A. Gerlach

Structure of a β-sheet model system in the gas phase: Analysis of the CO stretching vibrations

In this communication the first IR/R2PI (infrared/resonant two-photon ionisation) spectra of CO stretching vibrations are presented applied to a β-sheet model system. CO vibrations are extremely important to describe the structure of isolated peptides and especially the hydrogen bonding in β-sheet related structures in the gas phase.

β-sheet model systems in the gas phase: Structures and vibrations of Ac–Phe–NHMe and its dimer (Ac–Phe–NHMe)2

A combined experimental and theoretical study on the structure of the dipeptide model Ac–Phe–NHMe and its dimer (Ac–Phe–NHMe)2 is presented. In order to get a detailed vibrational analysis of all functional groups which are relevant to analyse the different structural arrangements, IR/R2PI spectra are recorded in the regions of the NH and the CO stretching vibrations. Force field calculations are used to scan the complex conformational landscape both of Ac–Phe–NHMe and the dimer (Ac–Phe–NHMe)2. Subsequent ab initio and DFT calculations on the most stable structures lead to predictions of the cluster geometries and their vibrational frequencies. Three isomers of the Ac–Phe–NHMe monomer have been assigned which contain either a β-sheet related configuration or hydrogen-bonded structures. The most prominent species has a β-sheet related conformation. The observed dimer contains a doubly hydrogen-bonded arrangement and turns out to be a β-sheet model system. In contrast to the β-sheet model (Ac–Phe–OMe)2 a different structural arrangement is found, connecting “the outer” CO and NH groups.

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.

Structures and rearrangement reactions of 4-aminophenol(H2O)1+ and 3-aminophenol(H2O)1+ clusters.

In this paper the structures of 4-aminophenol(H2O)1+ and 3-aminophenol(H2O)1+ clusters are investigated in molecular beam experiments by different IR/UV-double resonance techniques as well as the mass analyzed threshold ionization spectroscopy yielding both inter- and intramolecular vibrations of the ionic and neutral species. Possible structures are extensively calculated at the level of density functional theory (DFT) or at the ab initio level of theory. From the experimental and theoretical investigations it can be concluded that in the case of 4-aminophenol(H2O)1 one O-H...O hydrogen-bonded structure exists in the neutral cluster but two structures containing either an O-H...O or a N-H...O hydrogen-bonded arrangement are observed in the spectra of the ionic species. This observation is a result of an intramolecular rearrangement reaction within the ion which can only take place if high excess energies are used. A reaction path via the CH bonds is calculated and explains the experimental observations. In the case of 3-aminophenol(H2O)1+ only one O-H...O bound structure is observed both in the neutral and ionic species. Ab initio and DFT calculations show that due to geometrical and energetical reasons a rearrangement cannot be observed in the 3-aminophenol(H2O)1+ cluster ion.

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.

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.

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.