Infrared Multiphoton Dissociation Spectroscopy of Cationized Serine:  Effects of Alkali-Metal Cation Size on Gas-Phase Conformation.

The gas-phase structures of alkali-metal cation complexes of serine (Ser) are examined using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser, in conjunction with ab initio calculations. Spectra of Li(Ser) and Na(Ser) are similar and relatively simple, whereas Cs(Ser) includes distinctive new IR bands, and K(Ser) and Rb(Ser) exhibit intermediate behavior. Measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311G(d,p) level to identify the structures present in the experimental studies. On the basis of these experiments and calculations, the only conformations accessed for the complexes to the smaller alkali-metal cations, Liand Na, are charge-solvated structures involving tridentate coordination to the amine and carbonyl groups of the amino acid backbone and to the hydroxyl group of the side chain, M1N,CO,OH. For the cesiated complex, a band corresponding to a zwitterionic structure, ZWCO2-, is clearly visible. K(Ser) and Rb(Ser) exhibit evidence of the charge-solvated analogue of the zwitterions, M3COOH, in which the metal cation binds to the carboxylic acid group. Calculations indicate that the relative stability of the M3COOH structure is very strongly dependent on the size of the metal cation, consistent with the range of conformations observed experimentally. [ABSTRACT FROM AUTHOR]