Study of the structure and reactivity of the radical cations and anions of cysteine, its derivatives, and cysteine-containing peptides using ion-molecule reactions, infrared multi-photon dissociation spectroscopy, and density functional theory calculation

Due to the often transient nature of protein radicals in solution, there has been an increased interest in gaining a more fundamental understanding of the structure and reactivity of radicals in small model systems in the gas phase. With this objective in mind, the goal of this project has been to develop new methods for the study of radical ions in the gas phase using mass spectrometric techniques. In particular, we have employed ion-molecule reactions (IMR) for the study of radical ion reactivity, and infrared multiple photon dissociation (IRMPD) spectroscopy in conjunction with density functional theory (DFT) calculations for the determination of the structure of the radical ions.;The model systems which have been investigated in this project all contain cysteine or its derivatives, the motivation being to gain a fundamental understanding of sulfur radical chemistry in the gas phase. Generation of the sulfur radical of cysteine was accomplished by first nitrosylating the sulfur of the thiols in solution, followed by homolytic cleavage of the S-NO bond in the gas phase.;By introducing a volatile neutral species into the ion trap where the sulfur radical ion is being held, the reactivity of the radical ion towards the neutral specis was investigated. The reactions of several cysteine-containing radical cations and anions with neutrals such as dimethyldisulfide and allyl iodide were shown to produce similar products for both the cations and anions. This would suggest that the charge does not play a major role in the reactivity, and that the radical is the key component for the reaction to occur.;Kinetic plots generated for some of the ions studied (N-Ac-Cys, Gly-Cys, Cys-Gly, γ-Glu-Cys, γ-Glu-Cys-Gly) showed that after an initial display of radical ion-neutral reactivity the product formation leveled off, indicating that after a certain period of time the radical ion stopped reacting with the neutral. It was proposed that the reason for this change in reactivity was a result of radical migration, wherein the sulfur radical migrates (via hydrogen atom transfer) to an ?arbon position which is known to be unreactive towards these neutrals.;Data to support this theory of radical migration was obtained via IRMPD spectroscopy. Gas-phase IR spectra were obtained for each of the radical ions studied, and were compared to theoretical IR spectra generated by DFT calculations, this comparison allowed for the identification of the structure of each of the ions studied. The structural data indicated that for all of the systems that displayed a loss of reactivity, radical rearrangement to an ?arbon was occurring.;There were two species (cysteine and homocysteine) in which radical migration did not occur. For both of these radical cations it was shown that the pathway for rearrangement was relatively high in energy. The kinetic data of these two systems indicated that they had different rates of sulfur radical reactivity, which was explained by the degree of hydrogen bonding between the sulfur radical and an amine hydrogen. The increased hydrogen bonding in the radical cation of homocysteine led to a decrease in rate of reactivity compared to cysteine. This observation suggests that the cysteine sulfur radical is slightly more unstable and therefore more likely to cause damage to proteins via hydrogen atom abstraction than the sulfur radical of homocysteine.