title: Structure of protonated tryptophan dimer in the gas phase investigated by IRPD spectroscopy and theoretical calculations.
abstract: In addition to proton affinity (PA), side chains may also have an effect on the structures of amino acid complex ions in the gas phase. A previous study showed that the most stable isomer of Pro2 H(+) favored a salt-bridged structure. Tryptophan has a PA close to that of proline, but a quite different side chain. Thus, it will be interesting to discover whether the salt-bridged or charge-solvated conformation is energetically more favorable for Trp2 H(+) in the gas phase.
  
  
  
  The infrared photodissociation (IRPD) spectrum of Trp2 H(+) was obtained using a Fourier transform ion cyclotron resonance mass spectrometer equipped with a tunable OPO laser. The non-covalent cluster ions were generated by electrospray ionization. Structural optimization and frequency calculation of the selected isomers were performed at the M062X/6-311++G(d,p) level.
  
  
  
  The experimental IRPD spectrum of Trp2 H(+) was reported in the region of 2700-3750 cm(-1) . Theoretical calculations show that the most stable isomer has a charge-solvated structure. Its energy was found to be 9 kcal/mol lower than that of the most stable salt-bridged isomer. The experimental spectrum is consistent with the predicted spectra of the most stable charge-solvated structures. Temperature effect on the stability of isomers was also evaluated and it was revealed that the contribution from salt-bridged isomers can be neglected at a temperature of 300 K.
  
  
  
  Combining the method of IRPD spectroscopy with theoretical calculations, the structures of Trp2 H(+) were investigated. It is shown that the structures of Trp2 H(+) are dominated by charge-solvated forms. The results also indicate that the side chain may considerably affect the stability of the zwitterionic forms. Copyright © 2016 John Wiley & Sons, Ltd.
authors.affiliation: State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin, 300071, China.
authors.forename: Ruxia
authors.initials: R
authors.lastname: Feng
authors.affiliation: State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin, 300071, China.
authors.forename: Hong
authors.initials: H
authors.lastname: Yin
authors.affiliation: State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin, 300071, China.|Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin, 300071, China.
authors.forename: Xianglei
authors.initials: X
authors.lastname: Kong
mesh_terms: D019281:Dimerization; D005740:Gases; D007536:Isomerism; D011522:Protons; D013055:Spectrophotometry, Infrared / Q000379:methods*; D013696:Temperature; D014364:Tryptophan / Q000737:chemistry*
chemical_list: D005740:Gases; D011522:Protons; D014364:Tryptophan