Time-resolved spectroscopy of the excited electronic state of reaction centers of Rhodopseudomonas viridis

The spectral properties of the excited electronic state of the reaction centers of Rhodopseudomonas (Rps.) viridis are studied by dichroic transient absorption spectroscopy with sub-picosecond time resolution. The theoretical analysis of the experimental results allows the assignment of the transient absorption from two dimer bands of the special pair and show its excitonic coupling to other pigments

Orientation of chlorophylls within chloroplasts as shown by optical and electrochromic properties of the photosynthetic membrane

The effects on the optical properties of photosynthetic membranes caused by several types of chlorophyll differing in resonance frequency and in spatial disposition are theoretically analyzed. Using a method of moments and the linear dichroism spectrum of the lamellae, we evaluated the mean angle (phi) between the transition moment of each chlorophyll and the normal to the lamellae. We have confirmed that at about 695 nm the transition moment is in the plane of the lamellae, and outside it for chlorophyll b (phi approximately 48.6 degrees). By integrating over frequency the absorption variations affected by ionophores, we show that they may be ascribed to a Stark effect, and we analyze the dependence of this effect on the orientation of the chlorophylls. From this dependence and the degree of polarization of the Stark effect, we calculate the spatial fluctuations of the angle phi. The calculation shows that a definite value of phi corresponds to each resonance frequency of chlorophyl a found in vivo. This proves that the chlorophylls a are not oriented partly random. For chlorophylls b, on the other hand, phi may fluctuate by some 10 degrees about its mean value. The structural consequences of these results are discussed

A master equation theory of fluorescence induction, photochemical yield, and singlet-triplet exciton quenching in photosynthetic systems.

A master equation theory is formulated to describe the dependence of the fluorescence yield (phi) in photosynthetic systems on the number of photons (Y) absorbed per photosynthetic unit (or domain). This theory is applied to the calculation of the dependence of the fluorescence yield on Y in (a) fluorescence induction, and (b) singlet exciton-triplet excited-state quenching experiments. In both cases, the fluorescence yield depends on the number of previously absorbed photons per domain, and thus evolves in a nonlinear manner with increasing Y. In case a, excitons transform the photosynthetic reaction centers from a quenching state to a nonquenching state, or a lower efficiency of quenching state; subsequently, absorbed photons have a higher probability of decaying by radiative pathways and phi increases as Y increases. In case b, ground-state carotenoid molecules are converted to long-lived triplet excited-state quenchers, and phi decreases as Y increases. It is shown that both types of processes are formally described by the same theoretical equations that relate phi to Y. The calculated phi (Y) curves depend on two parameters m and R, where m is the number of reaction centers (or ground-state carotenoid molecules that can be converted to triplets), and R is the ratio phi (Y leads to infinity)/(Y leads to 0). The finiteness of the photosynthetic units is thus taken into account. The m = 1 case corresponds to the "puddle" model, and m leads to infinity to the "lake," or matrix, model. It is shown that the experimental phi (Y) curves for both fluorescence induction and singlet-triplet exciton quenching experiments are better described by the m leads to infinity cases than the m = 1 case

Analysis of picosecond laser induced fluorescence phenomena in photosynthetic membranes utilizing a master equation approach.

A Pauli master equation is formulated and solved to describe the fluorescence quantum yield, phi, and the fluorescence temporal decay curves. F(t), obtained in picosecond laser excitation experiments of photosynthetic systems. It is assumed that the lowering of phi with increasing pulse intensity is due to bimolecular singlet exciton annihilation processes which compete with the monomolecular exciton decay processes; Poisson statistics are taken into account. Calculated curves of phi as a function of the number of photon hits per domain are compared with experimental data, and it is concluded that these domains contain at least two to four connected photosynthetic units (depending on the temperature), where each photosynthetic unit is assumed to contain approximately 300 pigment molecules. It is shown that under conditions of high excitation intensities, the fluorescence decays approximately according to the (time)1/2 law