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

The initial electron transfer steps in pigment modified reaction cente rs, where bacteriopheophytin is replaced by plant pheophytin (R26.Phe- a RCs) have been investigated over a wide temperature range by femtose cond time-resolved spectroscopy. The experimental data obtained in the maximum of the bacteriochlorophyll anion band at 1020 nm show the exi stence of a high and long-lived population of the primary acceptor P+B A- even at 10 K. The data suggest a stepwise electron transfer mechani sm with B-A 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 l evels, approximately 450 cm(-1) below P. A Gaussian distribution (wit h sigma = 400 cm(-1)) of the Delta G values has to be assumed to accou nt 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.