Light collection and harvesting processes in bacterial photosynthesis investigated on a picosecond time scale

Abstract

Fluorescence lifetimes have been determined for four strains of Rhodopseudomonas sphaeroides. Chromatophore samples were excited with a single picosecond flash, and the fluorescence was detected with a streak camera. The decay times are 100 psec in strains 2.4.1 and Ga, and 300 psec in the carotenoidless strain R-26. These times are related to the transfer of energy from the light-harvesting antenna pigment molecules to the photochemical reaction center. In strain PM-8 dpl, which lacks reaction centers, the lifetime is 1.1 nsec. In addition, we have obtained curves relating the quantum yield of fluorescence to the photon density of the excitation pulse. These curves can be fit with a simple model that relates excitonic processes to properties of the photosynthetic unit and that qualitatively describes differences between the mutant strains. Picosecond laser techniques are now being widely applied in this field of biophysics, especially in the area of photosynthesis. Picosecond flash-photolysis techniques have identified transitions occurring in the primary photochemical reaction in photosynthetic bacteria (1, 2), and streak-camera measurement techniques have been used to measure rapid transfer of excitations between chlorophyll molecules in vivo (3-6). Other recent biophysical applications of picosecond or subnanosecond techniques include measurements on hemoglobin (7), visual pigments (8), and DNA (9-11).This paper is concerned with a determination by picosecond techniques of the kinetics with which the light-harvesting antenna pigment molecules of photosynthetic bacteria transfer energy to the photochemical reaction center. We have used two approaches to this problem. In the first approach, the sample is excited with a 20-psec pulse of low intensity and the fluorescence lifetime is measured. This time reflects the rate of depopulation of excited singlet states of bacteriochlorophyll in the antenna, mainly due to their capture by the reaction center. In the second approach, high-intensity pulses are used to generate nonlinear optical effects. High intensities produce a high density of excited states, leading to exciton annihilation processes, as Mauzerall (12) has demonstrated with nanosecond pulses and Campillo et al. (13) with picosecond pulses. Such processes have now been studied by several groups (6, 12-18). When two or more singlet excitations are simultaneously present in the photosynthetic unit (or within a certain diffusion radius), they can migrate into the vicinity of one another, and singlet-singlet annihilation can take place by the reactions Si +Si Sn + SO [1] Sn -"Si + heat [21 Eq. 1 represents a bimolecular fusion process in which a molecule in the first excited singlet state SI transfers its energy to a similarly excited neighboring molecule, promoting it to a higher singlet state S,. Because higher excited singlet states in large molecules such as bacteriochlorophyll almost always decay nonradiatively to the S1 state, as shown in Eq. 2, the net result is the loss of one excited singlet...

Energy migration in purple bacteria. The criterion for discrimination between migration- and trapping-limited photosynthetic units

“…The critical distance for excitation migration (/~r) was calculated in the works of Zankel et al (1974) to be 80 A for three-dimensional and 89 A (Campillo et al 1977) for two-dimensional BChl ensembles. Bearing in mind important findings in Zuber et al (1987), the second value, 89A, will be used below.…”

“…Now let us estimate ~ = 3*. The K~ values were obtained in a number of experiments with purple bacteria (Godik and Borisov 1977) for Rhodospirillum rubrum (Sebban et al 1984), for Rhodobacter sphaeroides (Campillo et al 1977) and for RCfree mutant PM-8. It was measured in these works as 1.6 x 10 ° > K~ > 109s -1.…”

The emission of chlorophyll in vivo

“…In this respect it is of interest to recall that, for chromatophores from various bacteria at the FO level, fluorescence lifetimes of the order of 100-300 ps have been reported [14,34,35] and that they are comparable to the kinetic of primary charge stabilization in the isolated RCs. This model also suggests that the rate-limiting step for the exciton lifetime is neither at the level of energy transfer within the antenna nor limited by the trapping of the exciton by the RC, but is determined by the stabilization of the negative charge on the quinone acceptor.…”

Excitation trapping and primary charge stabilization in Rhodopseudomonas viridis cells, measured electrically with picosecond resolution

“…1 were analyzed by assuming the consecutive reactions Antenna*-PHQ -ly P+H-Q -b. P+HQ-, [1] where k, and k2 are the rate constants. The time-dependent concentrations of the states P+H-Q and P+HQ-will be denoted n1(t) and n2(t), respectively.…”

“…In the case of the photosynthetic bacterium Rhodopseudomonas viridis, the rate of this process is not known. However, in other purple bacteria (R. sphaeroides and Rhodospirillum rubrum), a time constant of 50-100 ps has been deduced by analysis of fluorescence decay kinetics (1)(2)(3).…”