Probing secondary structures of end-capped tripeptide, BOC–Phe–Aib–Leu–OMe by combined study of infrared spectroscopy and quantum chemistry calculation

Abstract

We report conformational analysis of an end-protected tripeptide, BOC–Phe–Aib–Leu–OMe in CCl₄ solution and solid crystal at room temperature by infrared spectroscopy and quantum chemistry calculation. The measured spectra in those physical conditions appear different, which has been attributed to different secondary structures of the peptide in the two phases. Our analysis reveals that the peptide has only one secondary structure in the crystal, but in the solution there are two different secondary structures. While the spectrum in solution shows excellent correspondence with the superposition of predicted spectra of the two most favored gas-phase secondary structures, the spectrum of crystal shows best correspondence with the predicted spectrum of a relatively higher energy (∼6 kcal/mol) secondary structure. However, the latter in crystal can gain maximum stability by intermolecular hydrogen bonding. The secondary structure of the peptide in crystal that we have deduced agrees nicely with the one suggested by X-ray crystallographic study.