Dynamics of energy conversion in reaction center core complexes of the green sulfur bacterium Prosthecochloris aestuarii at low temperature.

Excited-state and electron-transfer dynamics at cryogenic temperature in reaction center core (RCC) complexes of the photosynthetic green sulfur bacterium Prosthecochloris aestuarii were studied by means of time-resolved absorption spectroscopy, using selective excitaton of bacteriochlorophyll (BChl) a and of chlorophyll (Chl) a 670. The results indicate that the BChls a of the RCC complex form an excitonically coupled system. Relaxation of the excitation energy within the ensemble of BChl a molecules occurred within 2 ps. A time constant of about 25 ps was ascribed to charge separation. Absorption changes in the 670 nm region, where Chl a 670 absorbs, were fairly complicated. They showed various time constants and were dependent on the wavelength of excitation and they did not lead to a simple picture of the electron acceptor reaction. Energy transfer from Chl a 670 to BChl a occurred with a time constant of 1.5 ps. However, upon excitation of Chl a 670 the amount of oxidized primary electron donor, P840(+), formed relative to that of excited BChl a was considerably larger than upon direct excitation of BChl a. This indicates the existence of an alternative pathway for charge separation which does not involve excited BChl a.