Modelling the bacterial photosynthetic reaction center.VI.Use of density-functional theory to determine the nature of the vibronic coupling between the four lowest-energy electronic states fo the special-pair radical cation

摘要

It is now over ten years since the first FTIR spectra were recorded of the radical catin of the special-pair a dimer of bacteriochlorophyll molecules that forms the promary electron donor responsible for promary charge separation in bacterial photoosynthesis While spectra of this type promise to reveal much concerming te role of te special pair electron donor in photosynthesis attempts to meodel and interpret tham have ben limited by poor knowledge of the vibratoinally specific aspects fo the electron-phonon couping and have thus been restricted to crude model calculation only we developtechniques through which density-functional theorycan be employed to evaluate most of th unknown properties This includes symmetric-mode displacements antisymmetric-mode vibronic coupling constants and interatate electonic couplings evaluated for interactions between the four lowest-energy states of the special-pair cation radical the ground state the primary hole-transfer state and states involving these two combined with SHOMO to HOMO transitions Geometry optimizatios are performed for all four states of the dimer while vibratioonal analyses are obtained for the first two vibronic coupling constants are extracted rom analysis of stole infrared transition moments usignHerzberg-Teller theory Quantitatively these results are emplloyed in the subsequent paper in this series to simulated the oberved spectra Qualitatively these results indicate that(1) vibronic coupling occurs through a large number of antisymmetric modes of the dimer rather than through a small numbe fo strongly active modes (2) the role of symmetric bibratinal motions of the dimer is only minor (3) that the active symmetric modes are significant in number and low in frequency (4) that vibronic couping between the hole-transfer state and the SHOMO to HOMO state is relatively weal and influences spectra only near reconance and (5) that the calculated electronic couplings are qualitatively realistic adn may provide an explansation for the much weaker couping observed in chlorophyll-containing reacin centers.