Cysteine Radical/Metal Ion Adducts: A Gas-Phase Structural Elucidation and Reactivity Study Michael Lesslie, [a] Justin Kai-Chi Lau, [b, c] John T. Lawler, [a] K. W. Michael Siu, [b, c] Vincent Steinmetz, [d] Philippe Maître, [d] Alan C. Hopkinson,* [b] and Victor Ryzhov* [a] Introduction Cysteine is one of the most redox-active amino acid residues in proteins. [1] It plays an essential role at the active site of sev- eral enzymes, including pyruvate formate lyase, [2] class I–IV ri- bonucleotide reductases, [3] and benzylsuccinate synthases. [4] The ability of the sulfur functionality to perform such redox re- actions results in the cysteine motif being present in potent antioxidant compounds, most notably glutathione, which can reach concentrations of approximately 10 mm in mammalian cells. [5] Moreover, at much lower concentrations in the body, [6] free cysteine also functions as a potent antioxidant. [7] It is known that interactions with metal cations at the active sites of redox-active enzymes is crucial to the function of pro- teins. [3b] In mammalian cells, the free concentration of common alkali metals is in the 1–100 mm range. [8] Coordination of these metals to cysteine or the cysteine residue in glutathione in either the reduced, oxidized, or radical form might have out- standing effects on their function as antioxidants. Therefore, the effect of metal coordination on the cysteine residue is of notable interest. Generally, characterization of metal-bound amino acids is of interest as interaction between amino acid side chains and metal ions in peptide and protein systems is known to occur extensively. [9] Knowledge of the binding energies and structural dynamics of individual amino acids coordinating with alkali metals can act as a building block for understanding the inter- actions of these metals with larger peptides and proteins. As such, the effects of alkali metals on the structure of cysteine have been studied previously, both theoretically and experi- mentally, in the gas phase. [10] Ground-state conformers of group 1A ions with cysteine were found to be primarily charge-solvated tridentate species. [10a, c] Interestingly, for the larger ions (K + and Rb + ), the bidentate carboxylic acid bound conformers were found at comparable energies to the triden- tate form. [10c] The bond energies of metal ion coordination de- crease going down the group and appear to correlate inversely to the size of the ion and the charge density. [10c] The theoreti- cal studies were supported by infrared multiple-photon disso- ciation (IRMPD) experiments. It was reported that for the small- er ions (Li + and Na + ) only the tridentate charge-solvated struc- ture was seen. [10b] However, for the larger ions (K + , Rb + , and Cs + ), more complex spectra were found, thus suggesting a mix- ture of conformers including the bidentate carboxylic acid bound form. [10b] Whereas multiple studies of the structure of metal ion/ amino acid (or peptide) complexes have been published, [11] The formation and investigation of sulfur-based cysteine radi- cals cationized by a group 1A metal ion or Ag + in the gas phase are reported. Gas-phase ion–molecule reactions (IMR) and infrared multiple-photon dissociation (IRMPD) spectrosco- py revealed that the Li + , Na + , and K + adducts of the cysteine radical remain S-based radicals as initially formed. Theoretical calculations for the three alkali metal ions found that the lowest-energy isomers are C a -based radicals, but they are not observed experimentally owing to the barriers associated with the hydrogen-atom transfer. A mechanism for the S-to-C a radi- cal rearrangement in the metal ion complexes was proposed, and the relative energies of the associated energy barriers were found to be Li + > Na + > K + at all levels of theory. Relative to the B3LYP functional, other levels of calculation gave signifi- cantly higher barriers (by 35–40 kJ mol 1 at MP2 and 44– 47 kJ mol 1 at the CCSD level) using the same basis set. Unlike the alkali metal adducts, the cysteine radical/Ag + complex re- arranged from the S-based radical to an unreactive species as indicated by IMRs and IRMPD spectroscopy. This is consistent with the Ag + /cysteine radical complex having a lower S-to-C a radical conversion barrier, as predicted by the MP2 and CCSD levels of theory. [a] M. Lesslie, J.T. Lawler, Dr. V. Ryzhov Department of Chemistry and Biochemistry Northern Illinois University DeKalb, IL 60115 (USA) E-mail : ryzhov@niu.edu [b] Dr. J. K.-C. Lau, Dr. K. W. M. Siu, Dr. A. C. Hopkinson Department of Chemistry and Centre for Research in Mass Spectrometry York University, Toronto, ON M3J 1P3 (Canada) E-mail : ach@yorku.ca [c] Dr. J. K.-C. Lau, Dr. K. W. M. Siu Department of Chemistry and Biochemistry University of Windsor, Windsor, ON N9B 3P4 (Canada) [d] Dr. V. Steinmetz, Dr. P. Maître Laboratoire de Chimie Physique, UniversitØ Paris-Sud UMR8000 CNRS, 91405 Orsay (France) Supporting information for this article can be found under http:// dx.doi.org/10.1002/cplu.201500558. ChemPlusChem 2016, 81, 444 – 452 # 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 444 Full Papers DOI: 10.1002/cplu.201500558