Impact of the metal ion and microsolvation on the structure and vibrations in a small model peptide

The impact of alkali metal ions (Li⁺, Na⁺, and K⁺) on the structure and vibrations in metal ion-diglycine complexes with up to two water molecules (M⁺GlyGly-nH₂O) is examined using cryogenic infrared action spectroscopy, conformer-specific IR-IR spectroscopy, H₂O/D₂O isotopic substitution experiments, and scaled, harmonic DFT calculations. While the main conformer is identical for all metal ions for the bare complex (M⁺GlyGly), the conformers observed upon the addition of water are dependent on the nature of the metal ion and the impact of solvation is distinctly larger for the Li⁺ than for the K⁺ ion. The number of solvent molecules needed to change the initial peptide conformation differs between the metal ions and is smaller for Li⁺ than for the larger metal ions. The comparison of the spectra upon the sequential addition of water reveals how solvation impacts the electric field strength of the metal ions in these clusters. In all cases, the addition of water molecules reduces the strength of the interaction between the metal ion and the peptide and consequently, reduces the strength of the electric field induced in the peptide.