Spectroscopic signatures of gas-phase helices: Ac-Phe-(Ala)5-Lys-H+ and Ac-Phe-(Ala)10-Lys-H+

We apply gas-phase double resonance spectroscopic techniques to two helical peptides, Ac-Phe-(Ala)5-Lys-H+ and Ac-Phe- (Ala)10-Lys-H+, using electrospray ionization combined with photofragmentation in a cold, 22-pole ion trap. Despite the large size of the molecules, the electronic and infrared spectra are sharp, allowing us to identify transitions in the ultraviolet spectra associated with individual conformers, and to assign vibrations in the infrared spectra to local-mode N-H oscillators. We establish the spectral features in the infrared that are associated with the formation of gas-phase helices.