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 ofpheophytin-modifiedRhodobacter sphaeroides R-26 reaction centers leads to a reversible and irreversible appearance of the state P+B A ' monitored by the measurements of the 1020-nm band which is characteristic of the radical anion band of bacteriochlorophyll monomer B A ' . The reversible appearance of the state P+B A ' is characterized by two (130 and 320 em") 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+B A ' potential energy surfaces. The irreversible formation of the state P+B A ' with a time constant of 3 ps is accompanied by oscillations with frequencies of 9 and 33 cm'l. These results show that the nuclear vibration modes (9 and 33 ern") on the p* potential energy surface have near their bottom the intersection with the P+B A ' surface as well. The electron transfer time betweenP* and Bx was estimated to be in the range of700 fs. This is larger than the wavepacket intersection time for the 130 cm'l mode (~100 fs) and comparable with that for the 33 cm'l mode (~600 fs). Therefore the reversible electron transfer with small amplitude can be observed for the 130 cm'l mode, while the irreversible one is allowed for the 33 cm'l mode that is probably important for the primary charge separation. 709

Mechanism of Charge Separation and Stabilization of Separated Charges in Reaction Centers ofChloroflexusaurantiacusand of YM210W(L) Mutants ofRhodobactersphaeroidesExcited by 20 fs Pulses at 90 K

“…To study these possibilities, femtosecond (fs) coherent spectroscopy and the frequency analysis of vibration and rotation of the molecular groups involved in charge separation have recently been applied. [21][22][23][24][25][26][27][28][29] This approach in combination with site-directed mutagenesis of the RC protein offers an opportunity to address the problem of stabilization of separated charges as well.…”

Nuclear Wave Packet Motion between P* and P+BA- Potential Surfaces with a Subsequent Electron Transfer to HA in Bacterial Reaction Centers at 90 K. Electron Transfer Pathway

“…It was found that in RCs the wave packet is initially formed by femtosecond light pulses on the 130-140 cm -1 potential energy hypersurface of P*. The subsequent motion on this surface leads the wave packet to the intersection region between P* and P + B Alocated close to the long-wavelength side (935 nm) of the stimulated emission band of P* (3,(31)(32)(33)(34). At 120 fs and mostly at 380 fs delays the wave packet reflects from the P* surface wall and is not stabilized onto the P + B Asurface at room temperature.…”

WAVE PACKET MOTIONS COUPLED TO ELECTRON TRANSFER IN REACTION CENTERS OF CHLOROFLEXUS AURANTIACUS

“…sphaeroides R-26 RCs by measuring oscillations in the kinetics of the absorbance increase at 1,020 nm that is characteristic for B − A radical anion formation. 5,24,[68][69][70][71] The oscillating motions of the wave packet on the P * potential energy surface towards the long-wavelengthstimulated emission region (935 nm) were shown to be accompanied by a reversible electron transfer from P * to B A . This results in modulated formation of the 1,020-nm absorption band of B − A with the initial period of about 260 fs.…”

“…63,64 Coherent components have also been observed in the kinetics of population of the product charge-separated states, P + B − A and P + H − A . [67][68][69][70][71][72] Modulation of the population of the P + B − A state has been detected in Rb. sphaeroides R-26 RCs by measuring oscillations in the kinetics of the absorbance increase at 1,020 nm that is characteristic for B − A radical anion formation.…”

“…This results in modulated formation of the 1,020-nm absorption band of B − A with the initial period of about 260 fs. [68][69][70][71] The first approaches of the wave packet to the intercrossing area ("transition zone") between the P * and P + B − A potential energy surfaces at 120 fs and 380 fs lead to only partial stabilization of the separated charges in P + B − A . The stabilization process is completed within ∼1.5 ps at cryogenic temperatures due possibly to a reorientation of the polar OH group of tyrosine M210, resulting in a dynamic lowering of the free energy of P + B − A with respect to that of P * .…”