Accessing the Vibrational Signatures of Amino Acid Ions Embedded in Water Clusters

We present an infrared predissociation (IRPD) study of microsolvated GlyH⁺(H₂O) _n and GlyH⁺(D₂O) _n clusters, formed inside of a cryogenic ion trap via condensation of H₂O or D₂O onto the protonated glycine ions. The resulting IRPD spectra, showing characteristic O-H and O-D stretches, indicate that H/D exchange reactions are quenched when the ion trap is held at 80 K, minimizing the presence of isotopomers. Comparisons of GlyH⁺(H₂O) _n and GlyH⁺(D₂O) _n spectra clearly highlight and distinguish the vibrational signatures of the water solvent molecules from those of the core GlyH⁺ ion, allowing for quick assessment of solvation structures. Without the aid of calculations, we can already infer solvation motifs and the presence of multiple conformations. The use of a cryogenic ion trap to cluster solvent molecules around ions of interest and control H/D exchange reactions is broadly applicable and should be extendable to studies of more complex peptidic ions in large solvated clusters.