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 mum fingerprint region, using a cold ion trap in combination with table-top lasers. The study has revealed at least three conformational states of GS populated under our experimental conditions, 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 computations of peptide structure.