Resonance-enhanced multiphoton ionization spectroscopy of dipeptides

We report resonance-enhanced multiphoton ionization (REMPI) spectroscopy of laser-desorbed, jet-cooled dipeptides, containing either tyrosine or phenylalanine as an aromatic chromophore (C). The single amino acids have multiple origins that we interpret as arising from two types of conformations, with the carboxyl group either anti or gauche with respect to the ring. Spectra for dipeptides of the form X-C, for example, Ala-Tyr, are similar to those of the corresponding single amino acid. On the other hand, spectra for dipeptides of the form C-X, for example, Tyr-Ala, show significant changes in the peaks, which we associate with gauche conformations. This observation can be understood in terms of an interaction between the carboxyl terminus and the ring, associated with the molecule assuming a folded conformation.