49 research outputs found

    Photoinduced Charge Separation Processes in Supramolecular Triad Systems

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    The nonadiabatic electron transfers in donor-acceptor-acceptor systems are investigated using three potential energy surfaces and two reaction coordinates via the stochastic Liouville equation to describe time evolution of the three excited electronic states: (1) D*-A-A, (2) D+\text{}^{+}-A−\text{}^{-}-A and (3) D+\text{}^{+}-A-A−\text{}^{-}. The electronic dephasing processes are taken into account phenomenologically in terms of dephasing constants. The couplings between surfaces are effective along the intersections of pairs of surfaces in the two-dimensional coordinate space. Special situations occur in the reaction coordinate space when three surfaces are nearly degenerate. The interplay between the sequential electron transfer processes and the superexchange process is analysed for different: reorganization energies, electronic coupling, free energies for the electron transfer, dephasing rates, and temperature. The time dependent contribution of the superexchange process to the charge separation in the triad system is analysed using the time dependent rate functions. It is shown that in the nonadiabatic limit of electron transfer the influence of the electronic dephasing processes for low barrier reactions can be accounted for by appropriate changes in the reorganization energies. The present model is compared with the experimental results concerning the charge separation in the bacterial photosynthetic reaction centers

    Identification of initiation sites for T4 lysozyme folding using CD and NMR spectroscopy of peptide fragments

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    Using CD and 2D H-1 NMR spectroscopy, we have identified potential initiation sites for the folding of T4 lysozyme by examining the conformational preferences of peptide fragments corresponding to regions of secondary structure. CD spectropolarimetry showed most peptides were unstructured in water, but adopted partial helical conformations in TFE and SDS solution. This was also consistent with the H-1 NMR data which showed that the peptides were predominantly disordered in water, although in some cases, nascent or small populations of partially folded conformations could be detected. NOE patterns, coupling constants, and deviations from random coil Her chemical shift values complemented the CD data and confirmed that many of the peptides were helical in TFE and SDS micelles. In particular, the peptide corresponding to helix E in the native enzyme formed a well-defined helix in both TFE and SDS, indicating that helix E potentially forms an initiation site for T4 lysozyme folding. The data for the other peptides indicated that helices D, F, G, and H are dependent on tertiary interactions for their folding and/or stability. Overall, the results from this study, and those of our earlier studies, are in agreement with modeling and IID-deuterium exchange experiments, and support an hierarchical model of folding for T4 lysozyme

    Ultrasonography: a method used for pregnancy imaging of the fire salamander (Salamandra salamandra)

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    Ultrasound imaging has more frequently been used in veterinary medicine of amphibians and reptiles. In this study, we have verified the usefulness of ultrasound imaging in pregnancy determination of the fire salamander Salamandra salamandra. We have also undertaken to estimate the number of larvae and their developmental stage directly in the oviducts. Three gravid females from Lower Silesia (southern Poland) were examined. Due to the small size of the scanned animals, and the particular arrangement of embryos in the oviducts and ultrasound beams dispersal, the method proved to be inaccurate. Therefore, the minimum number of well-visualized larvae was determined. The maximum number of larvae was established on the basis of the visible fragments of embryos. After birth, we found that the number of larvae born was included in the „min-max” range in only one case. In the remaining two salamanders the number of larvae was higher than estimated in 3 to 7 individuals. The results showed that ultrasound imaging allows the minimum number of larvae in salamander; oviducts to be specified. However, total length measurements were possible only for single and clearly visible embryos
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