Excited States of the Bacteriochlorophyll b Dimer of Rhodopseudomonas viridis: A QM/MM Study of the Photosynthetic Reaction Center That Includes MM Polarization

We present a hybrid quantum mechanical/molecular mechanical (QM/MM) model for microscopic solvation effects that includes polarizability in the MM region (QM/MMpol). QM/MMpol treatment of both ground and excited states is presented in the formalism. We present QM/MMpol analysis of the ground and electronic excited states of the bacteriochlorophyll b dimer (P) of the photosynthetic reaction center (RC) of Rhodopseudomonas viridis using the INDO/S method. The static-charge potential from the MM model of the RC alone causes Q sub(y1) to have significantly better agreement with the Stark effect results than isolated P. However, consideration of the protein polarization potential is further required to obtain more complete agreement with Stark effect experiments. Thus, we calculate a Q sub(y1) transition energy at 10826 cm super(-1) with a ground to excited state change in dipole moment of 4.8 D; an absorption Stark effect angle of 43 degree ; a net shift of 0.15 electrons from the L subunit to the M subunit of P; and a linear dichroism angle (between the transition moment of Q sub(y1) and the pseudo-C sub(2) axis of the RC) of 81 degree . These results are in good agreement with experiment. Interestingly, we find that net CT increase is greater for Q sub(y1) than for the second excited state of P (Q sub(y2)), a result that we anticipated in an early model dimer study.