title: Infrared spectroscopy of arginine cation complexes: direct observation of gas-phase zwitterions.
abstract: The structures of cationized arginine complexes [Arg + M]+, (M = H, Li, Na, K, Rb, Cs, and Ag) and protonated arginine methyl ester [ArgOMe + H]+ have been investigated in the gas phase using calculations and infrared multiple-photon dissociation spectroscopy between 800 and 1900 cm-1 in a Fourier transform ion cyclotron resonance mass spectrometer. The structure of arginine in these complexes depends on the identity of the cation, adopting either a zwitterionic form (in salt-bridge complexes) or a non-zwitterionic form (in charge-solvated complexes). A diagnostic band above 1700 cm-1, assigned to the carbonyl stretch, is observed for [ArgOMe + H]+ and [Arg + M]+, (M = H, Li, and Ag), clearly indicating that Arg in these complexes is non-zwitterionic. In contrast, for the larger alkali-metal cations (K+, Rb+, and Cs+) the measured IR-action spectra indicate that arginine is a zwitterion in these complexes. The measured spectrum for [Arg + Na]+ indicates that it exists predominantly as a salt bridge with zwitterionic Arg; however, a small contribution from a second conformer (most likely a charge-solvated conformer) is also observed. While the silver cation lies between Li+ and Na+ in metal-ligand bond distance, it binds as strongly or even more strongly to oxygen-containing and nitrogen-containing ligands than the smaller Li+. The measured IR-action spectrum of [Arg + Ag]+ clearly indicates only the existence of non-zwitterionic Arg, demonstrating the importance of binding energy in conformational selection. The conformational landscapes of the Arg-cation species have been extensively investigated using a combination of conformational searching and electronic structure theory calculations [MP2/6-311++G(2d,2p)//B3LYP/6-31+G(d,p)]. Computed conformations indicate that Ag+ is di-coordinated to Arg, with the Ag+ chelated by both the N-terminal nitrogen and Neta of the side chain but lacks the strong M+-carbonyl oxygen interaction that is present in the tri-coordinate Li+ and Na+ charge-solvation complexes. Experiment and theory show good agreement; for each ion species investigated, the global-minimum conformer provides a very good match to the measured IR-action spectrum.
authors.affiliation: Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada.
authors.forename: Matthew W
authors.initials: MW
authors.lastname: Forbes
authors.forename: Matthew F
authors.initials: MF
authors.lastname: Bush
authors.forename: Nick C
authors.initials: NC
authors.lastname: Polfer
authors.forename: Jos
authors.initials: J
authors.lastname: Oomens
authors.forename: Robert C
authors.initials: RC
authors.lastname: Dunbar
authors.forename: Evan R
authors.initials: ER
authors.lastname: Williams
authors.forename: Rebecca A
authors.initials: RA
authors.lastname: Jockusch
mesh_terms: D001120:Arginine / Q000031:analogs & derivatives* / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D005740:Gases / Q000737:chemistry*; D008670:Metals / Q000737:chemistry*; D015394:Molecular Structure; D012996:Solutions; D017550:Spectroscopy, Fourier Transform Infrared
chemical_list: D002412:Cations; D005740:Gases; D008670:Metals; D012996:Solutions; C026512:arginine methyl ester; D001120:Arginine