The nature of the lower excited state of the special pair of bacterial photosynthetic reaction center of Rhodobacter Sphaeroides and the dynamics of primary charge separation

Dynamics of the Excited State in Photosynthetic Bacterial Reaction Centers

“…It populates mixed states that have contributions from all the bacteriochlorophylls and bacteriopheophytins of the complex, including many different vibronic levels of P L and P M . The finding that the HOMO concentrates largely on P L but the LUMO on P M ,,,, reflects this mixing. The reaction center’s long-wavelength band peaks about 1000 cm –1 to the red of the Q y band of monomeric bacteriochlorophyll at room temperature and has the unusual property of moving further to the red at low temperatures. − Although exciton interactions undoubtedly play a major role in determining the position of the band, and the temperature dependence of the spectrum could possibly be explained by effects of thermal contraction or anharmonic vibrational modes on these interactions, , mixing of exciton and CT transitions probably provides the most economical explanation of these features along with the width and shape of the band and observations from Stark, IR, and hole-burning and linear-dichroism spectroscopy. ,,,− However, this rationalization of the reaction center’s spectroscopic properties requires the 0–0 level of the diabatic P L + P M – CT state to be above the lowest exciton transition in energy.…”

“…Probably the simplest explanation of the pump-probe and 2D spectroscopic signals that suggest conversion of P* to P L + P M – or, more accurately, suggest a time-dependent increase in the CT character of the excited state is a nuclear relaxation that favors transfer of negative charge to P M . , Rotation of this bacteriochlorophyll’s acetyl group or axial histidine ligand might play such a role. , Nuclear relaxations, however, require transfer of energy to the surroundings, which typically occurs on longer time scales of 1–10 ps. The persistence of vibrational coherences for several picoseconds after impulsive excitation provides a measure of the rate of this process in reaction centers. ,− Although a nuclear relaxation that proceeds through a conical intersection could be exceptionally rapid, most of the IR absorption bands associated with P have linewidths on the order of 20 cm –1 , − indicating that the excited vibrational states probably survive for several picoseconds.…”

“…When it is excited by light, P transfers an electron to a neighboring bacteriochlorophyll (B L ), forming a radical pair (P + B L – ) that sends an electron on to a chain of secondary carriers. Several investigators have proposed that the initial reaction proceeds in two discrete steps. − They suggest that excitation first promotes P to its lowest exciton state (P*), after which an electron moves from P L to P M to form an internal charge-transfer (CT) state (P L + P M – ), and that the anionic radical P M – then transfers an electron to B L : …”

“…During the first 200 fs after the reaction centers are excited in their long-wavelength absorption band, stimulated emission from the excited state weakens and shifts to the red, while the absorbance increases around 1080 nm and in IR vibrational bands between 1000 and 1600 cm –1 . − ,,− These events precede absorbance changes that signal reduction of B L and the next electron acceptor, which takes several picoseconds. Although identifications of the putative intermediate as P L + P M – have been tentative, quantum calculations indicate that a CT state mixes strongly with the P* exciton state and that electron density moves from P L to P M when the dimer is excited. ,− This movement of charge could facilitate transfer to B L because B L is closer to P M than to P L . ,− Rapid electronic relaxation from P* to P L + P M – also has been suggested as a possible explanation for the long-wavelength absorption band’s broad homogeneous width and relatively small cross sections for resonance Raman absorption. − …”