Conformation-Specific Spectroscopy of Asparagine-Containing Peptides: Influence of Single and Adjacent Asn Residues on Inherent Conformational Preferences

The infrared and ultraviolet spectra of a series of capped asparagine-containing peptides, Ac-Asn-NHBn, Ac-Ala-Asn-NHBn, and Ac-Asn-Asn-NHBn, have been recorded under jet-cooled conditions in the gas phase in order to probe the influence of the Asn residue, with its −CH2–C(═O)–NH2 side chain, on the local conformational preferences of a peptide backbone. The double-resonance methods of resonant ion-dip infrared (RIDIR) spectroscopy and infrared–ultraviolet hole-burning (IR–UV HB) spectroscopy were used to record single-conformation spectra in the infrared and ultraviolet, respectively, free from interference from other conformations present in the molecular beam. Ac-Asn-NHBn spreads its population over two conformations, both of which are stabilized by a pair of H-bonds that form a bridge between the Asn carboxamide group and the NH and C═O groups on the peptide backbone. In one the peptide backbone engages in a 7-membered H-bonded ring (labeled C7eq), thereby forming an inverse γ-turn, stabilized by a C6...