COE on PCPM: Research


THEORETICAL STUDIES ON ELECTRON TRANSFER AT PHOTOSYNTHETIC REACTION CENTERS
Kozo AOKI, Osamu KITAO, and Tetsuji Ogawa

Special Department of COE Project, National Institute of Materials and Chemical Research,
Tsukuba, Ibaraki 305-8565 JAPAN

This page is organized by COE on PCPM project.


Plants and photosynthetic bacteria convert solar energy into chemical energy with a quantum yield of almost 100 %. The initial stage of the conversion occurs at photosynthetic reaction center (PRC), where the system transfers electrons generated by solar light toward a determined direction along a determined route with a high speed of 4 ps for 20 A at room temperature. Since Deisenhofer et al. reported the structural data of Rhodopseudomonas viridis in 1984 [1], many studies have treated the electron transfer (ET) reaction, and left several problems unsolved; the most important issue is unidirectonality of charge separation. 
At the PRC, the protein consists of C-, L-, M-, and H-subunits, and the pigments include a pair of bacteriochlorophyll b dimer (P), two molecules of accessory bacteriochlorophyll b (B), two molecules of bacteriopheophytin b (H), and two quinone (Q) molecules. We call the pigment line as Qm-Hm-Bm-P-Bl-Hl-Ql. The Qm stands Q in the M-subunit, and the other definitions are also same manner. Electrons go through from P to Ql exclusively in spite of both possibility of L- and M-subunit form consideration on only pigments. The ratio is about 100:1 (P to Hl is preferable). Because this mechanism suppresses the electron recombination at the radical pair of the PRC, this system can convert solar light to chemical energy very efficiently. Many researchers consider protein controls the ET, and we have investigated this point from theoretical point of view [2].


In order to study the ET problem, we adopted charge equilibration method (QEq) by Rappe and Goddard [3] to calculate atomic partial charges depending on the structure, and generalized this method to QEq-CS (charge separation) [4]. These theories solve non- symmetric and dense linear equation with a dimension of total numbers of atoms for a few iterations, and determine the partial charges so that we can reproduce quantum chemistry calculation for the system. 
Although the PRC leads us to the linear equation with a dimension of over twenty thousands, we can solve this problem with less than 2 minutes per one iteration on HITACHI SR8000 16 node (1 node is 8 GFLOPS, 8G) of Tsukuba Advanced Computing Center, Agency of Industrial Science and Technology.
We constructed PRC structure based on X-ray structure data [5], and calculated electrostatic energy of PRC by QEq before ET and QEq-CS after ET. The initial state of ET the photoexcited P, P*, and this state is higher by 1.25 eV than the ground P state [6]. As summarized in Table 1, all four ET (P* to Bl, Bm, Hl, and Hm) are exothermic and the P* to Hl is most favorable. Because energy expression of QEq and QEq-CS is a sum of one- and two-bodies for atomistic suffixes, this table includes both terms. Although the sudden photoexcited ET makes the charge separation system unstable via the one-body term, the two-body term stabilized the charge separation system. The traditional simulations use the fixed charges during ET, they cannot express the stabilization by the two-body term.


In the QEq-CS method, we define both donor and acceptor regions. We calculated the difference of partial charge along each ET and investigated the contents for each case. When the donor region loses one electron, the neighbor region increases the electron density a little bit. At the same time, the acceptor region gains an electron, and the neighbor region decreases the electron density. These reorganizations depend on the specific position of several atoms within each region; our method can describe the realistic change. On the bases of QEq and QEq-CS, we checked the partial charge distribution of the PRC. These noticed us minus charge distribution just above P, and plus charge distribution beside Hl. With this contents, we summarized the minus charge distribution pushes electrons, and the plus charge distribution pulls the electrons, and proposed the "push-pull mechanism" [7]. We consider this mechanism converts solar light to chemical energy with a high efficiency at the earliest stage of photosynthetic reaction, and gives some ideas to artificial photosynthesis materials.


 
Table 1. Change of electrostatic energy along each ET (at X-ray structure; in eV)
ET One-body Two-body Total
P* to Bl -0.974 +1.079 +0.105
P* to Bm -0.828 +0.929 +0.101
P* to Hl -0.120 +0.084 -0.036
P* to Hm -0.130 +0.160 +0.030
All values are based on the energies from P* to All (except Donor)

We have done this study under "COE project on Photoreaction Control and Photofunctional Materials" and "Promote Research Projects for High Performance Computing, Tsukuba Advanced Computing Center, Agency of Industrial Science and Technology". We thank Prof. William A. Goddard for giving O.K. an original QEq code, and Drs. Osamu Tatebe, Satoshi Sekiguchi, Akira Okada, and Nobuaki Miura for valuable comments.

 

 
 

[1] J.Deisenhofer, O.Epp, K.Miki, R.Huber, and H.Michel, J.Mol.Biol., 180, 385 (1984).
[2] O.Kitao, H.Ushiyama, and N.Miura, J.Chem.Phys. , 110,2936(1999).
[3] A.K.Rappe and W.A.Goddard III, J.Phys.Chem., 95,3358(1991).
[4] O. Kitao, N.Miura, and H.Ushiyama, Theochem, 461-462,239-247(1999).
[5] O.Kitao and K.Aoki, submitted for Japanese Journal of Chem.Info. and Comp. Science.
[6] J.Breton, Biochim.Biophys.Acta, 810,235(1985).
[7] O.Kitao, K.Aoki, and T.Ogawa, to be submitted.


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