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

The gas phase structures of cationized histidine (His), including complexes with Li+, Na+, K+, Rb+, and Cs+, 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/6-311+G(d,p) (Li+, Na+, and K+ complexes) and B3LYP/HW*/6-311+G(d,p) (Rb+ and Cs+ complexes) levels of theory, where HW* indicates that the Hay-Wadt effective core potential with additional polarization functions was used on the metals. Single point energy calculations were carried out at the B3LYP, B3P86, and MP2(full) levels using the 6-311+G(2d,2p) basis set. 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+(His) and Na+(His), is a charge-solvated, tridentate structure where the metal cation binds to the backbone carbonyl oxygen, backbone amino nitrogen, and nitrogen atom of the imidazole side chain, [CO,Nα,N1], in agreement with the predicted ground states of these complexes. Spectra of the larger alkali metal cation complexes, K+(His), Rb+(His), and Cs+(His), have very similar spectral features that are considerably more complex than the IRMPD spectra of Li+(His) and Na+(His). For these complexes, the bidentate [CO,N1] conformer in which the metal cation binds to the backbone carbonyl oxygen and nitrogen atom of the imidazole side chain is a dominant contributor, although features associated with the tridentate [CO,Nα,N1] conformer remain, and those for the [COOH] conformer are also clearly present. Theoretical results for Rb+(His) and Cs+(His) indicate that both [CO,N1] and [COOH] conformers are low-energy structures, with different levels of theory predicting different ground conformers.