H.S. Mahal

Scavenging of reactive oxygen radicals by resveratrol: antioxidant effect

Pulse radiolysis of resveratrol was carried out in aqueous solutions at pH ranging from 6.5 to 10.5. The one-electron oxidized species formed by the N•3 radicals at pH 6.5 and 10.5 were essentially the same with λmax at 420 nm and rate constant varying marginally (k = (5−6.5) × 109 dm3 mol−1 s−1). The nature of the transients formed by NO•2, NO• radical reaction at pH 10.5 was the same as that with N•3, due to the similarity in decay rates and the absorption maximum. Reaction of •OH radical with resveratrol at pH 7 gives an absorption maximum at 380 nm, attributed to the formation of carbon centered radical. The repair rates for the thymidine and guanosine radicals by resveratrol were approx. 1 × 109 dm3 mol−1 s−1, while the repair rate for tryptophan was lower by nearly an order of magnitude (k = 2 × 108 dm3 mol−1 s−1). The superoxide radical anion was scavenged by resveratrol, as well as by the Cu–resveratrol complex with k = 2 × 107 and 1.5 × 109 dm3 mol−1 s−1, respectively. Its reduction potential was also measured by cyclic voltammetry.

Discrete ionization of two different short-living conformers of selenophenol by rapid free electron transfer to solvent parent radical cations

Abstract In the pulse radiolysis of solutions of selenophenol in n -butyl chloride, selenophenol radical cations and phenylselenyl radicals are generated as direct products of the ion–molecule reaction between the solvent parent radical cations and selenophenol. This effect is explained with an electron-transfer phenomenon where in each encounter situation the electron jump proceeds so extremely rapid that different transient conformers of the scavenger molecule must be taken into account, differing in the phases of the molecular torsion vibration of the C–SeH bond. Quantum-chemical calculations resulted in a very low activation energy of less than 1 kcal mol −1 for the rotation of this bond and a favored geometry where the Se–H group is twisted against the aromatic moiety by an angle of 90°.

One-electron redox reactions of troxerutin in aqueous solutions

The oxidation of flavonoids is of great interest because of their action as antioxidants with the ability to scavenge radicals by means of electron-transfer processes. The redox reactions of the flavonoid derivative troxerutin, (2-[3,4-bis-(2-hydroxyethoxy) phenyl]-3[[6-deoxy-α-L-manno-pyranosyl)-β-(D-glucopyranosyl]-oxy]-5-hydroxy-7-(2-hydroxyethoxy)-4H-1-benzo-pyran-4-one), were investigated over a wide range of conditions, using pulse radiolysis and cyclic voltammetry. The oxidation mechanism proceeds in sequential steps. One-electron redox potentials for troxerutin were found to be +1.196, +0.846 and −0.634 V vs. NHE.

Radical scavenging reactions of chlorogenic acid: a pulse radiolysis study

At near neutral pH (approx. 5.5), the OH-adduct of chlorogenic acid (CGA), formed on pulse radiolysis of N2O-saturated aqueous CGA solutions (λmax = 400 and 450 nm) with k = 9 × 109 dm3 mol−1 s−1, rapidly eliminates water (k = 1 × 103 s−1) to give a resonance-stabilized phenoxyl type of radical. Oxygen rapidly adds to the OH-adduct of CGA (pH 5.5) to form a peroxyl type of radical (k = 6 × 107 dm3 mol−1 s−1). At pH 10.5, where both the hydroxyl groups of CGA are deprotonated, the rate of reaction of · OH radicals with CGA was essentially the same as at pH 5.5, although there was a marked shift in the absorption maximum to approx. 500 nm. The CGA phenoxyl radical formed with more specific one-electron oxidants, viz., Br2·− and N3· radicals show an absorption maximum at 385 and 500 nm, k ranging from 1–5.5 × 109 dm3 mol−1 s−1. Reactions of other one-electron oxidants, viz., NO2·, NO· and CCl3OO· radicals, are also discussed. Repair rates of thymidine, cytidine and guanosine radicals generated pulse radiolytically at pH 9.5 by CGA are in the range of (0.7–3) × 109 dm3 mol−1 s−1.

Anomalous high reactivity of formyl and acetone ketyl radicals with uracil and its derivatives

Abstract CO 2 · − and ( CH 3 ) 2 · COH radicals apparently react with uracil and its derivatives, containing two carbonyl groups, with high rate constants ( k =10 10 dm 3 mol −1 s −1 ). Various mechanistic aspects of such reactions have been probed. The effect of pH and buffer concentration on the initial formation of absorbance points to some keto-enol tautomerism-type fast-reaction between OH − and the substrate, followed by a slower relaxation to the original equilibrium. The radical anions of these compounds react with methyl viologen mainly via an electron transfer reaction with a high rate constant value (4–9)×10 9 dm 3 mol −1 s −1 .

Electron transfer (oxidation) of complexes between bifunctional phenols and DMSO in non-polar surroundings

Pulse-radiolysis reactions were performed to study the effect of hydrogen bonding to dimethyl sulfoxide (DMSO) on the oxidation of dihydroxy benzene and biphenyl diols to phenoxyl radicals. It was observed that with DMSO as the hydrogen-bond acceptor, the oxidation process proceeds via proton-coupled electron transfer in the case of hydroquinone. For resorcinol, DMSO acts in a similar way as in the case of hydroquinone. For other biphenols, viz., 2,2′- and 4,4′-biphenyl diols, it was found that DMSO had no effect on the electron transfer. The results are explained based on the ionization potential and structure of the phenol derivatives which probably depends on the rotation of the OH bond causing different electron distribution in the transient conformation.

The fluorescence of tryptophan in N,N-dimethylformamide-water mixtures

Abstract The fluorescence of tryptophan in aqueous solutions containing N,N-dimethylformamide (DMF) in concentrations varying from 1 to 10 M has been studied using both steady-state techniques and lifetime measurements. Stern-Volmer plots as well as the variations in τ 1 emphasise the importance of exciplex emission at higher concentrations of DMF.

Complexation of gold and silver nanoparticles with radiolytically-generated radicals

The reactivity of (SCN)˙−2, Br˙−2 and phenoxyl radicals in aqueous solution, generated by pulse radiolysis, was studied on gold and silver nanoparticle surfaces. The spectral changes associated with chemical interaction of radicals with metal nanoparticles were elucidated using time-resolved absorption spectroscopy. The resultant complexes of (SCN)˙−2 and Br˙−2 with Au nanoparticles have shown a strong absorption band at 390 nm and 260 nm, respectively. The stability of the particles after adsorption of SCN−, Br− and phenols was measured by steady-state absorption. It has been shown that the concentration of CTAB plays a significant role in the stabilization of particles in the presence of an adsorbate.