TEMPERATURE DEPENDENCE OF THE PRIMARY ELECTRON TRANSFER REACTION IN PIGMENT-MODIFIED BACTERIAL REACTION CENTERS

The initial electron transfer steps in pigment modified reaction centers, where bacteriopheophytin is replaced by plant pheophytin (R26.Phe-a RCs) have been investigated over a wide temperature range by femtosecond time-resolved spectroscopy. The experimental data obtained in the maximum of the bacteriochlorophyll anion band at 1020 nm show the existence of a high and long-lived population of the primary acceptor P+BA− even at 10 K. The data suggest a stepwise electron transfer mechanism with BA as primary acceptor also in the low temperature domain. A detailed data analysis suggests that the pigment modification leads to a situation with almost isoenergetic primary and secondary acceptor levels, approximately 450 cm−1 below P*. A Gaussian distribution (with σ = 400 cm −1) of the ΔG values has to be assumed to account for the strong dispersive character of the kinetics in this sample. Based on these assumptions, a model is presented that reproduces the observed kinetics, heterogeneity and temperature dependence.