Holger Fricke

Investigation of Secondary Structure Elements by IR/UV Double Resonance Spectroscopy : Analysis of an Isolated β-Sheet Model System

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.

Structure of the tripeptide model Ac–Val–Tyr(Me)–NHMe and its cluster with water investigated by IR/UV double resonance spectroscopy

In this paper the structures of the isolated tripeptide model Ac–Val–Tyr(Me)–NHMe (Val = valine, Tyr = tyrosine) and its cluster with water are investigated by mass-, isomer- and state-selective IR/UV double resonance spectroscopy. From the IR spectra both in the region of the NH and CO stretching vibrations and in combination with force field and ab initio calculations it can unambiguously be derived that the peptide contains a stretched, β-sheet related structure. Thus the peptide serves as an ideal candidate for β-sheet model systems. By adding one water molecule to the peptide the β-sheet related structure seems to be preserved with a water molecule being attached to the NHMe group.

Structure of a β-sheet model system in the gas phase: Analysis of the fingerprint region up to 10 μm

In this communication we report on the conformational analysis of side-chains in the β-sheet model system (Ac-Phe-OMe)2 by applying IR/R2PI (infrared/resonant 2-photon ionisation) spectroscopy in the fingerprint region up to 10 μm, 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.

IR spectroscopy applied subsequent to a proton transfer reaction in the excited state of isolated 3-hydroxyflavone and 2-(2-naphthyl)-3-hydroxychromone

IR/R2PI-spectroscopy has been applied to the electronic ground and electronically excited states of 3-hydroxyflavone (3-HF) and 2-(2-naphthyl)-3-hydroxychromone (2-NHC) in a supersonic jet yielding direct structural information on the educt and product of a proton transfer reaction. We show that IR spectra of the electronically excited states can be recorded subsequent to a photoinduced chemical reaction, in this case a proton transfer. In combination with DFT and TDDFT calculations structural assignments are performed.

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

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.

Isolated β-Turn Model Systems Investigated by Combined IR/UV Spectroscopy

The functionality of bioactive molecules sensitively depends on their structure. For the investigation of intrinsic structural properties, molecular beam experiments combined with laser spectroscopy have proven to be a suitable tool. Herein we present an analysis of the two isolated tripeptide model systems Ac-Phe-Tyr(Me)-NHMe and Boc-Phe-Tyr(Me)-NHMe. For this purpose, mass-selective combined IR/UV spectroscopy is applied to both substances in a molecular beam experiment. The comparison of the experimental data with DFT calculations, including different functionals as well as dispersion corrections, allows an assignment of both tripeptide models to β-turns formed independently from the protection groups and supported by the interaction of the two aromatic chromophores.

Proton/hydrogen-transfer coordinate of 2,5-dihydroxybenzoic acid investigated in a supersonic beam: combined IR/UV spectroscopy in the S0, S1, and D0 states.

As a model system for intramolecular proton/hydrogen-transfer coordinates, the structure of 2,5-dihydroxybenzoic acid is investigated for the ground, first electronically excited and also the ionic state. Combined IR/UV spectroscopy in molecular-beam experiments is applied and the experimental results are interpreted by the application of DFT and CASPT2 methods. No proton or hydrogen transfer is observed, but evidence is given for a hydrogen dislocation of the intramolecular hydrogen bond in the S(1) state and to lesser extent in the D(0) state. To obtain direct information on the proton/hydrogen-transfer coordinate, IR spectra are recorded both in the region of the OH and especially the CO stretching vibrations by also applying two new variants of combined IR/UV spectroscopy for the S(1) and D(0) states. The CO groups are directly involved in the hydrogen bond and, in contrast to the hydrogen-bonded OH groups, the CO stretching frequencies can be observed in all electronic states.