Clusters of a protected amino acid with pyrazole derivatives:: β-sheet model systems in the gas phase

In this paper we report on the structure of isolated clusters formed by the protected amino acid Ac-Phe-OMe and pyrazole or 5-methyl-3-aminopyrazole (MAP). Ac-Phe-OMe contains phenylalanine (Phe) which is protected at the terminal positions by introducing an acetyl and a methyl group. From our previous gas phase investigations it is known that the conformation of Ac-Phe-OMe is similar to the one of an amino acid in a beta-sheet structure. Experimental studies in the liquid phase show that pyrazole derivatives are suitable agents to form very stable aggregates with beta-sheet model peptides in order to prevent an aggregation with other peptide molecules. The structures of the Ac-Phe-OMe/(pyrazole, MAP) clusters are investigated by applying Resonant 2-Photon Ionization (R2PI) and infrared/R2PI spectroscopy as well as Hartree-Fock and DFT calculations. Different structural arrangements of the clusters. are discussed. In the case of MAP at least two different isomers are observed experimentally.