Zoltán Filus

Quinone-dependent delayed fluorescence from the reaction center of photosynthetic bacteria.

Millisecond delayed fluorescence from the isolated reaction center of photosynthetic bacteria Rhodobacter sphaeroides was measured after single saturating flash excitation and was explained by thermal repopulation of the excited bacteriochlorophyll dimer from lower lying charge separated states. Three exponential components (fastest, fast, and slow) were found with lifetimes of 1.5, 102, and 865 ms and quantum yields of 6.4 x 10(-9), 2.2 x 10(-9), and 2.6 x 10(-9) (pH 8.0), respectively. While the two latter phases could be related to transient absorption changes, the fastest one could not. The fastest component, dominating when the primary quinone was prereduced, might be due to a small fraction of long-lived triplet states of the radical pair and/or the dimer. The fast phase observed in the absence of the secondary quinone, was sensitive to pH, temperature, and the chemical nature of the primary quinone. The standard free energy of the primary stable charge pair relative to that of the excited dimer was -910 +/- 20 meV at pH 8 and with native ubiquinone, and it showed characteristic changes upon pH and quinone replacement. The interaction energy ( approximately 50 meV) between the cluster of the protonatable groups around GluL212 and the primary semiquinone provides evidence for functional linkage between the two quinone binding pockets. An empirical relationship was found between the in situ free energy of the primary quinone and the rate of charge recombination, with practical importance in the estimation of the free energy levels from the easily available lifetime of the charge recombination. The ratio of the slow and fast components could be used to determine the pH dependence of the free energy level of the secondary stable charge pair relative to that of the excited dimer.

Comparison of Energetics of P*Q A → P + Q A - and P*Q B → P + Q B - Charge Separation by Detection of Delayed Fluorescence of the Bacteriochlorophyll Dimer in Reaction Centers of Rb. sphaeroides

Flash absorbed by the reaction center (RC) initiates electron transfer from the excited bacteriochlorophyll dimer (P*) to the adjacent cofactors (bacteriopheophytin I, primary quinone QA and secondary quinone Q8) (Fig. 1). The free energy levels of the charge separated states P+Q A - and P+Q B - relative to that of the P*Q excited state can be directly determined from delayed luminescence (DL) of P* (1–3) and related to results obtained from proton binding measurements (2–4). Open image in new window Figure 1 Origin of the leakage-type delayed fluorescence and the free energy levels of the redox states P+QH- and PQAQ, relative to that of the excited (P*Q) state of the RC with approximate numerical values of the rate constants. Note that there is no direct recombination from charge separated state P+Q B - .