Theoretical studies on the mechanism of primary electron transfer in the photosynthetic reaction center of Rhodobacter sphaeroides

The mechanism of the primary electron transfer (ET) process in the photosynthetic reaction center (PRC) of Rhodobacter sphaeroides has been studied with quantum chemistry method of ab initio density functional theory (DFT) (B3LYP/6-31G) based on the optimized X-ray crystallographic structure. The calculation was carried out on different structural levels. The electronic structure of pigment molecules was first studied, and then the influence of the neighboring protein was taken into account at three approximation levels: (a) the surrounding proteins were treated as a homogeneous medium with a uniform dielectric constant (SCRF); (b) both the influence of axial coordination of His to the special pair P and ABChl as, and the hydrogen bonds between related residues and P and also BPhas were included; and (c) the influence of the electronic structure of the protein subunit chains as a whole was studied. The results suggest that: (1) according to the composition of the HOMO and LUMO of P, there might be a charge-separated state of (BChl(L) (+)BChl(M)(-)) for the excited state of P; (2) to treat the protein surroundings as a homogeneous medium is not sufficient. Different interactions between pigment molecules and related residues play different roles in the ET process; (3) the axial coordination of His to P raises the E-LUMO of P greatly, and it is very important for the ET process to occur in the PRC of wild-type bacterium; the axial coordination of His to ABChl as also raises their E-LUMO significantly; (4) the hydrogen-bonds between amino acid residues and P and also BPh as depress the E-LUMO of the pigment molecules to some extent, which makes the E-LUMO of P lower than those of ABChlas, and the E-LUMO of BPh a(L) lower than that of BPh a(M). Consequently, the ET process from P to BPh aL does not, according to our calculation model, occur via ABChl aL. The possibility of the ET pathway from P to BPh a(L) via ABChl a(L) was discussed; (5) the frontier orbitals of protein subunit chains L and M are localized at the random coil area and the alpha-helix areas, respectively. Results mentioned above support the fact that the ET process proceeds in favourable circumstances along the branch L.