β-sheet model systems in the gas phase:: Structures and vibrations of Ac-Phe-NHMe and its dimer (Ac-Phe-NHMe)2

A combined experimental and theoretical study on the structure of the dipeptide model Ac-Phe-NHMe and its dimer (Ac-Phe-NHMe)2 is presented. In order to get a detailed vibrational analysis of all functional groups which are relevant to analyse the different structural arrangements, IR/R2PI spectra are recorded in the regions of the NH and the C=O stretching vibrations. Force field calculations are used to scan the complex conformational landscape both of Ac-Phe-NHMe and the dimer (Ac-Phe-NHMe)(2). Subsequent ab initio and DFT calculations on the most stable structures lead to predictions of the cluster geometries and their vibrational frequencies. Three isomers of the Ac-Phe-NHMe monomer have been assigned which contain either a beta-sheet related configuration or hydrogen-bonded structures. The most prominent species has a beta-sheet related conformation. The observed dimer contains a doubly hydrogen-bonded arrangement and turns out to be a beta-sheet model system. In contrast to the beta-sheet model (Ac-Phe-OMe)(2) a different structural arrangement is found, connecting "the outer" C=O and NH groups.