Radical cations of some water-soluble organoselenium compounds: Insights from pulse radiolysis studies

Radiation-Induced Oxidation Reactions of 2-Selenouracil in Aqueous Solutions: Comparison with Sulfur Analog of Uracil

One-electron oxidation of 2-selenouracil (2-SeU) by hydroxyl (^●OH) and azide (^●N₃) radicals leads to various primary reactive intermediates. Their optical absorption spectra and kinetic characteristics were studied by pulse radiolysis with UV-vis spectrophotometric and conductivity detection and by the density functional theory (DFT) method. The transient absorption spectra recorded in the reactions of ^●OH with 2-SeU are dominated by an absorption band with an λ_max = 440 nm, the intensity of which depends on the concentration of 2-SeU and pH. Based on the combination of conductometric and DFT studies, the transient absorption band observed both at low and high concentrations of 2-SeU was assigned to the dimeric 2c-3e Se-Se-bonded radical in neutral form (2^●). The dimeric radical (2^●) is formed in the reaction of a selenyl-type radical (6^●) with 2-SeU, and both radicals are in equilibrium with K_eq = 1.3 × 10⁴ M⁻¹ at pH 4 (below the pK_a of 2-SeU). Similar equilibrium with K_eq = 4.4 × 10³ M⁻¹ was determined for pH 10 (above the pK_a of 2-SeU), which admittedly involves the same radical (6^●) but with a dimeric 2c-3e Se-Se bonded radical in anionic form (2^●−). In turn, at the lowest concentration of 2-SeU (0.05 mM) and pH 10, the transient absorption spectrum is dominated by an absorption band with an λ_max = 390 nm, which was assigned to the ^●OH adduct to the double bond at C5 carbon atom (3^●) based on DFT calculations. Similar spectral and kinetic features were also observed during the ^●N₃-induced oxidation of 2-SeU. In principle, our results mostly revealed similarities in one-electron oxidation pathways of 2-SeU and 2-thiouracil (2-TU). The major difference concerns the stability of dimeric radicals with a 2c-3e chalcogen-chalcogen bond in favor of 2-SeU.

Effects of Selenium Species on the Belousov-Zhabotinsky Reaction

The effects of selenium species on the Belousov-Zhabotinsky (B-Z) reaction were investigated by adding them to the system before and during oscillation. When selenium species were added into the system before oscillation, sodium selenite prolonged the induction period, whose effect was strong as sodium selenite could consume malonic acid to prohibit the accumulation of bromomalonic acid. For selenomethionine and selenocystine, their effects were derived from their reaction with ^•CH₂COOH and ^•Br₂^- producing a radical cation of selenoamino acids, which prohibited the accumulation of bromomalonic acid. Here, the selenium atoms in selenoamino acids, as reactive centers, took part in the redox reaction. As a result, the induction period was prolonged. However, as a diselenide, selenocystine can reduce bromate in acidic medium, which led to shortening of the induction period. Therefore, the effect of selenocystine on the induction period was the result of two opposite effects. Nanoselenium shortened the induction period in a certain concentration range because bromate was directly reduced by nanoselenium and the accumulation of bromomalonic acid was promoted. Furthermore, the dose perturbation effect was investigated by the injection of nanoselenium during oscillation. It was found that the amplitude was increased or decreased in a dose-dependent fashion when nanoselenium was injected at peak or trough of the time-dependent redox potential curve.

Free radical induced selenoxide formation in isomeric organoselenium compounds: the effect of chemical structures on antioxidant activity

Formation of selenoxides improves the antioxidant activity of organoselenium compounds and should be considered as an important marker in the design of new selenium based antioxidants.

Reaction of low-molecular-mass organoselenium compounds (and their sulphur analogues) with inflammation-associated oxidants

Selenium is an essential trace element in mammals, with the majority specifically encoded as seleno-L-cysteine into a range of selenoproteins. Many of these proteins play a key role in modulating oxidative stress, via either direct detoxification of biological oxidants, or repair of oxidised residues. Both selenium- and sulphur-containing residues react readily with the wide range of oxidants (including hydrogen peroxide, radicals, singlet oxygen and hypochlorous, hypobromous, hypothiocyanous and peroxynitrous acids) that are produced during inflammation and have been implicated in the development of a range of inflammatory diseases. Whilst selenium has similar properties to sulphur, it typically exhibits greater reactivity with most oxidants, and there are considerable differences in the subsequent reactivity and ease of repair of the oxidised species that are formed. This review discusses the chemistry of low-molecular-mass organoselenium compounds (e.g. selenoethers, diselenides and selenols) with inflammatory oxidants, with a particular focus on the reaction kinetics and product studies, with the differences in reactivity between selenium and sulphur analogues described in the selected examples. These data provide insight into the therapeutic potential of low-molecular-mass selenium-containing compounds to modulate the activity of both radical and molecular oxidants and provide protection against inflammation-induced damage. Progress in their therapeutic development (including modulation of potential selenium toxicity by strategic design) is demonstrated by a brief summary of some recent studies where novel organoselenium compounds have been used as wound healing or radioprotection agents and in the prevention of cardiovascular disease.

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

Formation of a stable five membered transient in seleno bis(propanoic acid) reduces its radiolytic degradation and enhances its peroxyl radical scavenging activity.

One-electron redox processes in a cyclic selenide and a selenoxide: a pulse radiolysis study

One-electron redox reactions of cyclic selenium compounds, DL-trans-3,4-dihydroxy-1-selenolane (DHS(red)), and DL-trans-3,4-dihydroxy-1-selenolane oxide (DHS(ox)) were carried out in aqueous solutions using nanosecond pulse radiolysis, and the resultant transients were detected by absorption spectroscopy. Both *OH radical and specific one-electron oxidant, Br(2)(*-) radical reacted with DHS(red) to form similar transients absorbing at 480 nm, which has been identified as a dimer radical cation (DHS(red))(2)(*+). Secondary electron transfer reactions of the (DHS(red))(2)(*+) were studied with 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS(2-)) and superoxide (O(2)(*-)) radicals. The bimolecular rate constants for the electron transfer reaction between (DHS(red))(2)(*+) with ABTS(2-) was determined as 2.4 +/- 0.4 x 10(9) M(-1) s(-1). From this reaction, the yield of (DHS(red))(2)(*+) formed on reaction with *OH radical was estimated in the presence of varying phosphate concentrations. (DHS(red))(2)(*+) reacted with O(2)(*-) radical with a bimolecular rate constant of 2.7 +/- 0.1 x 10(9) M(-1) s(-1) at pH 7. From the same reaction, the positive charge on (DHS(red))(2)(*+) was confirmed by the kinetic salt effect. HPLC analysis of the products formed in the reaction of (DHS(red))(2)(*+) with O(2)(*-) radicals showed formation of the selenoxide, DHS(ox). In order to know if a similar mechanism operated during the reduction of DHS(ox), its reactions with e(aq)(-) were studied at pH 7. The rate constant for this reaction was determined as 5.6 +/- 0.9 x 10(9) M(-1) s(-1), and no transient absorption could be observed in the wavelength region from 280 to 700 nm. It is proposed that the radical anion (DHS(ox))(*-) formed by a one-electron reduction would get protonated to form a hydroxyl radical adduct, which in presence of proton donors, would undergo dehydration to form DHS(*+). Evidence for this mechanism was obtained by converting DHS(*+) to (DHS(red))(2)(*+) with the addition of DHS(red) to the same system. Quantum chemical calculations provided supporting evidence for some of the redox reactions.

Antioxidant capacity of Macaronesian traditional medicinal plants

The use of many traditional medicinal plants is often hampered by the absence of a proper biochemical characterization, essential to identify the bioactive compounds present. The leaves from five species endemic to the Macaronesian islands with recognized ethnobotanical applications were analysed: Apollonias barbujana (Cav.) Bornm., Ocotea foetens (Ainton) Baill, Prunus azorica (Mouill.) Rivas-Mart., Lousã, Fern. Prieto, E. Días, J.C. Costa & C. Aguiar, Rumex maderensis Lowe and Plantago arborescens Poir. subsp. maderensis (Dcne.) A. Hans. et Kunk.. Since oxidative stress is a common feature of most diseases traditionally treated by these plants, it is important to assess their antioxidant capacity and determine the molecules responsible for this capacity. In this study, the antioxidant capacity of these plants against two of the most important reactive species in human body (hydroxyl and peroxyl radicals) was determined. To trace the antioxidant origin total phenol and flavonoid contents as well as the polyphenolic profile and the amount of trace elements were determined. There was a wide variation among the species analysed in what concerns their total leaf phenol and flavonoid contents. From the High Performance Liquid Chromatography (HPLC) electrochemically detected peaks it was possible to attribute to flavonoids the antioxidant capacity detected in A. barbujana, O. foetens, R. maderensis and P. azorica extracts. These potential reactive flavonoids were identified for A. barbujana, R. maderensis and P. azorica. For R. maderensis a high content (7 mg g-1 dry weight) of L-ascorbic acid, an already described antioxidant phytomolecule, was found. A high content in selenomethionine (414.35 microg g-1 dry weight) was obtained for P. arborescens subsp. maderensis extract. This selenocompound is already described as a hydroxyl radical scavenger is reported in this work as also possessing peroxyl radical scavenging capacity. This work is a good illustration of different phytomolecules (flavonoids, organic acids and selenocompounds), presents in leaves of the five traditional medicinal plants endemic to Macaronesia, all exhibiting antioxidant properties.