Mass Spectrometric Behavior of 5-S-Cysteinyldopa and Structurally Related Phenolic Compounds. Fragmentation Susceptibility of the Alkylthioether Bond Under Electron Impact and Fast Atom Bombardment Conditions

Abstract

The marked proclivity of 5-S-cysteinyldopa (1) and related phenolic alkylthioethers to undergo UV-induced desulfurization via primary photohomolytic cleavage of the CH 2 -S bond prompted an investigation of the mass spectrometric behavior of these compounds with a view to obtaining information on the intrinsic susceptibility to dissociation of the alkylthioether bond under non-irradiative conditions. Dealkylative fission of the carbon-sulfur bond was found to be an important dissociative channel in the fragmentation patterns of 1 and related compounds, including cysteinylhydroquinone (2), 4-S-cysteaminylphenol (3) and 4-S-cysteaminylcatechol (4) under electron impact conditions. The mode of fission and relative susceptibility of the CH 2 -S bond were, however, profoundly different in cysteaminyl vs. cysteinyl compounds. In particular, the former showed a marked tendency to undergo simple homolytic fission, whereas in the latter dealkylative C-S bond cleavage occurred largely via an intramolecular rearrangement mechanism, and proved highly sensitive to protonation-induced effects, as evidenced in fast atom bombardment experiments. Desulfurization processes with neat loss of the alkylthio residue, like those occurring upon UV irradiation, were virtually absent in the fragmentation patterns of 3 and 4, and provided only a minor contribution to those of 1 and 2. These and other data reported open fresh prospects for future studies on the chemistry and mass spectrometric behavior of 1 and structurally related phenolic compounds of biological relevance.