ENERGY-LEVEL OF P- WITH RESPECT TO P-ASTERISK FOUND FROM RECOMBINATION FLUORESCENCE MEASUREMENTS IN PHEOPHYTIN MODIFIED REACTION CENTERS(B)

Recent studies of reaction centers from Rhodobacter. sphaeroides (R-26 ), in which bacteriopheophytins a were replaced by plant pheophytins a , have shown that, at low temperatures, the excited state of the prima ry electron donor P is converted to the state P+BA- A (where B-A is a bacteriochlorophyll a monomer in the branch A) which has a long lifet ime (about 600 ps). This allows the direct measurement of the free ene rgy difference between P and P+BA- using the temperature dependence o f the recombination fluorescence from P+BA-. The data show that P+BA- is located below P by 550 +/- 30 mV. Therefore, the primary conversio n of P leads to the formation of P+BA-, which is below P* in energy a nd is a real intermediate in the electron transfer.