Sensitized Photooxidation of S-Methylglutathione in Aqueous Solution: Intramolecular (S∴O) and (S∴N) Bonded Species

Abstract

Nanosecond laser flash photolysis was used to generate sulfur radical cations of the thioether, S-methylglutathione (S-Me-Glu), via the one-electron oxidation of this thioether by triplet 4-carboxybenzophenone. The purpose of this investigation was to follow the neighboring group effects resulting from the interactions between the sulfur radical cationic sites and nearby lone-pair electrons on heteroatoms within the radical cation, especially the electron lone-pairs on heteroatoms in the peptide bonds. The tripeptide, S-Me-Glu, offers several possible competing neighboring group effects that are characterized in this work. Quantum yields of the various radicals and three-electron bonded (both intramolecular and intermolecular) species were determined. The pH dependence of photoinduced decarboxylation yields was used as evidence for the identification of a nine-membered ring, sulfur-nitrogen, three-electron bonded species. The mechanisms of the secondary reactions of the radicals and radical cations were characterized by resolving their overlapping transient-absorption spectra and following their kinetic behavior. In particular, sulfur-oxygen and sulfur-nitrogen three-electron bonded species were identified where the oxygen and nitrogen atoms were in the peptide bonds.

Tandem mass spectrometry and infrared spectroscopy as a tool to identify peptide oxidized residues

“…12,13 Several free radicals are formed with various 2-centre 3-electron bonds between the sulphur radical cation and a nitrogen atom of the molecule, or the sulphur atom of another molecule of GS-Me. 12,13 Several free radicals are formed with various 2-centre 3-electron bonds between the sulphur radical cation and a nitrogen atom of the molecule, or the sulphur atom of another molecule of GS-Me.…”

Oxidation studies of a novel peptide model N-acetyl-3-(methylthio)propylamine

“…As can be seen in the Figure 2, the use of 4 input components (αS, (>SNH) + , SS + and >SOH) gives an excellent fit to the experimental data. Knowing the mechanism for similar systems from previous studies, [8,[29][30][31] the use of αS, SS + and >SOH, reference absorption spectra is quite obvious, however, one can ask if it is necessary to use an additional (>SNH) + component to resolve the spectra (since the electron density on the acetylated amino group is certainly much lower compared to that of -NH 2 ). The calculations have shown it is absolutely necessary to use a component absorbing in the 350-400 nm range (without this input, the fit was really very poor).…”

“…The lone electron pairs are necessary to form the three-electron sulfur-nitrogen bonds under consideration. Such threeelectron bonding can also be achieved with the participation of the carbonyl oxygen atom [16,31,32], however, this would require the formation of a thermodynamically and kinetically [33] less favorable seven-membered ring compared to a five-membered ring with the nitrogen atom. On the other hand, it has been recently reported [32] that such (SO) + formation gains the energy benefit of 0.04 eV in cyclic Met-Met dipeptides.…”

A Long Story of Sensitized One‐Electron Photo‐oxidation of Methionine

“…A different mechanism of photo‐oxidation occurs in S‐methylglutathione. At low pH, there is no decarboxylation, which was interpreted as a sign of the lack of formation of a nine‐membered intramolecular ring (S ∴ N) + involving the terminal amine 25. Spectral analysis of the transient could indicate the formation of five‐membered‐ring (S ∴ N) + radical cations involving the nitrogen of the peptide bond.…”

“…At high pH (close to 11), the decarboxylation yield reached 0.25. In addition, there was spectral evidence for the formation of intramolecular (S ∴ O) + species 25…”