Premium
Structure of protonated tryptophan dimer in the gas phase investigated by IRPD spectroscopy and theoretical calculations
Author(s) -
Feng Ruxia,
Yin Hong,
Kong Xianglei
Publication year - 2016
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.7615
Subject(s) - chemistry , protonation , ion , dimer , mass spectrometry , proton affinity , spectroscopy , electrospray ionization , fourier transform ion cyclotron resonance , analytical chemistry (journal) , crystallography , computational chemistry , organic chemistry , physics , chromatography , quantum mechanics
Rational 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 Pro 2 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 Trp 2 H + in the gas phase. Methods The infrared photodissociation (IRPD) spectrum of Trp 2 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. Results The experimental IRPD spectrum of Trp 2 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. Conclusions Combining the method of IRPD spectroscopy with theoretical calculations, the structures of Trp 2 H + were investigated. It is shown that the structures of Trp 2 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.