S∴N and S∴O three-electron-bonded radicals and radical cations in aqueous solutions

Abstract

Intramolecularly formed S∴N three-electron-bonded radical cations have been identified as transient intermediates in the ′OH radical-induced oxidation of 3-(methylthio)propylamine, methionine, and methionine ethyl ester. A similarly bonded radical is also indicated in the reduction of dehydromethionine by hydrated electrons. Intramolecular S∴O-bonded intermediates are suggested to be formed in the ˙OH-induced oxidation of 4-(methylthio)butyric acid and S-methylcysteine. All these three-electron-bonded species exhibit optical absorptions with maxima in the 380–400 nm range. The observable S∴O-bonded transient from S-methylcysteine is presumed to be protonated at oxygen. Favourable steric structures, particularly five-membered rings, significantly increase the probability of formation and the overall stability of a particular three-electron bond. Under comparable conditions, i.e. structure, substituents, consecutive chemistry, etc., the S∴O bond appears to be considerably weaker than corresponding S∴N and S∴S bonds.