Electron Transfer in the Reaction Center of the Photosynthetic Bacterium Rb. sphaeroides R-26 Measured by Transient Absorption in the Blue Spectral Range

Abstract

Electron transfer in the reaction center of the purple photosynthetic bacterium Rb. sphaeroides R-26 has been studied at room temperature by transient absorption spectroscopy with a time resolution of 120 fs. Measurements of absorption changes were performed in the range from 400 nm up to 680 nm, after excitation with a laser pulse of 80 fs duration within the absorption band of the bacteriochlorophyll at 800 nm. The excited state of the primary donor, characterized by the absorbance changes extending over the whole spectral range investigated, appeared within 120 fs and gave rise to the bleaching of the Q_x absorption band of bacteriochlorophyll at 600 nm, increased further by electron transfer to bacteriopheophytin in ∼3 ps. Photoreduction of the bacteriopheophytin acceptor detected at 546 nm and 670 nm proceeded with the same time constant. Multiphase absorbance changes were relatively the largest in the blue spectral range between 415 nm and 450 nm. Apart from the immediate absorbance increase due to excitation of the primary donor, another fast increasing phase was detected characterized by a wavelength dependent time constant—from ∼5.5 ps at 415 nm to ∼1.9 ps at 450 nm. Both the photooxidised primary donor and photoreduced bacteriopheophytin contributed to the amplitude of this phase. The electron transfer from the reduced bacteriopheophytin to a quinone acceptor was observed as a decrease in the intensity of the transient absorption bands at about 422 nm and at 670 nm, with disappearance of the bleaching at 546 nm and increase of the bleaching at 600 nm, all in ∼200 ps.