Infrared multiphoton dissociation spectroscopy of cationized threonine: effects of alkali-metal cation size on gas-phase conformation.

The gas-phase structures of alkali-metal cation complexes of threonine (Thr) are examined using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser in conjunction with quantum chemical calculations. Spectra of Li+(Thr) and Na+(Thr) are similar and relatively simple, whereas K+(Thr), Rb+(Thr), and Cs+(Thr) include distinctive new IR bands. Measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311+G(d,p) level to identify the structures present in the experimental studies. For the smaller metal cations, the spectra match those predicted for charge-solvated structures in which the ligand exhibits tridentate coordination, M1[N,CO,OH], binding to the amide and carbonyl groups of the amino acid backbone and to the hydroxyl group of the side chain. K+(Thr), Rb+(Thr), and Cs+(Thr) exhibit evidence of the charge-solvated complex, M3[COOH], in which the metal cation binds to the carboxylic acid group. Evidence for a small population of the zwitterionic analogue of this structure, ZW[CO2-], is also present, particularly for the Cs+ complex. Calculations indicate that the relative stability of the M3[COOH] structure is very strongly dependent on the size of the metal cation, consistent with the range of conformations observed experimentally. The present results are similar to those obtained previously for the analogous M+(Ser) complexes, although there are subtle distinctions that are discussed.