Fluorescence and Energy Transfer in Photosynthesis

Fluorescence decay kinetics of chlorophyll in photosynthetic membranes

The absorption of light by the pigments of photosynthetic organisms results in electronic excitation that provides the energy to drive the energy-storing light reactions. A small fraction of this excitation gives rise to fluorescence emission, which serves as a sensitive probe of the energetics and kinetics of the excited states. The wavelength dependence of the excitation and emission spectra can be used to characterize the nature of the absorbing and fluorescing molecules and to monitor the process of sensitization of the excitation transfer from one pigment to another. This excitation transfer process can also be followed by the progressive depolarization of the emitted radiation. Using time-resolved fluorescence rise and decay kinetics, measurements of these processes can now be characterized to as short as a few picoseconds. Typically, excitation transfer among the antenna or light harvesting pigments occurs within 100 psec, whereupon the excitation has reached a photosynthetic reaction center capable of initiating electron transport. When this trap is functional and capable of charge separation, the fluorescence intensity is quenched and only rapidly decaying kinetic components resulting from the loss of excitation in transit in the antenna pigment bed are observed. When the reaction centers are blocked or saturated by high light intensities, the photochemical quenching is relieved, the fluorescence intensity rises severalfold, and an additional slower decay component appears and eventually dominates the decay kinetics. This slower (1-2 nsec) decay results from initial charge separation followed by recombination in the blocked reaction centers and repopulation of the excited electronic state, leading to a rapid delayed fluorescence component that is the origin of variable fluorescence. Recent growth in the literature in this area is reviewed here, with an emphasis on new information obtained on excitation transfer, trapping, and communication between different portions of the photosynthetic membranes.