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Article

Infrared Spectroscopy of Arginine Cation Complexes: Direct Observation of Gas-Phase Zwitterions

Supporting Info

Matthew W. Forbes,^† Matthew F. Bush,^‡ Nick C. Polfer,^§ Jos Oomens,^§ Robert C. Dunbar, Evan R. Williams,^‡ and Rebecca A. Jockusch*^†

Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada, Department of Chemistry, University 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 Reserve University, Cleveland, Ohio 44106

J. Phys. Chem. A, 2007, 111 (46), pp 11759–11770

DOI: 10.1021/jp074859f

Publication Date (Web): November 1, 2007

†

University of Toronto.

‡

University of California, Berkeley.

FOM Institute for Plasma Physics “Rijnhuizen”.

Present address: Department of Chemistry, University of Florida, Gainesville, Florida 32611.

Case Western Reserve University.

Author to whom correspondence should be addressed. E-mail: rebecca. jockusch@utoronto.ca.

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 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.