Bacteriochlorophyll fluorescence of purple bacteria at low redox potentials. The relationship between reaction center triplet yield and the emission yield.

This work describes fluorescence yield measurements in suspensions of strains of Rhodospirillum rubrum and Rhodopseudomonas sphaeroides in which the iron . quinone complex (X) was chemically reduced (state [PIX-]; P is the reaction center bacteriochlorophyll dimer, I is the long wavelength bacteriopheophytin), and compares these with the fluorescence observed when all the traps are open (state [PIX]) and with the fluorescence observed when all the traps are closed (state [P+IX]). At 77 K the amplitude and the shape of the fluorescence emission spectrum in [PIX-] are identical to those observed in [PIX]. This is a strong indication that all the extra fluorescence observed at room temperature in [PIX-] is, in fact, caused by an efficient back reaction [P+I-X-] leads to [PIX-]. Using an equation similar to the original Vredenberg-Duysens relationship (Vredenburg, W.J. and Duysens, L.N.M. (1963) Nature 197, 355-357) but now assuming that a single reaction center has a probability pt of trapping an excitation and (1--pt) of re-emitting it to the surroundings, we are able to calculate pt as a function of the temperature by measuring the fluorescence in [PIX], [PIX-] and [P+IX] as a function of the temperature. The calculated pt values agree reasonably well with triplet yields measured in isolated reaction centers. Finally, we have measured the reaction center triplet yield (PTR) in intact systems and we have shown that the sum of the triplet yield and the remaining loss processes (PL) in the antenna bacteriochlorophyll including the bacteriochlorophyll dimer (such as fluorescence, internal conversion or direct triplet formation) is approximately constant; if we assume that at 77 K the only process which occurs in the reaction center is the formation of a reaction center triplet, than PTR + PL=1. The energy barrier between [PIX-] and [P+I-X-] was estimated to be 0.11--0.15 eV for a set of preparations.