24 - J. Phys. Chem. A 2007, 111, 11759-11770 11759 Infrared...

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Infrared Spectroscopy of Arginine Cation Complexes: Direct Observation of Gas-Phase Zwitterions Matthew W. Forbes, ² Matthew F. Bush, Nick C. Polfer, §, Jos Oomens, § Robert C. Dunbar, | Evan R. Williams, and Rebecca A. Jockusch* Department of Chemistry, Uni V ersity of Toronto, Toronto, Ontario M5S 3H6, Canada, Department of Chemistry, Uni V ersity of California at Berkeley, Berkeley, California 94720-1460, FOM Institute for Plasma Physics “Rijnhuizen”, Edisonbaan 14, 3439 MN Nieuwegein, The Netherlands, and Chemistry Department, Case Western Reser V e Uni V ersity, Cle V eland, Ohio 44106 Recei V ed: June 21, 2007; In Final Form: August 14, 2007 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] + ) 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 N η 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.
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24 - J. Phys. Chem. A 2007, 111, 11759-11770 11759 Infrared...

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