Structure of Protonated Threonine Dimers in the Gas Phase: Salt-Bridged or Charge-Solvated?

For homodimers of amino acids, their salt-bridged structures are gradually stabilized as the proton affinity of the component amino acid increases. Threonine has a proton affinity value located in the middle of the list of 20 natural amino acids. Thus, identifying whether the most stable isomer of protonated threonine dimer (Thr(2)H(+)) has a charge-solvated or salt-bridged structure is important and helpful for understanding the structures of other homodimers. By combining infrared photodissociation (IRPD) spectroscopy and theoretical calculations, the structures of Thr(2)H(+) were investigated. Based on calculations at the M062X/6-311++G(d,p)//M062X/6-311++G(d,p) level, the most stable isomer of Thr(2)H(+) was computed to be a charge-solvated structure, with an energy 3.87 kcal/mol lower than the most stable salt-bridged isomer. The predicted infrared spectrum is in good agreement with the experimental spectrum. To evaluate the temperature effect on the distribution of different isomers, the relative concentrations of the six isomers of Thr(2)H(+) were calculated at different temperatures, according to their partition functions and enthalpies. The results show that the isomers are dominated by charge-solvated structures at a temperature of 300 K.