Formation of bacteriochlorophyll anion band at 1020 nm produced by nuclearwavepacket motion in bacterial reaction centers

Formation of a nuclear wavepacket on the potential energy surface of the pr imary electron donor P* induced by 30-fs excitation of pheophytin-modified Rhodobacter sphaeroides R-26 reaction centers leads to a reversible and irr eversible appearance of the state P+BA- monitored by the measurements of th e 1020-nm band which is characteristic of the radical anion band of bacteri ochlorophyll monomer B-A(-) The reversible appearance of the state P+BA- is characterized by two (130 and 320 cm(-1)) vibration modes and related to t he creation of maximal value of the Franck-Condon factor when the wavepacke t is near an intersection of the P* and P+BA- potential energy surfaces. Th e 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 res ults show that the nuclear vibration modes (9 and 33 cm(-1)) on the P* pote ntial energy surface have near their bottom the intersection with the P+BA- surface as well. The electron transfer time between P* and B-A was estimat ed to be in the range of 700 fs. This is larger than the wavepacket interse ction time for the 130 cm(-1) mode (similar to 100 fs) and comparable with that for the 33 cm(-1) mode (similar to 600 fs). Therefore the reversible e lectron transfer with small amplitude can be observed for the 130 cm(-1) mo de, while the irreversible one is allowed for the 33 cm(-1) mode that is pr obably important for the primary charge separation.