title: Experimental and theoretical investigation of the proton-bound dimer of lysine.
abstract: The structure of the proton-bound lysine dimer has been investigated by infrared multiple photon dissociation (IRMPD) spectroscopy and electronic structure calculations. The structures of different possible isomers of the proton-bound lysine dimer have been optimized at the B3LYP/6-31 + G(d) level of theory and IR spectra calculated using the same computational method. Based on relative Gibbs free energies (298 K) calculated at the MP2/aug-cc-pVTZ//B3LYP/6-31 + G(d) level of theory, LL-CS01, and followed closely (1.1 kJ mol(-1)) by LL-CS02 are the most stable non-zwitterionic isomers. At the MP2/aug-cc-pVTZ//6-31 + G(d) and MP2/aug-cc-pVTZ//6-31 + (d,p) levels of theory, isomer LL-CS02 is favored by 3.0 and 2.3 kJ mol(-1), respectively. The relative Gibbs free energies calculated by the aforementioned levels of theory for LL-CS01 and LL-CS02 are very close and strongly suggest that diagnostic vibrational signatures found in the IRMPD spectrum of the proton-bound dimer of lysine can be attributed to the existence of both isomers. LL-ZW01 is the most stable zwitterionic isomer, in which the zwitterionic structure of the neutral lysine is well stabilized by the protonated lysine moiety via a very strong intermolecular hydrogen bond. At the MP2/aug-cc-pVTZ//B3LYP/6-31 + G(d), MP2/aug-cc-pVTZ//6-31 + G(d) and MP2/aug-cc-pVTZ//6-31 + G(d,p) levels of theory, the most stable zwitterionic isomer (LL-ZW01) is less favored than LL-CS01 by 7.3, 4.1 and 2.3 kJ mol(-1), respectively. The experimental IRMPD spectrum also confirms that the proton-bound dimer of lysine largely exists as charge-solvated isomers. Investigation of zwitterionic and charge-solvated species of amino acids in the gas phase will aid in a further understanding of structure, property, and function of biological molecules.
authors.affiliation: Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1.
authors.forename: Ronghu
authors.initials: R
authors.lastname: Wu
authors.forename: Richard A
authors.initials: RA
authors.lastname: Marta
authors.forename: Jonathan K
authors.initials: JK
authors.lastname: Martens
authors.forename: Kris R
authors.initials: KR
authors.lastname: Eldridge
authors.forename: Terry B
authors.initials: TB
authors.lastname: McMahon
mesh_terms: D004151:Dipeptides / Q000737:chemistry*; D013058:Mass Spectrometry / Q000379:methods*; D011522:Protons; D013055:Spectrophotometry, Infrared / Q000379:methods*; D013816:Thermodynamics; D014732:Vibration
chemical_list: D004151:Dipeptides; D011522:Protons; C024575:lysyllysine