title: Infrared multiple photon dissociation spectroscopy of cationized histidine: effects of metal cation size on gas-phase conformation.
abstract: 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,N(α),N(1)], 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,N(1)] 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,N(α),N(1)] conformer remain, and those for the [COOH] conformer are also clearly present. Theoretical results for Rb(+)(His) and Cs(+)(His) indicate that both [CO,N(1)] and [COOH] conformers are low-energy structures, with different levels of theory predicting different ground conformers.
authors.affiliation: Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, United States.
authors.forename: Murat
authors.initials: M
authors.lastname: Citir
authors.forename: Christopher S
authors.initials: CS
authors.lastname: Hinton
authors.forename: Jos
authors.initials: J
authors.lastname: Oomens
authors.forename: Jeffrey D
authors.initials: JD
authors.lastname: Steill
authors.forename: P B
authors.initials: PB
authors.lastname: Armentrout
mesh_terms: D005740:Gases / Q000737:chemistry*; D006639:Histidine / Q000737:chemistry*; D008670:Metals / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D017785:Photons; D013055:Spectrophotometry, Infrared / Q000379:methods*
chemical_list: D005740:Gases; D008670:Metals; D006639:Histidine