title: Infrared spectroscopy of cationized lysine and epsilon-N-methyllysine in the gas phase: effects of alkali-metal ion size and proton affinity on zwitterion stability.
abstract: The gas-phase structures of protonated and alkali-metal-cationized lysine (Lys) and epsilon-N-methyllysine (Lys(Me)) are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser, in conjunction with ab initio calculations. IRMPD spectra of Lys.Li(+) and Lys.Na(+) are similar, but the spectrum for Lys.K(+) is different, indicating that the structure of lysine in these complexes depends on the metal ion size. The carbonyl stretch of a carboxylic acid group is clearly observed in each of these spectra, indicating that lysine is nonzwitterionic in these complexes. A detailed comparison of these spectra to those calculated for candidate low-energy structures indicates that the bonding motif for the metal ion changes from tricoordinated for Li and Na to dicoordinated for K, clearly revealing the increased importance of hydrogen-bonding relative to metal ion solvation with increasing metal ion size. Spectra for Lys(Me).M(+) show that Lys(Me), an analogue of lysine whose side chain contains a secondary amine, is nonzwitterionic with Li and zwitterionic with K and both forms are present for Na. The proton affinity of Lys(Me) is 16 kJ/mol higher than that of Lys; the higher proton affinity of a secondary amine can result in its preferential protonation and stabilization of the zwitterionic form.
authors.affiliation: Department of Chemistry, University of California, Berkeley, California 94720-1460, USA.
authors.forename: Matthew F
authors.initials: MF
authors.lastname: Bush
authors.forename: Matthew W
authors.initials: MW
authors.lastname: Forbes
authors.forename: Rebecca A
authors.initials: RA
authors.lastname: Jockusch
authors.forename: Jos
authors.initials: J
authors.lastname: Oomens
authors.forename: Nick C
authors.initials: NC
authors.lastname: Polfer
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Saykally
authors.forename: Evan R
authors.initials: ER
authors.lastname: Williams
mesh_terms: D002412:Cations; D005740:Gases / Q000737:chemistry*; D008239:Lysine / Q000031:analogs & derivatives* / Q000737:chemistry*; D008672:Metals, Alkali; D013055:Spectrophotometry, Infrared
chemical_list: D002412:Cations; D005740:Gases; D008672:Metals, Alkali; C005205:epsilon-N-methyllysine; D008239:Lysine