Spectroscopy and conformational preferences of gas-phase helices.

We describe here a study of the spectroscopy of two peptides that we expect to be helical, Ac-Phe-(Ala)(5)-Lys-H(+) and Ac-Phe-(Ala)(10)-Lys-H(+), and one that we expect to be globular, Ac-Lys(H(+))-Phe-(Ala)(10), with the goal of identifying the spectral features characteristic of their secondary structure. Conformation-specific IR-UV double resonance spectroscopy in a cold ion trap, together with nitrogen-15 isotopic substitution, allow us to identify four conformers of the smaller helix. Infrared spectra in the OH and amide NH stretch regions, together with theoretical calculations, provide diagnostics of the presence of helical structure as well as details of the specific hydrogen bonding patterns within the helix. The assigned vibrational spectra presented here provide a benchmark for the ability of theory to predict the spectrum of a helical peptide.