Primary charge separation within P870* in wild type and heterodimer mutants in femtosecond time domain

Primary charge separation dynamics in the reaction center (RC) of purple bacterium Rhodobacter sphaeroides and its P870 heterodimer mutants have been studied using femtosecond time-resolved spectroscopy with 20 and 40 fs excitation at 870 nm at 293 K. Absorbance increase in the 1060–1130 nm region that is presumably attributed to PAδ+ cation radical molecule as a part of mixed state with a charge transfer character P*(PAδ+PBδ−) was found. This state appears at 120–180 fs time delay in the wild type RC and even faster in H(L173)L and H(M202)L heterodimer mutants and precedes electron transfer (ET) to BA bacteriochlorophyll with absorption band at 1020 nm in WT. The formation of the PAδ+BAδ− state is a result of the electron transfer from P*(PAδ+PBδ−) to the primary electron acceptor BA (still mixed with P*) with the apparent time delay of ~ 1.1 ps. Next step of ET is accompanied by the 3-ps appearance of bacteriopheophytin a− (HA−) band at 960 nm. The study of the wave packet formation upon 20-fs illumination has shown that the vibration energy of the wave packet promotes reversible overcoming of an energy barrier between two potential energy surfaces P* and P*(PAδ+BAδ−) at ~ 500 fs. For longer excitation pulses (40 fs) this promotion is absent and tunneling through an energy barrier takes about 3 ps. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

► New band with λ ≥ 1050 nm was attributed to charge separation inside of P* of RC.
► 1020-nm band was registered with a delay of ~ 1 ps after excitation in WT RC.
► Coherent oscillation in WT RC at 940 and 1020 nm was observed upon 20-fs excitation.