,c ontaining 5mm SeROH and 10-100 mm ABTS 2À were pulse irradiated (under these conditions the radicals are predominantly of (> Se\Se <) + radicalt ype) and by using the extinction coefficient of ABTSC À (e 645nm = 1.3 10 4 m À1 cm À1 )a nd by applying Schuler'sf ormula.…”

“…[7,8] We have been investigating the nature and reactivity of selenium-centered radicalsgenerated by hydroxyl (COH) radicalreactions on av ariety of organoselenium compounds ranging from simple selenourea to bifunctional seleno-aminoacids like selenomethione and selenocystine. [9][10][11][12] For all the chosen compounds, the COH radical wasf ound to react with the selenium atom;t he nature, stabilitya nd reactivity of the resulting selenium radicals are modulated by nonbonding interactions betweens elenium and the neighboring heteroatom. [9][10][11] Another important property of seleniumc ompounds is their abilityt oe xhibit GPx-like activity where it catalyzes the reduction of hydrogen peroxide, am olecular oxidanta nd ROS.…”

“…[9][10][11][12] For all the chosen compounds, the COH radical wasf ound to react with the selenium atom;t he nature, stabilitya nd reactivity of the resulting selenium radicals are modulated by nonbonding interactions betweens elenium and the neighboring heteroatom. [9][10][11] Another important property of seleniumc ompounds is their abilityt oe xhibit GPx-like activity where it catalyzes the reduction of hydrogen peroxide, am olecular oxidanta nd ROS. [13,14] For ad esigned organoselenium compound to be an antioxidant, it should not only show GPx-like activity buta lso exhibit free-radical reactions andt he reaction products of such reactions are recyclable.…”