128884-32-2Relevant academic research and scientific papers
Mechanism of sulfoxide formation through reaction of sulfur radical cation complexes with superoxide or hydroxide ion in oxygenated aqueous solution
Miller, Brian L.,Williams, Todd D.,Christian,Sch?neich
, p. 11014 - 11025 (1996)
We have characterized and quantified several pathways which transform aliphatic sulfur radical cations into sulfoxides in aqueous solution. Sulfur radical cations were produced photochemically via one-electron photooxidation through triplet 4-carboxybenzophenone. Sulfur radical cations and superoxide yield sulfoxide, confirmed by oxygen product isotope effects and an inhibitory role of superoxide dismutase. On the basis of competition experiments with superoxide dismutase the rate constant for the reaction between dimethylsulfide radical cations and superoxide was derived as (2.3 ± 1.2) x 1011 M-1 s-1. A demetalated variant of superoxide dismutase did not inhibit superoxide mediated sulfoxide formation, confirming the importance of an active site of the enzyme for inhibition. The stoichiometry of 2 equiv of sulfoxide per reaction of superoxide with a sulfur radical cation suggests a pathway like the singlet oxygen mediated sulfoxide formation, i.e., via a persulfoxide intermediate formed via (i) direct coupling of superoxide with the sulfur radical cation or (ii) electron transfer followed by addition of the product singlet oxygen to a nonoxidized sulfide. In aqueous solution the persulfoxide may add water to yield a hydroperoxy sulfurane prior to its reaction with a second nonoxidized sulfide. At pH values larger than 9, hydroxide ion starts to compete with superoxide for sulfur radical cations and reacts with the persulfoxide or hydroperoxy sulfurane intermediates, initiating less effective pathways of sulfoxide formation. One pathway involves the formation of hydroxysulfuranyl radicals and their reaction with oxygen, supported by product and solvent isotope effects. Besides superoxide and hydroxide-mediated sulfoxide formation there is an additional route involving methylthiomethylperoxyl radicals. Based on oxygen product isotope effects, the latter appear to transfer oxygen onto the sulfide rather than reacting via electron transfer.
4-Carboxybenzophenone-sensitized photooxidation of sulfur-containing amino acids. Nanosecond laser flash photolysis and pulse radiolysis studies
Bobrowski, Krzysztof,Marciniak, Bronislaw,Hug, Gordon L.
, p. 10279 - 10288 (1992)
Sulfur-containing amino acids were oxidized via photosensitization by 4-carboxybenzophenone (CB) in neutral aqueous solutions. The mechanism of this reaction was investigated with flash photolysis and pulse radiolysis techniques. The rate constants were determined for the quenching of the CB triplet state by 12 amino acids (with variable relative location and number of terminal functions COOH and NH2 with respect to the sulfur atom) and were found to be 1.8 × 108-2.9 × 109 M-1s-1. Time-resolved transient spectra accompanying the quenching events were assigned to the triplet state, the ketyl radical of CB, the radical anion of CB, and the (S∴S)+ radical cations of some of the amino acids. The presence of the radical ions showed the nature of the quenching process to be electron-transfer in character. Two temporally distinct processes were observed for ketyl radical formation. A fast component occurred on a nanosecond time scale. It is ascribed to electron transfer from the sulfur atom to the triplet state of CB followed by (1) the diffusion apart of the charge-transfer (CT) complex and (2) the intramolecular proton transfer within CT complex. The first process was the more efficient one and led to the formation of sulfur-centered radical cations and ketyl radical anions which undergo fast protonation. A slower formation of ketyl radicals occurred on a microsecond time scale and is characterized by pseudo-first-order rate constants, which depend linearly on the CB ground-state concentration (k ～ 109 M-1 s-1). This dark reaction is assigned to the one-electron reduction of CB by the α-aminoalkyl radicals produced from the free-radical cation of the amino acids as a result of intramolecular electron transfer from the carboxyl group to the sulfur-centered radical cation followed by decarboxylation. Yields were determined for some of the processes: the limiting overall quantum yields of ketyl radical (0.4-1.4), the quantum yields of ketyl radical formed in the primary photochemical process and in the dark reaction, and quantum yields of ketyl radical anion. A detailed mechanism for the CB-sensitized photooxidation of sulfur-containing amino acids is proposed and discussed.
Mechanism of 4-carboxybenzophenone-sensitized photooxidation of methionine-containing dipeptides and tripeptides in aqueous solution
Marciniak,Hug,Bobrowski,Kozubek
, p. 13560 - 13568 (2007/10/02)
The mechanism of 4-carboxybenzophenone (CB)-sensitized photooxidation of methionine-containing dipeptides (Met-Gly and Gly-Met) and tripeptides (Met-Gly-Gly, Gly-Met-Gly, and Gly-Gly-Met) was investigated using nanosecond flash photolysis and steady-state photolysis. The rate constants for quenching of the CB triplet by sulfur-containing peptides were determined to be in the range (1.8-2.3) × 109 M-1 S-1 for neutral and alkaline solutions. The presence of the various electron-transfer intermediates accompanying the CB triplet quenching events was identified through the use of a multiple-regression procedure that was used to resolve the experimental transient spectra into components. The intermediates identified were the CB ketyl radical anion, the CB ketyl radical, intermolecularly (S.·.S)-bonded radical cations, and intramolecularly (S.·.N)-bonded radical cations derived from peptides. The types of intermediates were found to depend on the pH of the solution and on the location of the methionine unit with respect to the terminal functions. The quantum yields of all the transients and the kinetics of their formation and decay were measured by flash photolysis, and quantum yields of CO2 formation were measured by steady-state photolysis. A detailed mechanism of the CB-sensitized photooxidation of methionine-containing peptides was discussed and compared to that for methionine.
