Infrared multiple photon dissociation spectroscopy of cationized cysteine: Effects of metal cation size on gas-phase conformation

Abstract

The gas-phase structures of cationized cysteine (Cys) including complexes with Li +, Na +, K +, Rb +, and Cs +, as well as protonated Cys, are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by a free electron laser, in conjunction with quantum-chemical calculations. To identify the structures present in the experimental studies, measured IRMPD spectra are compared to spectra calculated at B3LYP/6-311G(d,p) (H +, Li +, Na +, and K + complexes) and B3LYP/HW*/6-311G(d,p) (Rb + and Cs + complexes) levels of theory, where HW* indicates that the Hay–Wadt effective core potential was used on the metals. On the basis of these experiments and calculations, the only conformation that reproduces the IRMPD action spectra for the complexes of the smaller alkali metal cations, Li +(Cys) and Na +(Cys), is a charge-solvated, tridentate structure where the metal cation binds to the amine and carbonyl groups of the amino acid backbone and the sulfur atom of the side chain, [N,CO,S], in agreement with the predicted ground states of these complexes. For the larger alkali metal cation complexes, K +(Cys), Rb +(Cys), and Cs +(Cys), the spectra have very similar spectral features that are considerably more complex than the IRMPD spectra of Li +(Cys) and Na +(Cys). For these complexes, the bidentate [COOH] conformer, in which the metal cation binds to both oxygens of the carboxylic acid group, is a dominant contributor, although features associated with the tridentate [N,CO,S] conformer remain and those for the zwitterionic [CO 2 −] conformer are also clearly present. Theoretical results for Rb +(Cys) and Cs +(Cys) indicate that both [COOH] and [N,CO,S] conformers are low-energy structures. For H +(Cys), the IRMPD action spectrum is reproduced by [N,CO] conformers, in which the protonated amine group hydrogen bonds to the carbonyl oxygen atom and the sulfur atom of the amino acid side chain. Several low-energy [N,CO] conformers that differ only in their side-chain orientations are found and therefore have very similar predicted spectra.