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

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Abstract
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 β-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 β-sheet model system. This is the first observation of such a model system in the gas phase.