Induction and anisotropy of fluorescence of reaction center from photosynthetic bacterium Rhodobacter sphaeroides

Abstract

Submillisecond dark-light changes of the yield (induction) and anisotropy of fluorescence under laser diode excitation were measured in the photosynthetic reaction center of the purple bacterium Rhodobacter sphaeroides. Narrow band (1–2 nm) laser diodes emitting at 808 and 865 nm were used to selectively excite the accessory bacteriochlorophyll (B, 800 nm) or the upper excitonic state of the bacteriochlorophyll dimer (P₋, 810 nm) and the lower excitonic state of the dimer (P₊, 865 nm), respectively. The fluorescence spectrum of the wild type showed two bands centered at 850 nm (B) and 910 nm (P₋). While the monotonous decay of the fluorescence yield at 910 nm tracked the light-induced oxidation of the dimer, the kinetics of the fluorescence yield at 850 nm showed an initial rise before a decrease. The anisotropy of the fluorescence excited at 865 nm (P₋) was very close to the limiting value (0.4) across the whole spectral range. The excitation of both B and P₋ at 808 nm resulted in wavelength-dependent depolarization of the fluorescence from 0.35 to 0.24 in the wild type and from 0.30 to 0.24 in the reaction center of triple mutant (L131LH–M160LH–M197FH). The additivity law of the anisotropies of the fluorescence species accounts for the wavelength dependence of the anisotropy. The measured fluorescence yields and anisotropies are interpreted in terms of very fast energy transfer from ¹B* to ¹P₋ (either directly or indirectly by internal conversion from ¹P₊) and to the oxidized dimer.