Structures of Ac–Trp–OMe and its dimer (Ac–Trp–OMe)2 in the gas phase: influence of a polar group in the side-chain

Abstract

In order to determine the quality of different chromophores to form β-sheet model systems, this paper reports on the structure of the isolated amino acid Ac–Trp–OMe and its dimer. The amino acid is protected at the terminal positions by introducing an acetyl and an O-methyl group. Only one isomer with a linear β-sheet related structure is obtained for the monomer. In contrast to similar systems containing the Phe chromophore, no β-sheet model system is obtained for the dimer. Due to the presence of polar NH groups in the side-chain of the indole moiety the minimum energy structure of the dimer contains hydrogen bonds between the NH groups of indole and C=O groups of the backbone. The structures are derived from the frequencies obtained from IR/R2PI spectra applied both to the region of the N–H and C=O stretching vibrations and a theoretical approach including force field and ab initio calculations. The force field calculations are used to explore the various possibilities on the potential energy surfaces of monomer and dimer structures. The combination with ab initio and DFT calculations yields an assignment of the resulting structures.