Cysteine radical cation: A distonic structure probed by gas phase IR spectroscopy

The interest in the radical cations of amino acids is twofold. On the one hand, these species are relevant in enzymatic catalysis and in oxidative damage of proteins. On the other hand, as constituents of peptides and proteins, they aid the mass spectrometric characterization of these biomolecules, yielding diagnostic fragmentation patterns and providing complementary information with respect to the one obtained from even electron ions. The cysteine radical cation has been obtained by S-NO bond cleavage of protonated S-nitrosocysteine and thoroughly characterized by IRMPD spectroscopy, both in the 1000-2000 cm^-1 range (the highly structurally diagnostic, so-called ' fingerprint’ range) and in the 2900-3700 cm^-1 spectral range, encompassing O-H and N-H stretching vibrations. In this way the distonic structure in which the charge is on the NH₃ group and the spin is on the sulfur atom is unambiguously demonstrated. This tautomer is a local minimum on the potential energy surface, at 29.7 kJ mol^-1 with respect to the most stable tautomer, a captodative structure allowing extensive delocalization of charge and spin.