Ab initio quantum chemical study electron transfer processes in the photosynthetic reaction center and scanning tunneling microscopy

Using a diabatic state formalism and pseudospectral numerical methods, we have developed an efficient ab initio quantum chemical approach to the calculation of electron transfer matrix elements for large molecules. The theory is developed at the Hartree-Fock level and validated by comparison with results in the literature for small systems. The method and the code developed within PSGVB suite of electronic structure programs have been applied to study the electron transfer process in two systems: the bacterial photosynthetic reaction center and the scanning tunneling microscope (STM). The calculated electronic couplings between chromophores on the L-side in the reaction center agree remarkably well with parameters obtained from recent quantum dynamical modeling of experimental data assuming an explicit intermediate mechanism. We have also computed couplings on the M-side of the reaction center, and find that the interaction of the primary donor to the M-side intermediate bacteriochlorophyll is quite small due to destructive interference of the two localized coupling matrix elements. This may explain the slow rate of electron transfer down the M-side of the reaction center. The calculated STM contrasts (modeled by the square of the tunneling matrix elements) of a series of functionalized alkanes are compared to the experimental images, most of the experimental observations can be qualitatively explained. More complicated theoretical considerations for the future work are given.;In addition, using PSGVB together with an accurate numerical Poisson-Boltzmann solver, we have carried out ab initio calculation of the redox potentials of bacteriochlorophyll and bacteriopheophytin in solution at the Hartree-Fock level. These results have implications for calculation of redox energies of the chromophores in the bacterial photosynthetic reaction center which are essential in understanding the electron transfer mechanism.