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

Abstract

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.

Cold‐Ion Spectroscopy Reveals the Intrinsic Structure of a Decapeptide

“…[19] Cooling sample molecules to sufficiently low temperatures ( % 10 K) allows vibrational resolution in the UV and IR spectra of GS. [20] High resolution in the UV spectrum enables the use of IR/UV double-resonance detection [12,13,[21][22][23] for conformerselective measurements of IR spectra. We recently demonstrated application of this approach for spectroscopy of GS in the 6 mm region.…”

Conformation-Specific Spectroscopy of Peptide Fragment Ions in a Low-Temperature Ion Trap

“…Scanning an IR depletion laser generates selective vibrational spectra of only the labeled conformer [48][49][50][51]. By allowing selective and separate characterization of different fragment ion structures and conformations, such spectra provide the most stringent benchmarks to constrain theoretical calculations [52,53].…”

Characterizing the Intramolecular H-bond and Secondary Structure in Methylated GlyGlyH+ with H2 Predissociation Spectroscopy

“…This spectroscopic data is most often available through the integration of tunable infrared laser photodissociation with mass spectrometry and, in most cases, spectra of the mass-selected ions are obtained under ambient temperature conditions using infrared multiple photon dissociation (IRMPD) [28][29][30][31][32][33]. A recent variation on this scheme [34], yielding a resolution enhancement of 5 to 10 times that which is typically reported, involves cooling the ions close to 10 K [35][36][37] and "tagging" them with a weakly bound mass "messenger" using cryogenic ion trapping techniques [38][39][40]. The resulting spectra are obtained in a linear regime that is readily compared with calculated band patterns for ions frozen into their zero-point energy levels.…”