Biomedical subjects
Publications and source records attributed to Thomas Häber.
2,2,2-Trifluoroethanol molecules synchronize their transiently chiral gauche configurations upon dimerization in supersonic jet expansions, while they avoid an energetically competitive heteroconfigurational hydrogen bonded dimer topology predicted by extensive quantum chemical calculations.
High-resolution IR absorption spectra of supersonically cooled ethyl radicals (Trot approximately 20 K) have been obtained in a slit supersonic jet discharge expansion, revealing first rotationally resolved data for CH-stretch excitation of the methyl group. Three different vibrational bands are observed, one parallel (k=0<--0) and two perpendicular (/k/1<--0), which for a nearly decoupled methyl rotor framework would correspond to symmetric and (nearly degenerate) asymmetric CH-stretch excitations. However, the splitting between the two asymmetric CH-stretch excitations is anomalously large (approximately 125 cm(-1)), signaling the presence of interactions between the CH2 radical moiety and the opposing CH bond on the methyl group. This suggests an improved zeroth-order vibrational description as an isolated CH stretch, strongly redshifted by hyperconjugation, with localized vibrations in the remaining CH bonds split into symmetric and asymmetric stretches. Such a dynamical picture highlights a remarkably strong coupling between methyl CH-stretch vibrations and C-C torsional geometry and begins to elucidate discrepancies with previous matrix observations.
An apparatus for detailed study of quantum state-resolved inelastic energy transfer dynamics at the gas-liquid interface is described. The approach relies on supersonic jet-cooled molecular beams impinging on a continuously renewable liquid surface in a vacuum and exploits sub-Doppler high-resolution laser absorption methods to probe rotational, vibrational, and translational distributions in the scattered flux. First results are presented for skimmed beams of jet-cooled CO(2) (T(beam) approximately 15 K) colliding at normal incidence with a liquid perfluoropolyether (PFPE) surface at E(inc) = 10.6(8) kcal/mol. The experiment uses a tunable Pb-salt diode laser for direct absorption on the CO(2) nu(3) asymmetric stretch. Measured rotational distributions in both 00(0)0 and 01(1)0 vibrational manifolds indicate CO(2) inelastically scatters from the liquid surface into a clearly non-Boltzmann distribution, revealing nonequilibrium dynamics with average rotational energies in excess of the liquid (T(s) = 300 K). Furthermore, high-resolution analysis of the absorption profiles reveals that Doppler widths correspond to temperatures significantly warmer than T(s) and increase systematically with the J rotational state. These rotational and translational distributions are consistent with two distinct gas-liquid collision pathways: (i) a T approximately 300 K component due to trapping-desorption (TD) and (ii) a much hotter distribution (T approximately 750 K) due to "prompt" impulsive scattering (IS) from the gas-liquid interface. By way of contrast, vibrational populations in the CO(2) bending mode are inefficiently excited by scattering from the liquid, presumably reflecting much slower T-V collisional energy transfer rates.
We present resonant two-photon ionization (R2PI), IR-UV, and UV-UV double resonance spectra of jet-cooled 2-aminopurine (2AP) as well as Fourier transform infrared (FTIR) gas phase spectra. 2AP is a fluorescing isomer of the nucleobase adenine. The results show that there is only one tautomer of 2AP which absorbs in the wavelength range 32,300-34,500 cm(-1). The comparison with the calculated IR spectra of 9H- and 7H-2AP points to 9H-2AP as the dominating tautomer in the gas phase but the spectra are too similar to allow an unambiguous assignment to the respective tautomer. Hence, we determined vertical and adiabatic excitation energies of both tautomers employing combined density functional theory and multi-reference configuration interaction techniques. For the 0-0 band of the first 1pipi* transition of 9H-2AP we obtain a theoretical value of 32,328 cm(-1), in excellent agreement with the band origin of our R2PI spectrum at 32,371 cm(-1). The first singlet pipi* transition of the less stable 7H-2AP tautomer is predicted to be red-shifted by about 1700 cm(-1) with respect to the corresponding transition in 9H-2AP. From the absence of experimental bands in the energy region between 30,300 and 32,350 cm(-1) we conclude that 7H-2AP is not present to an appreciable extent in the molecular beam. Our calculations yield nearly equal energies for the 1npi* and 1pipi* minima of isolated 2AP, similar to the situation in adenine. The hitherto existing argument that the energetic order of states is responsible for the different spectroscopic properties of these isomers therefore does not hold. Rather, vibronic levels close to the origin of the 1pipi* transition cannot access the conical intersection between the 1pipi* and S(0) states along a puckering coordinate of the six-membered ring, in contrast to the situation in electronically excited 9H-adenine. As a consequence, a rich vibrational structure can be observed in the R2PI spectrum of 2AP whereas the spectrum of 9H-adenine breaks off at low energies.
To understand the intrinsic properties of peptides, which are determined by factors such as intramolecular hydrogen bonding, van der Waals bonding and electrostatic interactions, the conformational landscape of isolated protein building blocks in the gas phase was investigated. Here, we present IR-UV double-resonance spectra of jet-cooled, uncapped peptides containing a tryptophan (Trp) UV chromophore in the 1000-2000 cm(-1) spectral range. In the series Trp, Trp-Gly and Trp-Gly-Gly (where Gly stands for glycine), the number of detected conformers was found to decrease from six (Snoek et al., PCCP, 2001, 3, 1819) to four and two, respectively, which indicates a trend to relaxation to a global minimum. Density functional theory calculations reveal that the O-H in-plane bending vibration, together with the N-H in-plane bend ing and the peptide C=O stretching vibrations, is a sensitive probe to hydrogen bonding and, thus, to the folding of the peptide backbone in these structures. This enables the identification of spectroscopic fingerprints for the various conformational structures. By comparing the experimentally observed IR spectra with the calculated spectra, a unique conformational assignment can be made in most cases. The IR-UV spectrum of a Trp-containing nonapeptide (Trp-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu) was recorded as well and, although the IR spectrum is less well-resolved (and it probably results from different isomers), groups of amide I (peptide C=O stretching) and amide II (N-H in-plane bending) bands can still be recognised, in agreement with predictions at the AM1 level.