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

► Tri-peptide, Z-Pro-Leu-Gly-NH2 and Z-Pro-Leu-Gly-OH are investigated in gas phase. ► IR dip spectroscopy and quantum chemical calculations are employed. ► IR dip spectra were obtained in the N-H and O-H stretching frequency region. ► Impact of NH2 capping on overall structure is determined. ► C-terminal OH group remains free whereas NH2 group participates in hydrogen bonding.