Electron Transfer in Porphyrin Complexes in Different Solvents

The electron transfer in different solvents is investigated for systems consisting of donor, bridge and acceptor. It is assumed that vibrational relaxation is much faster than the electron transfer. Electron transfer rates and final populations of the acceptor state are calculated numerically and in an approximate fashion analytically. In wide parameter regimes these solutions are in very good agreement. The theory is applied to the electron transfer in ${\rm H_2P-ZnP-Q}$ with free-base porphyrin (${\rm H_2P}$) being the donor, zinc porphyrin (${\rm ZnP}$) the bridge, and quinone (${\rm Q}$) the acceptor. It is shown that the electron transfer rates can be controlled efficiently by changing the energy of the bridging level which can be done by changing the solvent. The effect of the solvent is determined for different models.Comment: 28 pages + 5 figures, submitted to J. Phys. Chem. For more details see the Ph. D. thesis in quant-ph archive http://xxx.lanl.gov/abs/quant-ph/000100

Structural Basis for Allosteric Regulation in the Major Antenna Trimer of Photosystem II

The allosteric regulation of protein function proves important in many life-sustaining processes. In plant photosynthesis, LHCII, the major antenna complex of Photosystem II, employs a delicate switch between light harvesting and photoprotective modes. The switch is triggered by an enlarged pH gradient (ΔpH) across the thylakoid membranes. Using molecular simulations and quantum calculations, we show that ΔpH can tune the light-harvesting potential of the antenna via allosteric regulation of the excitonic coupling in chlorophyll-carotenoid pairs. To this end, we propose how the LHCII excited state lifetime is coupled to the environmental conditions. In line with experimental findings, our theoretical model provides crucial evidence toward the elucidation of the photoprotective switch of higher plants at an all-atom resolution

Chebyshev Expansion Applied to Dissipative Quantum Systems

To determine the dynamics of a molecular aggregate under the influence of a strongly time-dependent perturbation within a dissipative environment is still, in general, a challenge. The time-dependent perturbation might be, for example, due to external fields or explicitly treated fluctuations within the environment. Methods to calculate the dynamics in these cases do exist though some of these approaches assume that the corresponding correlation functions can be written as a weighted sum of exponentials. One such theory is the hierarchical equations of motion approach. If the environment, however, is described by a complex spectral density or if its temperature is low, these approaches become very inefficient. Therefore, we propose a scheme based on a Chebyshev decomposition of the bath correlation functions and detail the respective quantum master equations within second-order perturbation theory in the environmental coupling. Similar approaches have recently been proposed for systems coupled to Fermionic reservoirs. The proposed scheme is tested for a simple two-level system and compared to existing results. Furthermore, the advantages and disadvantages of the present Chebyshev approach are discussed

Interatomare Austauschenergien und Potentiale zwei- und dreiatomiger Systeme

In this thesis the interactions in two and three atomic molecules are investigated. The interaction energies can mainly be traced back to atomic parameters. Because of this fact, atomic wave functions are discussed in the beginning of the thesis. In the foreground of the interest are the asymptotic behavior and the normalization constants occuring in this occasion. For alkali atoms, the normalization constants can be determined exactly with pseudo-potential methods. For atoms and ions with two electrons, a model wave function is developed which has the correct asymptotic behavior, the correct behavior for small electron-electron distances and for small electron-nucleus distances. The percentage of the correlation energies calculated with this parameter-free wave function are in between 95% for H"- and 85% for Ne"8"+. In order to determine the normalization constants of helium as well as of the other rare gas atoms, configuration interaction calculations are performed. The exchange energy of two atomic van der Waals molecules is written as the product of the exchange energy of single electron pair and a angular momentum factor. Parallel to this, the same factor is determined with a simple counting rule. The exchange energies of the single electorn pairs are calculated by the surface integral method. The Generalized Heitler-London theory of Tang and Toennies is extended to many-electron systems and applied to He_2, Ne_2 and Ar_2282 refs.Available from TIB Hannover: RA 1396(1996,6) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Role of water during the extrusion of substrates by the efflux transporter AcrB

The major efflux system in Escherichia coli is the tripartite complex AcrAB-TolC. Its homotrimeric transporter AcrB is polyspecific and extrudes antibiotics out of the bacterium. This extrusion is performed via a functional rotation, in which each monomer assumes a particular conformation. In the present study, targeted molecular dynamics simulations have been employed to obtain a molecular level understanding of the transport process. A particular focus is put on the role of water molecules in this extrusion process. It is shown that the water flows from the binding pocket toward the exit gate in the extrusion step and helps the substrate to move along this path. These results are underpinned by a detailed analysis of the electrostatic interaction energy. Furthermore, the role of water for the polyspecificity of the transporter is discussed

Electron transfer in porphyrin complexes in different solvents

Submitted to J. Phys. Chem.Consiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7 Rome / CNR - Consiglio Nazionale delle RichercheSIGLEITItal

Effects of the F610A mutation on substrate extrusion in the AcrB transporter: explanation and rationale by molecular dynamics simulations

The tripartite efflux pump AcrAB-TolC is responsible for the intrinsic and acquired multidrug resistance in Escherichia coli. Its active part, the homotrimeric transporter AcrB, is in charge of the selective binding of substrates and energy transduction. The mutation F610A has been shown to significantly reduce the minimum inhibitory concentration of doxorubicin and many other substrates, although F610 does not appear to interact strongly with them. Biochemical study of transport kinetics in AcrB is not yet possible, except for some β-lactams, and other techniques should supply this important information. Therefore, in this work, we assess the impact of the F610A mutation on the functionality of AcrB by means of computational techniques, using doxorubicin as substrate. We found that the compound slides deeply inside the binding pocket after mutation, increasing the strength of the interaction. During subsequent conformational alterations of the transporter, doxorubicin was either not extruded from the binding site or displaced along a direction other than the one associated with extrusion. Our study indicates how subtle interactions determine the functionality of multidrug transporters, since decreased transport might not be simplistically correlated to decreased substrate binding affinity