6374
J. Phys. Chem. 1995,99, 6374-6386
Excited States of the Bacteriochlorophyll b Dimer of Rhodopseudomonas viridis: A QM/MM Study of the Photosynthetic Reaction Center That Includes MM Polarization Mark A. Thompson* and Gregory K. Schenter Environmental Molecular Sciences Laboratory, Pact@ Northwest Laboratory, Richland, Washington 99352 Received: November 30, 1994; In Final Form: January 27, 1995@
We present a hybrid quantum mechanicaYmolecular 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. We treat P and five adjacent amino acid side chains quantum mechanically, and the remainder of the protein, cofactors, and waters of crystallization with polarizable MM (325 QM atoms embedded in the field of 20 158 polarizable MM atoms). While dimer formation alone is enough to account for the majority of the monomer BChlb to P red-shift of the lowest electronic excited state of P (QyI),we demonstrate that explicit treatment of the protein is required to properly interpret the experimental Stark effect data that describe the charge transfer asymmetry of QYl. The static-charge potential from the MM model of the RC alone causes QYI 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,I transition energy at 10 826 cm-I with a ground to excited state change in dipole moment of 4.8 D; an absorption Stark effect angle of 43"; 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 QYland the pseudo-C* axis of the RC) of 81". These results are in good agreement with experiment. Interestingly, we find that net CT increase is greater for QYl than for the second excited state of P (Qy2), a result that we anticipated in an early model dimer study.
I. Introduction The initial events of photosynthesis involve a light-driven charge separation across a photosynthetic membrane.',2 The elucidation of the X-ray structures of the photosynthetic reaction centers (RC) of the photosynthetic bacteria Rhodopseudomonas v i r i d i ~ ~and 3 ~ Rhodobacter sphaeroides5,6 has motivated increased activity in experimental and theoretical studies of the photochemistry of these membrane-bound pigmendprotein c~mplexes.~-~~ The RC of Rps. viridis contains four bacteriochlorophyll b (BChlb) and two bacteriopheophytin b (BPheb) chromophores. In addition, there are two quinones, menaquinone and ubiquinone (QL and QM, respectively), and a nonheme Fe.334 Two molecules of BChlb are closely juxtaposed, with their pyrrole rings I overlapping at an average macrocycle separation of -3.3 A, and form the "special pair" or primary donor, hereafter referred to as P. The remaining chromophores form two branches, each consisting of an auxiliary BChlb, a BPheb, and a quinone. The chromophores of P and the branches are labeled L or M according to their association with either the L or M protein subunits of the RC complex. There is also a carotenoid residing near the M auxiliary BChlb. The RC protein consists of both the L and M subunits, which constitute the majority of the membrane-embedded protein, as well as a third subunit known as H, which resides mainly on the cytoplasmic side of the membrane with one a-helix anchored in the membrane. In Rps. viridis, there is also a cytochrome affixed to the periplasmic side of the RC complex near P. The cytochrome contains four Fe-hemes which act to reduce the photochemically oxidized P. There is a pseudo-C2 symmetry axis that extends from P to the @Abstractpublished in Advance ACS Abstracts, April 15, 1995
nonheme Fe (excluding the cytochrome). The entire RC complex is shown schematically in Figure 1. The primary donor, P, absorbs excitation energy either directly or through energy transfer from nearby antenna complexes and rapidly transfers an electron to the BPheb on the L branch (HL) in -3 ps at room temperature over a distance of -17 A. This electron transfer (ET) is followed by two slower ET steps, first to a nearby menaquinone (QL) on the L branch and then to a ubiquinone (QM) on the M branch.27 Despite the pseudo-2fold symmetry of the RC, ET proceeds only along the L branch. In a recent classical molecular mechanics (MM) study that included atom-centered polarizabilities, Parson, Chu, and Warshel showed that electrostatic interactions with the protein favor ET along the L branch.I5 Thompson and Zemer, using the quantum mechanical INDO/s method, have calculated an energetic preference for ET along the L branch, on the basis of both structural considerations and treatment of the protein as a simple homogeneous dielectric.' I More recently, Steffen, Lao, and Boxer described dielectric asymmetry in the RC by showing that dielectric screening of ET states is larger on the L branch relative to the M branch.23 In a molecular dynamics (MD) study of the RC, Marchi et al. also demonstrated preference for ET along the L branch of the RC of Rps. viridis.*O While much has been learned from studies of these RCs, key issues such as the mechanism of primary charge separation remain elusive. Specifically, the role of BL in the initial ET remains controv e r ~ i a l . 'Despite ~ ~ ~ ~ the results of the above experiments and theoretical studies, there is no general consensus on the reason for unidirectional ET along the L branch. Lastly, there remains a lack of firm understanding of the electronic excited states of the RC. The BChlb dimer has the optical absorption band of lowest energy in the RC and serves as the phototrap and primary donor
0022-3654/95/2099-6374$09.00/0 0 1995 American Chemical Society
Thompson and Schenter
6376 J. Phys. Chem., Vol. 99, No. 17,1995
t pseudo-C2 axis
-
Mg
PL
Figure 2. BChlb dimer (P) with phytyl tails truncated for clarity. The side view reflects the actual coordinates of P, while the top view is shown schematically. The subunits PL and PMhave been labeled. Also shown is the pseudo-CZ symmetry axis of the RC. The rings of each BChlb have been numbered in the usual fashion.77The polarization of each monomer Q, transition is approximately from the ring 111 nitrogen to the ring I nitrogen. Thus, we also schematically show the polarization axis of the dimer electronic excited state el,which contains the out of phase combination of the two monomer Q, transitions. In the schematic top view, we also show the vector connecting the centers of PL and PM.
The identity of the 850 nm shoulder as Qy2 has recently been firmly established by Raja et al. using nonphotochemical hole burning (NPHB).I6.l7 Experimentally, Stark effect spectroscopy has been used to show that the excited state properties of P reflect the lack of near-C2 symmetry of P.22,25,27-37 We show P, along with some useful vector quantities in Figure 2 which will be useful in orienting the reader during our discussions of Stark effect and linear dichroism results. Stark effect spectroscopy measures the effect of an applied external electric field on the absorption or emission spectrum of a molecule. The Stark effect spectrum can provide information on the change in dipole moment (Ap) associated with an optical transition. Also, Stark effect spectra provide information on the angle between 4 and the transition dipole moment for the absorption or emission.27 The lowest singlet excited state of P (Q,,) exhibits a large Stark effect relative to the monomers.22-25,27,37 The Stark effect results indicate a ground to excited state change in state dipole moment, IApl, for QYl of (8.2-10.5)f-I D at room temperature, where f = 1.0-1.4 to account for the local dielectric effect of the p r ~ t e i n . ~The ~ . ~value ~ of f is controversial, giving some uncertainty in the actual value of Ap for the absorbing species. Also, in recent experiments on Rb. sphaeroides., Middendort et al. demonstrated for the first time that there is a substantial contribution from higher-order Stark effects of P, most notably Aa, the change in polarizability between ground and excited state.37 Their Stark effect line-shape analysis now reveals new values of Ap lower than previous experiments that assumed the Stark effect arising entirely from A p . The angle between Ap and the transition moment of the absorption has a measured
value of
