Structures of the protected amino acid Ac-Phe-OMe and its dimer:: A β-sheet model system in the gas phase

We report on the structure of phenylalanine which is protected at the terminal positions by introducing an acetyl and a methyl group (Ac-Phe-OMe). The structures of Ac-Phe-OMe and its dimer are investigated by applying resonant 2-photon ionization (R2PI) and IR/R2PI spectroscopy as well as Hartree-Fock and density functional theory (DFT) calculations. In contrast to the R2PI spectrum of unprotected phenylalanine the spectrum of Ac-Phe-OMe exhibits only one prominent isomer. The geometry of this isomer can be correlated with a beta-sheet related structure. In the case of (Ac-Phe-OMe)(2) only hydrogen-bonded NH groups are observed, indicating a dimer which can easily be formed by two monomers building a beta-sheet model system. This is the first observation of such a model system in the gas phase.