Formation of Bacteriochlorophyll Anion Band at 1020 nm Produced by Nuclear Wavepacket Motion in Bacterial Reaction Centers

Abstract

Formation of a nuclear wavepacket on the potential energy surface of the primary electron donor P* induced by 30-fs excitation of pheophytin-modified Rhodobacter sphaeroides R-26 reaction centers leads to a reversible and irreversible appearance of the state P+BA− monitored by the measurements of the 1020-nm band which is characteristic of the radical anion band of bacteriochlorophyll monomer BA−. The reversible appearance of the state P+BA− is characterized by two (130 and 320 cm−1) vibration modes and related to the creation of maximal value of the Franck-Condon factor when the wavepacket is near an intersection of the P* and P+BA− potential energy surfaces. The irreversible formation of the state P+BA− with a time constant of 3 ps is accompanied by oscillations with frequencies of 9 and 33 cm−1. These results show that the nuclear vibration modes (9 and 3 3 cm−1) on the P* potential energy surface have near their bottom the intersection with the P+BA− surface as well. The electron transfer time between P* and BA was estimated to be in the range of 700 fs. This is larger than the wavepacket intersection time for the 130 cm−1 mode (∼100 fs) and comparable with that for the 33 cm−1 mode (∼600 fs). Therefore the reversible electron transfer with small amplitude can be observed for the 130 cm−1 mode, while the irreversible one is allowed for the 33 cm−1 mode that is probably important for the primary charge separation.