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

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 Ca²⁺ 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 Ca²⁺. 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.