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

Abstract

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 CO 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 β-sheet related configuration or hydrogen-bonded structures. The most prominent species has a β-sheet related conformation. The observed dimer contains a doubly hydrogen-bonded arrangement and turns out to be a β-sheet model system. In contrast to the β-sheet model (Ac–Phe–OMe)₂ a different structural arrangement is found, connecting “the outer” CO and NH groups.