Novel Reactions of One-Electron Oxidized Radicals of Selenomethionine in Comparison with Methionine

Abstract

Pulse radiolysis studies on hydroxyl (*OH) radical reactions of selenomethionine (SeM), a selenium analogue of methionine, were carried out, and the resultant transient radical cations and their subsequent reactions have been reported. At pH<3, the >Se*-OH radical adducts produced on reaction of SeM with *OH radical were converted to selenium centered radical cations (Se*+M), which react with another molecule of SeM to form dimer radical cation M(Se therefore Se)M+. At pH 7, the >Se*-OH radical adducts were converted to a monomer radical of the type (Se therefore N)M+ that acquires intramolecular stability through interaction with the lone pair of the N atom and this radical is denoted as SeM*+. SeM*+ decayed by first order kinetics, and the reduction potential of the couple SeM*+/SeM was determined to be 1.21+/-0.05 V vs NHE at pH 7. SeM*+ oxidized ABTS2- and TMPD with rate constants of (2.5+/-0.1)x10(8) and (6.1+/-0.2)x10(8) M(-1) s(-1), respectively, and reacted with hydroxide ion with a rate constant of (3.8+/-0.9)x10(5) M(-1) s(-1). SeM*+ reacts with molecular oxygen, and the rate constant for this reaction was determined to be (4.3+/-0.2)x10(8) M(-1) s(-1); similar reaction with methionine could not be observed experimentally. Like methionine radical cations, SeM*+ undergoes decarboxylation, although with lesser yield, to produce reducing 3-methyl-selenopropyl amino radicals (referred as alpha-amino radicals). The formation of these radicals was confirmed both by the estimation of the liberated CO2 and by one-electron reduction of MV2+, thionine, and PNAP. These results have been supported by quantum chemical calculations. Implications of these results in the biological role of SeM have also been briefly discussed.

p-Cyanophenylalanine and selenomethionine constitute a useful fluorophore–quencher pair for short distance measurements: application to polyproline peptides

“…As shown (Figure S3), the oxidation potential of Phe CN was measured to be 0.921 V vs. NHE at pH 7.0, a value similar to that of other aromatic amino acids. 43 Because the reduction potential of SeMet was previously determined to be 1.21 V vs NHE at pH 7.0, 44 we can use the following Rehm and Weller equation 41 to estimate the free energy change (Δ G ET ) of the corresponding electron transfer reaction: $$\mathrm{normal\Delta }{G}_{\mathrm{normalE}\mathrm{normalT}}={E_{1/2}}^{\text{ox}}-{E_{1/2}}^{\text{red}}-E_{\mathrm{s}}+\mathrm{normalC},$$ where E 1/2 ox is the half-wave oxidation potential of the electron donor Phe CN , E 1/2 red is the half-wave reduction potential of the electron acceptor SeMet, E s is the singlet excitation energy of the fluorophore Phe CN (280 nm), 5 and C is the Coulomb energy change described by: $$C=-\frac{e^2}{\epsilon a4\pi {\epsilon }_0},$$ where ε is the dielectric constant of water and a is the closest distance separation (7 Å). As expected, Δ G ET was estimated to be −4.7 eV, a value that is consistent with the notion that the Phe CN fluorescence is quenched by SeMet via an electron transfer mechanism.…”

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

“…2). 19 The yield of the transient at 360 nm decreased with increasing proton concentration, indicating that the 360 nm absorbance is due to pSerOH, which undergoes acid catalyzed dehydration to form selenium centred radical cation. 20 Therefore to verify the nature of the transient formed by cOH radical, the absorbance values at 360 nm (where maximum changes were observed) and 490 nm were monitored at different concentration of SeBA.…”

“…Further, similar experiments with 5 mM SeBA, generated transient absorption spectrum with absorption maxima at 320 nm and 490 nm at 1 ms aer the pulse (Fig. [15][16][17][18][19][20][21] The absorbance at 360 nm can be due to the selenuranyl radical (pSerOH) formed by OH adduct formation at the selenium centre or it may be due to intramolecularly stabilized selenium monomer radical species formed between the selenium centred radical cation and the negatively charged oxygen of the carboxylate functional group. In general, selenium radicals form dimer radical species and the yield of such species increases with increase in the concentration of the selenium compound.…”

The reaction of methionine with hydroxyl radical: reactive intermediates and methanethiol production

“…Importantly, Met ? Á OH may also end in decarboxylation (Bobrowski and Schöneich 1996;Mishra et al 2009). In our experimental setup, CO 2 signal in GC was overlapped by strong signal from air.…”

“…Á OH reaction has undergone periodical revisions during the past, to take into account various reactive intermediates and end products (Taniguchi et al 1972;Hiller et al 1981;Hiller and Asmus 1983;Miller et al 1998;Pogocki et al 2001;Nukuna et al 2001;Abedinzadeh 2001;Schöneich et al 2003;Mishra et al 2009). The identification of free radicals is a formidable task, due to their exceptionally short lifetime and versatility of reactive species produced in various systems (Halliwell and Whiteman 2004;Valko et al 2007).…”