Combined IR/UV spectroscopy on peptides, ß-sheet model systems and cyclopeptides
Highly selective IR/UV and UV/UV multiple resonance methods permit - in combination with mass spectrometry in a molecular beam experiment - the mass-, isomer-, and vibrational-selective analysis of microscopic processes. The investigation of model systems represents the basis for the understanding of structure and reactivity of larger aggregates (biomolecular aggregates, nano-particles, solutions).
A central research topic is the application of different IR/UV double resonance techniques for the investigation of intermolecular hydrogen bonds in peptides with different secondary structures, mainly β-sheet arrangements. On the one hand the driving forces to form a β-sheet as well as the structural changes due to successive peptide or/and solvent aggregation (microsolvation) is discussed. In cooperation with the group of Prof. Schrader (organic chemistry, University Duisburg-Essen) ideal template molecules should be found which aggregate with peptides like in an extended β-sheet. The cluster formation with the template molecules could e.g. inhibit the progressive generation of pathogenic β-sheet structures. This mechanism is discussed for the Creutzfeldt-Jakob disease or BSE (prion diseases). Pathogenic b-sheet arrangements are also discussed for the Alzheimer disease.
Figure 1: Substructures of a protein are analyzed selectively with the focus on relevant protein parts. Scheme: part of a β-sheet.
The application of mass-, isomer-, and state-selective IR/UV and UV/UV double resonance spectroscopy can lead to information both on the strength of intermolecular hydrogen bonds and on the geometry of the peptides by the frequencies of the NH, CO, and CH stretching vibrations. For the investigation of intermolecular interactions the gas-phase measurements are particularly suitable because the isolated model systems can be analyzed with respect to their mass, isomerization and vibrations without additional bands of the solvent. The detailed knowledge of bond strengths and geometry, which is achieved by our measurements, shall assist the synthesis of new template molecules.
Figure 2: Schematic structure of a molecular beam apparatus consisting of a linear time-of-flight mass spectrometer as well as two UV lasers and one or two IR laser systems for spectroscopy. The molecules and clusters which shall be examined are expanded into the vacuum by a pulsed nozzle, whereby noble gases serve as carrier gas. Resonant ionization of the examined species takes place by the UV lasers, and the (singly charged) ions are size-selected by the linear mass spectrometer. The signals received on the MCP detector are obtained time-resolved on a digital oscilloscope and registered by means of a PC. The OH, NH or CH and also CO stretching vibrations of the molecules can be excited by an IR laser. The excitation of the vibrations in the neutral or ionic ground or excited states is determined by the de- or increase of ion signals (for further details see list of publications and references in the publications). With respect to the use of a second IR laser, see section about electronially excited states.