The electronic spectra of small peptides in the gas phase

Abstract

The electronic spectra of the peptides Gly-Trp, Trp-Gly, and Trp-Gly-Gly have been observed in the gas phase. Solid samples of the peptide were introduced into a supersonic expansion by laser desorption, and both resonantly enhanced multiphoton ionization (MPI) spectra and laser-induced fluorescence spectra were observed. The MPI and fluorescence excitation spectra of these peptides had well resolved structure consisting, in part, of long, low-frequency vibrational progressions. In the case of Gly-Trp, emission spectroscopy was used to separate features in the MPI spectrum into contributions from two conformers of the molecule. The emission spectrum of one conformer contained only broad, red-shifted emission, while the emission spectrum of the second conformer contained both sharp and broad features. Although the MPI spectrum of the tripeptide Trp-Gly-Gly appeared to contain features from more than one conformer, all observed emission spectra from Trp-Gly-Gly were similar and contained only broad features. By analogy with the spectroscopy of tryptophan, the peptide spectra were interpreted in terms of mixing between two excited electronic states, one state arising from an intramolecular exciplex-like interaction.