Biochimica et Biophysica Acta 807 (1985) 221-229 221 Elsevier BBA41739 Kinetics of picosecond bacteriochlorophyll luminescence in vivo as a function of the reaction center state A.Yu. Borisov a, A.M. Freiberg b,,, V.I. Godik a, K.K. Rebane b and K.E. Timpmann b A.N. Belozersky Laboratory of Molecular Biology and Bioorganic Chemistry, Moscow State University, Moscow 117234 and b Institute of Physics of the Estonian SSR Academy of Sciences, Tartu 202400 (U.S.S.R.) (Received July 17th, 1984) (Revised manuscript received December 11 th, 1984) Key words: Bacterial photosynthesis; Luminescence; Reaction center; Excitation energy transfer Bacteriochiorophyll (BChl) luminescence lifetimes (~) were measured in purple bacteria Rhodospirillum rubrum and Rhodopseudomonas sphaeroides at low-excitation pulse energy with the use of a picosecond luminescence spectrochronograph of high sensitivity and high time-resolution. Average high-frequency excitation light density was changed from about 1 • 10 t3 photons • cm -2 • s -I up to 1 • 1017 photons ° cm -2 • s- i. Maximal energy density in a single pulse was in the range 10-14-10-10 J/cm 2, which completely rules out nonlinear exciton interactions. In this range ~ increased as a function of excitation light density from about 60 ps to 210 ps. Luminescence yield (~b) for the bacteria investigated measured under continuous or picosecond excitation changed in a similar manner as ~. The luminescence increase was shown to accompany the conversion of the reaction centers to the closed, photooxidized state. Luminescence decay of R. rubrum and Rps. sphaeroides chromatophores without any chemical additions was well approximated by a single exponential component both at low and at saturating intensities of exciting light. The time necessary for the primary charge separation to occur was shown to be 60 + 10 ps. The pairwise jump-time of excitation-energy transfer, as well as excitation-diffusion characteristics were estimated from these data. On the basis of life-time measurements in the state of active photosynthesis, the quantum yield of the primary charge separation in the reaction centers was estimated to be equal to 0.95 _+ 0.02. In intact cells as well as in chromatophores in the presence of reducing agents, a nanosecond component of emission decay was also observed. The relative amplitude of this component, being several percent of the picosecond one at low-excitation intensity levels, increased (2-3)-times with excitation density. Its life-time was estimated to be 3 _+ 1 ns. The nanosecond component appeared only under conditions when a part of the reaction centers were converted to the closed state PQ-. Introduction There is a convincing body of evidence that the two main functions that chlorophyll (Chl) or * To whom correspondence should be addressed. Abbreviations: Chl, chlorophyll; BChl, bacteriochlorophyll; TMPD, tetramethyl-p-phenylenediamine. bacteriochlorophyll (BChl) performs during photo- synthesis, viz., light absorption and excitation en- ergy transductions, are performed by two different types of Chl protein: light quanta are absorbed and collected by the so-called antenna or light- harvesting Chl, while excitation energy is trans- formed into an electrochemical form with par- ticipation of chlorophyllous pigments of the reac-