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 40fs excitation at 870nm at 293K. Absorbance increase in the 1060-1130nm region that is presumably attributed to P(A)(δ+) cation radical molecule as a part of mixed state with a charge transfer character P*(P(A)(δ+)P(B)(δ-)) was found. This state appears at 120-180fs 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 B(A) bacteriochlorophyll with absorption band at 1020nm in WT. The formation of the P(A)(δ+)B(A)(δ-) state is a result of the electron transfer from P*(P(A)(δ+)P(B)(δ-)) to the primary electron acceptor B(A) (still mixed with P*) with the apparent time delay of ~1.1ps. Next step of ET is accompanied by the 3-ps appearance of bacteriopheophytin a(-) (H(A)(-)) band at 960nm. 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*(P(A)(δ+)B(A)(δ-)) at ~500fs. For longer excitation pulses (40fs) this promotion is absent and tunneling through an energy barrier takes about 3ps. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.