128884-32-2Relevant articles and documents
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.
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.
