IRMPD Spectroscopic Study of Microsolvated [Na(GlyAla)]+ and [Ca(GlyAla–H)]+ and the Blue Shifting of the Hydrogen-Bonded Amide Stretch with Each Water Addition

Abstract

In this study, the structures of [Na(GlyAla)(H2O)](+) and [Ca(GlyAla-H)(H2O)n](+), (n = 1-3) solvated ion-molecule complexes (as well as the AlaGly isomers) were investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and with computational methods. Calculations showed that in the calcium clusters, the lowest-energy complex is the one in which the peptide is deprotonated at the carboxylic acid end and that Ca(2+) binds to both carboxylate oxygen atoms as well as the amide carbonyl oxygen. For the microsolvated structures, all three water molecules also bind directly to Ca(2+). For the singly, doubly, and triply solvated complexes, these structures are supported by experimental IRMPD spectra. For the [Na(GlyAla)(H2O)](+) complex, both carbonyl oxygen atoms, one from the intact carboxylic acid and one from the amide group, as well as the water molecule were found to be bound to the Na(+). In all of the spectra, a strong band is observed between 3300 and 3400 cm(-1) and is assigned to the amide N-H stretch, which is red-shifted due to hydrogen bonding with the amine nitrogen. The position of the hydrogen-bonded amide N-H stretch is experimentally and theoretically found to be sensitive to the number of water molecules; it is shown to blue shift upon successive hydrations.