Isolated ß-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 ß-turns formed independently from the protection groups and supported by the interaction of the two aromatic chromophores.

Structure of isolated xanthone in the T1 state obtained via combined UV/IR spectroscopy.

In order to extend combined UV/IR spectroscopy to triplet states, xanthone has been chosen as a model system due to its efficient intersystem crossing (ISC). The IR/R2PI (resonant two-photon ionisation) spectrum of the electronic ground state (S(0)) as well as the IR spectrum of the T(1) state have been recorded in a supersonic jet. We show that the IR spectrum of a triplet state can be recorded subsequent to an ISC. In combination with DFT and TDDFT calculations, structural assignments are performed and geometrical changes are identified.

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.