No abstract

In this communication we report on the conformational analysis of side-chains in the beta-sheet model system (Ac-Phe-OMe)(2) by applying IR/R2PI (infrared/resonant 2-photon ionisation) spectroscopy in the fingerprint region up to 10 microm, where in order to get information in this spectral region an extension of our new high energy and narrow band-width nanosecond IR laser system is presented.

An isolated beta-sheet model system is investigated in a molecular beam experiment by means of mass- and isomer-selective IR/R2PI double resonance spectroscopy as well as ab initio and DFT calculations. As the exclusive intermolecular assembly, a beta-sheet motif is formed by spontaneous dimerization of two isolated peptide molecules. This secondary structure is produced from the tripeptide model Ac-Val-Tyr(Me)-NHMe without any further environment to form the binding motif which is analyzed by both the characteristic amide A and I vibrations. The experimental and theoretical investigations yield the assignment to an antiparallel beta-sheet model. The result of this detailed spectroscopic analysis on an isolated beta-sheet model indicates that there are intrinsic properties of a beta-sheet structure which can be formed without a solvent or a peptidic environment.

In order to investigate the influence of hydration on the backbone of a peptide or protected amino acid, the successive aggregation of water to Ac-Phe-OMe is analysed by means of IR/UV double resonance spectroscopy. To achieve meaningful results the spectra have been recorded in the region of the amide A and OH stretching vibrations as well as the amide I/II modes. Comparison with ab initio and DFT calculations leads to size-selective structural assignments. Two isomers of the mono- and dihydrated clusters and one isomer of the trihydrated cluster are observed in the molecular beam leading to a formation of the first solvation shell of the backbone. In case of the trihydrated cluster the backbone geometry is remarkably changed compared to the structure of the monomer since a network of water molecules can be formed.