In this work, we describe two different methods for generating protonated S-nitrosocysteine in the gas phase. The first method involves a gas-phase reaction of protonated cysteine with t-butylnitrite, while the second method uses a solution-based transnitrosylation reaction of cysteine with S-nitrosoglutathione followed by transfer of the resulting S-nitrosocysteine into the gas phase by electrospray ionization mass spectrometry (ESI-MS). Independent of the way it was formed, protonated S-nitrosocysteine readily fragments via bond homolysis to form a long-lived radical cation of cysteine (Cys •ϩ ), which fragments under collision-induced dissociation (CID) conditions via losses in the following relative abundance order: •COOH ӷ CH 2 S Ͼ •CH 2 SH Ϸ H 2 S. Deuterium labeling experiments were performed to study the mechanisms leading to these pathways. DFT calculations were also used to probe aspects of the fragmentation of protonated S-nitrosocysteine and the radical cation of cysteine. NO loss is found to be the lowest energy channel for the former ion, while the initially formed distonic Cys•ϩ with a sulfur radical site undergoes proton and/or H atom transfer reactions that precede the losses of T here has been renewed interest in the gas-phase formation and reactions of radical ions of biomolecules. The motivation for these studies range from fundamental interest in species related to biological processes such as enzyme catalysis [1] and oxidative chemistry associated with damage to biomolecules such as DNA [2] and proteins [3] through to the potential for developing novel mass spectrometry based analytical applications [4]. With regards to the formation of radical ions of amino acids and peptides, several methods have been developed as alternatives to electron ionization (EI) [5]. These include UV photodissociation [6 -11] and ion-electron based techniques such as electron capture dissociation (ECD) [12][13][14][15]. Chemical-based methods that involve low-energy collisioninduced dissociation (CID) [16] of ions generated via electrospray ionization and which can thus be carried out on a wide range of mass spectrometers, merit special discussion. The first, pioneered by Siu, involves carrying out CID of ternary metal complexes to form radical cations of peptides via the redox reaction shown in eq 1 [17][18][19]. To date, doubly charged copper(II) ternary metal complexes (eq 1, where Metal ϭ Cu; L ϭ a range of neutral ligands, x ϭ 2, y ϭ 0) and singly charged metal(III) complexes (eq 1, where Metal ϭ Cr, Mn, Fe, and Co; L ϭ a salen ligand, x ϭ 3, y ϭ 2) have been used to study the formation and reactions of peptide radical ions, including recent contributions from Julia Laskin [20 -23].The second method involves carrying out CID on a cationized peptide containing a functional group with a weak bond, which is susceptible to bond homolysis. A number of different functional groups have been examined to date including: (1) N-terminal azo derivatives