Structure of protonated tryptophan dimer in the gas phase investigated by IRPD spectroscopy and theoretical calculations.

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 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.

METHODS: 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.

RESULTS: 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.

CONCLUSIONS: 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.