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.

A first-principles investigation of histidine and its ionic counterparts

“…Amino acids exist in the neutral, non-zwitterionic form in gas phase, while in solvent phase they generally exist as zwitterions. However, reports do indicate the presence of zwitterions of basic amino acids such as arginine and His even in gas phase [42,69]. Hence, the search for the zwitterionic form of His was carried out in both the gas and solvent phase at the B3LYP/6-31G(d) level of theory.…”

IRMPD spectroscopy of metalated flavins: structure and bonding of Mq+–lumichrome complexes (Mq+ = Li+–Cs+, Ag+, Mg2+)

“…This combined strategy has proven to be a powerful tool to characterize metal ion complexes of a large variety of (bio-)molecules. [21][22][23][24][25][26][27][28][29][30][31][32][33] Similar to protonated flavins, 18 the CO stretch modes serve as the main indicator of the metal binding sites. The comparison of the measured IRMPD spectra with the calculated linear absorption spectra provides insight into their respective structures and bonding mechanisms.…”

Characterizing the Intramolecular H-bond and Secondary Structure in Methylated GlyGlyH+ with H2 Predissociation Spectroscopy

“…Vibrational spectroscopy is emerging as a powerful means with which to elucidate such structures by revealing peptide conformations and protonation sites through theoretical analysis of the observed band patterns [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. This spectroscopic data is most often available through the integration of tunable infrared laser photodissociation with mass spectrometry and, in most cases, spectra of the mass-selected ions are obtained under ambient temperature conditions using infrared multiple photon dissociation (IRMPD) [28][29][30][31][32][33]. A recent variation on this scheme [34], yielding a resolution enhancement of 5 to 10 times that which is typically reported, involves cooling the ions close to 10 K [35][36][37] and "tagging" them with a weakly bound mass "messenger" using cryogenic ion trapping techniques [38][39][40].…”