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) mo del for microscopic solvation effects that includes polarizability in the MM region (QM/MMpol). QM/MMpol treatment of both ground and excite d states is presented in the formalism. We present QM/MMpol analysis o f the ground and electronic excited states of the bacteriochlorophyll b dimer (P) of the photosynthetic reaction center (RC) of Rhodopseudom onas viridis using the INDO/S method. We treat P and five adjacent ami no acid side chains quantum mechanically, and the remainder of the pro tein, cofactors, and waters of crystallization with Polarizable MM (32 5 QM atoms embedded in the field of 20 158 polarizable MM atoms). Whil e dimer formation alone is enough to account for the majority of the m onomer BCh1b to P red-shift of the lowest electronic excited state of P (Q(y1)), we demonstrate that explicit treatment of the protein is re quired to properly interpret the experimental Stark effect data that d escribe the charge transfer asymmetry of Q(y1). The static-charge pote ntial from the MM model of the RC alone causes Q(y1) to have significa ntly better agreement with the Stark effect results than isolated P. H owever, consideration of the protein polarization potential is further required to obtain more complete agreement with Stark effect experime nts. Thus, we calculate a Q(y1) transition energy at 10 826 cm(-1) wit h a ground to excited state change in dipole moment of 4.8 D; an absor ption Stark effect angle of 43 degrees; 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(y1) and the pseudo-C-2 axis of th e RC) of 81 degrees. These results are in good agreement with experime nt. Interestingly, we find that net CT increase is greater for Q(y1) t han for the second excited state of P (Q(y2)), a result that we antici pated in an early model dimer study.