The theoretical studies on the machanism of the primary electron transfer in the photosynthetic reaction center

The electron structure of the primary electron donor P960 in Rhodopseuodomo nas (Rps.) viridis photosynthetic reaction center (RC) and P680 in photosys tem II (PS II) RC have been studied with B3LYP DFT quantum chemical ah init io method at 3 - 21G level. The influence of the surrounding proteins and t he axial coordination of histindine residues on the electronic structure of the primary electron donor has also been studied. Finally, the mechanism o f the primary electron transfer (ET) is discussed. The results indicate: (1 ) the HOMO of super - molecule P960 - h in bacterial RC mainly consists of atomic orbitals of atoms located at the composition unit M. The correspondi ng LUMO mainly consists of atomic orbitals of atoms located at both L and M composition units. Both HOMO and LUMO of super - molecule P680 - h in PS I I RC mainly consist of atomic orbitals of atoms located at composition unit D1. These calculated results are helpful for understanding the experimenta l fact that the primary ET in RC of bacterial and PS ii mainly perform alon g L branch and D1 branch, respectively. (2) Although the His residues, whic h are axially coordinated to the primary electron donor P960 in the bacteri al RC, do not contribute to the compositions of HOMO and LUMO of super - mo lecule P960 - h, they make the E-LUMO Of P960 - h much greater than that of P960 and greater than those of ABChl and BPheo. This cause the primary ET in bacterial RC to proceed easily. Otherwise, it will be very difficult for the primary E;T to perform. For PSII RC, the case is similar. (3) The dist ance between the Mg ion in ABChl and the N in the His residues is similar t o that between the Mg ion in P960 and the N atom in the corresponding His, so that the His residues coordinating to ABChl must be taken into considera tion. At this condition, the primary ET in bacterial RC should be a two - s tep ET process from P960 - h to BPheo b(L) via ABChl b(L)(h) along the L br anch. For PS II RC, there does not exist the axial coordination of His to a ccessory chlorophyll a (AChl a), which is one of the important difference f rom the bacterial RC. At this condition, the primary ET in PSII RC should b e a one - step process from P680 - h to Pheo a along D1 protein, but the ET process from P680 h to Pheo a via AChl a along D1 protein could not be exc luded completely.