Gas phase IR spectra of tri-peptide Z-Pro-Leu-Gly: Effect of C-terminal amide capping on secondary structure

Three-residue peptides capped with benzyloxycarbonyl (Z-) group, Z-Pro-Leu-Gly–NH2 and Z-Pro-Leu-Gly–OH, are investigated by infrared (IR) spectroscopy, using supersonic-jet laser desorption technique, in the N–H and O–H stretching frequency ranges. The IR spectra show clear evidence of the formation of different hydrogen-bonding network in the two peptides. The possible gas phase structure is proposed from density functional theory calculations using cc-pVDZ basis set. The Z-Pro-Leu-Gly–OH in the gas phase forms successive γ-turn structure with free C-terminal carboxyl group whereas main structural element in Z-Pro-Leu-Gly–NH2 is β-turn with C-terminal NH2 group forming hydrogen bond. Structural information is employed to predict their binding capability in gas phase.