Leonas Valkunas

Singlet-singlet annihilation kinetics in aggregates and trimers of LHCII

Singlet-singlet annihilation experiments have been performed on trimeric and aggregated light-harvesting complex II (LHCII) using picosecond spectroscopy to study spatial equilibration times in LHCII preparations, complementing the large amount of data on spectral equilibration available in literature. The annihilation kinetics for trimers can well be described by a statistical approach, and an annihilation rate of (24 ps)(-1) is obtained. In contrast, the annihilation kinetics for aggregates can well be described by a kinetic approach over many hundreds of picoseconds, and it is shown that there is no clear distinction between inter- and intratrimer transfer of excitation energy. With this approach, an annihilation rate of (16 ps)(-1) is obtained after normalization of the annihilation rate per trimer. It is shown that the spatial equilibration in trimeric LHCII between chlorophyll a molecules occurs on a time scale that is an order of magnitude longer than in Photosystem I-core, after correcting for the different number of chlorophyll a molecules in both systems. The slow transfer in LHCII is possibly an important factor in determining excitation trapping in Photosystem II, because it contributes significantly to the overall trapping time.

Nonlinear annihilation of excitations in photosynthetic systems

The theory of the singlet-singlet annihilation in quasi-homogeneous photosynthetic antenna systems is developed further. In the new model, the following important contributions are taken into account: 1) the finite excitation pulse duration, 2) the occupation of higher excited states during the annihilation, 3) excitation correlation effects, and 4) the effect of local heating. The main emphasis is concentrated on the analysis of pump-probe kinetic measurements demonstrating the first two above possible contributions. The difference with the results obtained from low-intensity fluorescence kinetic measurements is highlighted. The experimental data with picosecond time resolution obtained for the photosynthetic bacterium Rhodospirillum rubrum at room temperature are discussed on the basis of this theory.

Picosecond processes in chromatophores at various excitation intensities

The aim of this paper is to review and discuss the results obtained by fluorescence and absorption spectroscopy of bacterial chromatophores excited with picosecond pulses of varying power and intensity. It was inferred that spectral and kinetic characteristics depend essentially on the intensity, the repetition rate of the picosecond excitation pulses as well as on the optical density of the samples used. Taking the different experimental conditions properly into account, most of the discrepancies between the fluorescence and absorption measurements can be solved. At high pulse repetition rate (>106 Hz), even at moderate excitation intensities (1010–1011 photons/cm2 per pulse), relatively long-lived triplet states start accumulating in the system. These are efficient (as compared to the reaction centers) quenchers of mobile singlet excitations due to singlet-triplet annihilation. The singlet-triplet annihilation rate constant in Rhodospirillum rubrum was determined to be equal to 10-9 cm3 s-1. At fluences >1012 photons/cm2 per pulse singlet-singlet annihilation must be taken into account. Furthermore, in the case of high pulse repetition rates, triplet-triplet annihilation must be considered as well. From an analysis of experimental data it was inferred that the singlet-singlet annihilation process is probably migration-limited. If this is the case, one has to conclude that the rate of excitation decay in light-harvesting antenna at low pumping intensities is limited by the efficiency of excitation trapping by the reaction center. The influence of annihilation processes on spectral changes is also discussed as is the potential of a local heating caused by annihilation processes. The manifestation of spectral inhomogeneity of light-harvesting antenna in picosecond fluorescence and absorption kinetics is analyzed.

The dependence of the degrees of sigmoidicities of fluorescence induction curves in spinach chloroplasts on the duration of actinic pulses in pump-probe experiments

Abstract Utilizing a pump-probe double flash method, the shapes of the fluorescence induction curves in spinach chloroplasts (0°C), and the induction ratio R = F M / F 0 ( F 0 and F M are the fluorescence yields when all PS II reaction centers are open, and closed, respectively) were studied as a function of: (1) the duration ( t 1 ) of the pump flash, (2) the number of pump flashes, (3) absence or presence of DCMU, and (4) the time interval Δt between the pump and the probe flashes. In the pump-probe technique, an actinic pulse (P 1 ) of different fluences closes a fraction q of the PS II reaction centers. After a variable time interval Δt a second, weak pulse (P 2 ), is used to measure the variable fluorescence yield F v . The shapes of the fluorescence curves ( F v vs. the fluence of the P 1 pulse) were analyzed in terms of the standard eqution F v = (1 − p ) q /(1 − pq ), where p is a parameter describing the connectivity between different photosynthetic units (Joliot, A. and Joliot, P. (1964) C. R. Acad. Sci. 13, 4622–4625). The shapes of the F v curves are found to depend only on the width ( t 1 ) of the pump pulses. For t 1 ⩽ 300 ps, the curves are exponential in shape with p = 0.0 and R t 1 in the millisecond range, and employing the same probe flash method for measuring the variable fluorescence, the F v curves assume the same familiar sigmoidal shapes as in the case of conventional steady-state illumination. With t 1 in the microsecond range ( t 1 ≈ 0.7−2 μ s), the F v curves are more nearly exponential than sigmoidal in shape with p values generally around ≈0.3 and R t 1 >/ 50 μ s, the fluorescence induction curves are sigmoidal in shape and, generally, p values of 0.55–0.60 are observed with R > 3. A sequential hit model (Valkunas, L., Geacintov, N.E., France, L. and Breton, J. (1991) Biophys. J. 59, 397–408), in which the PS II reaction centers evolve through different states characterized by differences in fluorescence yields, a process characterized by a dark-time t 1 or ≈2–50 μs between two successive hits, can account for these results. This model is consistent with the concept of interunit transfer of excitations. However, the shapes of the fluorescence induction curves cannot provide any information on the value of p within the context of this model.

On the role of spectral and spatial antenna inhomogeneity in the process of excitation energy trapping in photosynthesis

Abstract This paper addresses the temperature dependence of the rate and quantum yield of energy trapping by reaction centres (RCs) in the purple bacterium Rhodospirillum rubrum . A simple kinetic model is presented which takes into account the spectral inhomogeneity of the light-harvesting antenna. The description of the excitation decay kinetics at room temperature, using the excitation migration rate extracted from the singlet-singlet annihilation, indicates that the distance between the RC and the antenna is approximately 1.8 times the lattice spacing of the antenna (assuming a square lattice model). The spectral inhomogeneity of the light-harvesting antenna, as shown by the experimental data, becomes important only at low temperature (less than 100 K). The present model enables us to explain the kinetic measurements at various temperatures (300—4 K) and various initial states of the RCs and is consistent with the results from singlet-singlet annihilation measurements.