Excitation energy transfer between monomeric bacterioclorophylls in reaction centers of P-less mutant of purple photosynthetic bacteria.

Photosynthetic reaction centers (RCs) are unique natural converters of light energy into chemical energy. Studying the excitation dynamics of these RCs helps better understand the physical mechanisms underlying their organization and function, which may be useful in the creating of their artificial analogs. Using femtosecond differential absorption spectroscopy, we studied the early-time dynamics of excited states of bacteriochlorophyll BA and BB monomers in mutant RCs from purple bacteria Rhodobacter (Rb.) sphaeroides, which lack the bacteriochlorophyll P dimer due to the replacement of valine with arginine at residue L157. Charge separation is absent in these RCs, which cardinally simplifies the study of chromophore excitation dynamics. We found that upon excitation of RCs in the Qy absorption band of B formed by the closely lying bands of BA and BB, the spectrum ΔA(light - dark) rapidly shifts along the wavelength scale, and the direction and magnitude of this shift depend on the pump wavelength λpump and temperature T. At λpump = 780-785 nm, BA is mainly excited, and the spectrum ΔA shifts to the red side in ~ 300 fs by ~ 2.5 nm at 77 K and by ~ 1.5 nm at 295 K. At λpump = 815-820 nm, BB is mainly excited, and at T = 295 K the spectrum ΔA shifts to the blue side by ≤ 1 nm in ~ 300 fs, and this shift is absent at T = 77 K. Global analysis of the ΔA spectra showed that the time constant of these shifts varies from 104 fs at 77 K to 98 fs at 295 K. At 77 K, the red shift is accompanied by weak (~ 2%) oscillations that completely decay within ~ 500 fs. Modeling of the spectral dynamics using kinetic equations showed that the observed spectral shifts may be related to reversible energy transfer between ВА* and ВВ*. Modeling showed that the ratio of forward (ВА*→ ВВ*) and backward (ВВ*→ ВА*) energy transfer rates varies from 2.7 at 295 K to 4.4 at 77 K, which explains the small value of the blue shift of the spectrum ΔA upon excitation of BB. The high rate of energy transfer between BA and BB is an indicator of close interaction between the active (index A) and inactive (index B) branches within a single "supermolecule" formed by the RCs pigments.