Thorsten Hansen

Plasma Volume and Vascular Permeability during Hypoxia and Carbon Monoxide Exposure

The effect on plasma volume and capillary permeability to albumin of high. altitude (3454 m) and of carbon monoxide (CO) was investigated during acute exposure (12 hours) and more prolonged exposure (8 days). Prolonged exposure to CO resulted in a decrease or no change in plasma volume, while prolonged exposure to hypoxia showed a more marked decrease in plasma volume. Acute exposure to hypoxia gave no change or a decrease in plasma volume. No changes were found in plasma volume during acute exposure to CO, although the results probably were overestimated due to an increase in capillary permeability. Capillary permeability was unchanged during exposure to hypoxia. From the present results it seems justified to conclude that CO have a more pronounced effect on the permeability of the capillaries to albumin than hypoxia alone, although the explanation for this finding could be that the tissue hypoxia was more severe in the CO experiments than during hypoxia.

Orbital Topology Controlling Charge Injection in Quantum-Dot-Sensitized Solar Cells

Quantum-dot-sensitized solar cells are emerging as a promising development of dye-sensitized solar cells, where photostable semiconductor quantum dots replace molecular dyes. Upon photoexcitation of a quantum dot, an electron is transferred to a high-band-gap metal oxide. Swift electron transfer is crucial to ensure a high overall efficiency of the solar cell. Using femtosecond time-resolved spectroscopy, we find the rate of electron transfer to be surprisingly sensitive to the chemical structure of the linker molecules that attach the quantum dots to the metal oxide. A rectangular barrier model is unable to capture the observed variation. Applying bridge-mediated electron-transfer theory, we find that the electron-transfer rates depend on the topology of the frontier orbital of the molecular linker. This promises the capability of fine tuning the electron-transfer rates by rational design of the linker molecules.

Communication: Finding destructive interference features in molecular transport junctions

Associating molecular structure with quantum interference features in electrode-molecule-electrode transport junctions has been difficult because existing guidelines for understanding interferences only apply to conjugated hydrocarbons. Herein we use linear algebra and the Landauer-Büttiker theory for electron transport to derive a general rule for predicting the existence and locations of interference features. Our analysis illustrates that interferences can be directly determined from the molecular Hamiltonian and the molecule-electrode couplings, and we demonstrate its utility with several examples.

Frequency-Dependent Polarizabilities of Amino Acids as Calculated by an Electrostatic Interaction Model.

The frequency-dependent polarizability of the 20 essential amino acids has been calculated by an electrostatic interaction model where an Unsöld-type of model has been adopted for the frequency dependence. The interaction model has previously been parametrized from Hartree-Fock calculations on a set of molecules, and the model is in this work extended by sulfur parameters by including a set of 18 small sulfur compounds. The results for the amino acids by using the interaction model compare well with Hartree-Fock calculations with deviations of around 5% for the isotropic polarizability. Furthermore, the intrinsic (or optical) dielectric constant related to the polarizability has been calculated for three small proteins, ribonuclease inhibitor, lysozyme, and green fluorescent protein, adopting the interaction model. The results are consistent with the intrinsic dielectric constants found for proteins in the literature.

Multireference Excitation Energies for Bacteriochlorophylls A within Light Harvesting System 2.

Light-harvesting system 2 (LH2) of purple bacteria is one of the most popular antenna complexes used to study Natures way of collecting and channeling solar energy. The dynamics of the absorbed energy is probed by ultrafast spectroscopy. Simulation of these experiments relies on fitting a range of parameters to reproduce the spectra. Here, we present a method that can determine key parameters to chemical accuracy. These will eliminate free variables in the modeling, thus reducing the problem. Using MS-RASPT2/RASSCF calculations, we compute excitation energies and transition dipole moments of all bacteriochlorophylls in LH2. We find that the excitation energies vary among the bacteriochlorophyll monomers and that they are regulated by the curvature of the macrocycle ring and the dihedral angle of an acetyl moiety. Increasing the curvature lifts the ground state energy, which causes a red shift of the excitation energy. Increasing the torsion of the acetyl moiety raises the excited state energy, resulting in a blue shift of the excitation energy. The obtained results mark a giant leap for multiconfigurational multireference quantum chemical methods in the photochemistry of biological systems, which can prove instrumental in exposing the underlying physics of photosynthetic light-harvesting.

Controlling Two-Step Multimode Switching of Dihydroazulene Photoswitches

We present the synthesis and switching studies of systems with two photochromic dihydroazulene (DHA) units connected by a phenylene bridge at either para or meta positions, which correspond to a linear or cross-conjugated pathway between the photochromes. According to UV/Vis absorption and NMR spectroscopic measurements, the meta-phenylene-bridged DHA-DHA exhibited sequential light-induced ring openings of the two DHA units to their corresponding vinylheptafulvenes (VHFs). Initially, the VHF-DHA species was generated, and, ultimately, after continued irradiation, the VHF-VHF species. Studies in different solvents and quantum chemical calculations indicate that the excitation of DHA-VHF is no longer a local DHA excitation but a charge-transfer transition that involves the neighboring VHF unit. For the linearly conjugated para-phenylene-bridged dimer, electronic communication between the two units is so efficient that the photoactivity is reduced for both the DHA-DHA and DHA-VHF species, and DHA-DHA, DHA-VHF, and VHF-VHF were all present during irradiation. In all, by changing the bridging unit, we can control the degree of stepwise photoswitching.

3D spectroscopy of vibrational coherences in quantum dots: theory.

In semiconductor nanocrystals, called quantum dots (QD), electronic transition energies, phonon frequencies, and electron-phonon coupling strengths are all reported to depend on the size of the crystals. The size dependencies of the transition energies and the mode frequencies are well characterized and understood. At the same time, the electron-phonon coupling dependence on size is controversial-even the sign of the change is not settled. In this article, third-order response functions of a model QD resembling CdSe are calculated. The longitudinal optical (LO) mode is included as a relatively narrow Lorentzian contribution to the spectral density. A novel version of electronic 2D spectroscopy is investigated where a third Fourier transform is taken over a so-called population time, leading to 3D spectral representation. The amplitude and phase of the 2D cuts of the 3D spectral body around the LO mode frequency are analyzed. The analytical power and sensitivity of the cuts in determining the possible Huang-Rhys factor (electron-phonon coupling strength) and the LO mode frequency dependence on the QD size are investigated. Peak structures in the cuts with a tilt relative to the diagonal are identified as sensitive signatures for the size dependencies. The study elucidates the 3D representation of the electronic 2D spectroscopy as a powerful tool for obtaining insight into otherwise hardly accessible characteristics of the system.

Tracking molecular resonance forms of donor–acceptor push–pull molecules by single-molecule conductance experiments

The ability of molecules to change colour on account of changes in solvent polarity is known as solvatochromism and used spectroscopically to characterize charge-transfer transitions in donor–acceptor molecules. Here we report that donor–acceptor-substituted molecular wires also exhibit distinct properties in single-molecule electronics under the influence of a bias voltage, but in absence of solvent. Two oligo(phenyleneethynylene) wires with donor–acceptor substitution on the central ring (cruciform-like) exhibit remarkably broad conductance peaks measured by the mechanically controlled break-junction technique with gold contacts, in contrast to the sharp peak of simpler molecules. From a theoretical analysis, we explain this by different degrees of charge delocalization and hence cross-conjugation at the central ring. Thus, small variations in the local environment promote the quinoid resonance form (off), the linearly conjugated (on) or any form in between. This shows how the conductance of donor–acceptor cruciforms is tuned by small changes in the environment.

Microphotometry of single fibres from the quadriceps muscle in man

Two main types of muscle fibres were histochemically identified in the lateral portion of the human quadriceps muscle. Muscle samples were obtained by needle biopsies from a highly trained, a hypotrophied and a normal leg. Serial sections were histochemically stained for myofibrillar ATPase (pH 9.4, preincubation at pH 10.3), nicotinamide adenine dinucleotide tetrazolium oxidoreductase (NADHTR) and α-glycerophosphate dehydrogenase (α-GPD, E.C.1.1.2.1.). In the two fibre types identified in the ATPase stained sections, type I (ATPase negative) and type II (ATPase positive), microphotometry was used to quantify the oxidative (NADHTR) and glycolytic (α-GPD) enzyme activity of the single fibre. The correspondence to the ATPase ‘type’ was 100% when the activity of both metabolic enzymes were combined as a ‘metabolic profile’ for each fibre. This clear distinction was unaffected by the training state of the muscle. It is concluded that histochemical reaction for myofibrillar ATPase as well as the combination of...

Intermolecular Modes between LH2 Bacteriochlorophylls and Protein Residues: The Effect on the Excitation Energies

Light-harvesting system 2 (LH2) executes the primary processes of photosynthesis in purple bacteria; photon absorption, and energy transportation to the reaction center. A detailed mechanistic insight into these operations is obscured by the complexity of the light-harvesting systems, particularly by the chromophore-environment interaction. In this work, we focus on the effects of the protein residues that are ligated to the bacteriochlorophylls (BChls) and construct potential energy surfaces of the ground and first optically excited state for the various BChl-residue systems where we in each case consider two degrees of freedom in the intermolecular region. We find that the excitation energies are only slightly affected by the considered modes. In addition, we see that axial ligands and hydrogen-bonded residues have opposite effects on both excitation energies and oscillator strengths by comparing to the isolated BChls. Our results indicate that only a small part of the chromophore-environment interaction can be associated with the intermolecular region between a BChl and an adjacent residue, but that it may be possible to selectively raise or lower the excitation energy at the axial and planar residue positions, respectively.