A SINGLE PULSE PICOSECOND LASER STUDY OF EXCITON DYNAMICS IN CHLOROPLASTS

Abstract. Using single picosecond laser pulses at 610 nm, the fluorescence yield (φ) of spinach chloroplasts as a function of intensity ( I ) (10¹²-10¹⁶ photons/pulse/cm²) was studied in the range of 21–300 K. The quantum yield decreases with increasing intensity and the φ vs I curves are identical at the emission maxima of 685 and 735 nm. This result is interpreted in terms of singlet exciton-exciton annihilation on the level of the light-harvesting pigments which occurs before energy is transferred to the Photosystem I pigments which emit at 735 nm.
The yield φ is decreased by factors of 12 and 43 at 300 and 21 K, respectively. The shapes of the φ vs I curves are not well accounted for in terms of a model which is based on a Poisson distribution of photon hits in separate photosynthetic units, but can be satisfactorily described using a one-parameter fit and an exciton-exciton annihilation model. The bimolecular annihilation rate constant is found to be γ= (5–15) times 10^-9cm³s^-1 and to exhibit only a minor temperature dependence. Lower bound values of the singlet exciton diffusion coefficient (≥ 10^-3cm²s^-1), diffusion length (≥ 2 times 10^-6cm) and Förster energy transfer rates (≥ 3 ≥ 10¹⁰s^-1) are estimated from γ using the appropriate theoretical relationships.     

Dynamic and Structural Aspects of Energy Transport in Green Plants

In photosynthesis, only a small part of the pigment molecules (less than 1%) is directly involved in the primary photochemical reactions. These specialized molecules are located in definite centers (reaction centers: RC). The other pigment molecules form an antenna which first collects the light energy and then distributes the singlet excitation energy to the RC's. A RC and its (by relation of vicinity) associated part of the antenna are called a “photosynthetic unit” (PSU). The concept of PSU was born after the experiments of Emerson and Arnold¹ and has been developed in the thirties^2–3. Franck and Teller⁴ were the first to introduce in photosynthesis the concept of exciton migration. From that time an important theoretical work has been devoted to the problem of energy transfer in photosynthesis (see the review of Knox⁵). In this field the goal of the theoretician is to establish the master equation that governs teh excitation motion. The structural parameters of this equation have an important part from the biological point of view. As a matter of fact the collection antenna exhibits a short and a long distance degree of order. This spatial organization conditions the distribution of excitation energy to the Rc's. Consequently the study of energy transfer is inseparable from a structural analysis of the primary photochemical apparatus. In the following we will linit the discussion to the problem of gree plants.