Perturbing Peptide Cation-Radical Electronic States by Thioxoamide Groups: Formation, Dissociations, and Energetics of Thioxopeptide Cation-Radicals

Thioxodipeptides Gly-thio-Lys (GtK), Ala-thio-Lys (AtK), and Ala-thio-Arg (AtR) in which the amide group has been modified to a thioxoamide were made into dications by electrospray ionization and converted to cation-radicals, (GtK + 2H)+•, (AtK + 2H)+•, and (AtR + 2H)+•, by electron transfer dissociation (ETD) tandem mass spectrometry using fluoranthene anion-radical as an electron donor. The common and dominant dissociation of these cation-radicals was the loss of a hydrogen atom. The dissociation products were characterized by collision-induced dissociation (CID) multistage tandem mass spectrometry up to CID-MS5. The ground electronic states of several (GtK + 2H)+•, (AtK + 2H)+•, and (AtR + 2H)+• conformers were explored by extensive ab initio and density functional theory calculations of the potential energy surface. In silico electron transfer to the precursor dications, (GtK + 2H)2+, (AtK + 2H)2+, and (AtR + 2H)2+, formed zwitterionic intermediates containing thioenol anion-radical and ammonium catio...