Highly Resolved Spectra of Gas-Phase Gramicidin S: A Benchmark for Peptide Structure Calculations

We have measured a vibrationally resolved UV spectrum of doubly protonated gramicidin S (GS) in the gas phase and, subsequently, a highly resolved, conformer-specific IR spectrum in the 6 mu m fingerprint region, using a cold ion trap in combination with tabletop lasers. The study has revealted at least three conformational states of GS populated under our experimental conditons, with the major one showing evidence of a symmetric three-dimensional structure similar to that in the condensed phase. The derived qualitative constraints, along with the measured vibrational frequencies, serve as a benchmark for compuations of peptide structure.