Transfer and Trapping of Excitations in Plant Photosystems

Abstract

In this chapter we discuss the transfer and trapping of excitation energy by plant Photosystem I and Photosystem II. For both photosystems structural models are now available that allow detailed modeling of their spectroscopic and energy transfer properties. In the core of Photosystem I the energy transfer process is fast, and is characterized by a first passage time of less than 10 ps. On a similar time scale the excitation energy may be localized on the conspicuous red chlorophylls (Chi; i.e. Chls absorbing at lower energy than the photochemical trap, P700) present in Photosystem I. Slower equilibration with even redder pools of Chls occurs in competition with trapping of excitation. The overall decay time of the excitation takes several tens of ps, depending on the amount and color of the red Chls. Also within the core antenna proteins of Photosystem II, CP43 and CP47, the energy transfer is fast. However, due to the large distance between the Chls of CP43 and CP47 and the cofactors in the D1D2 reaction center, the overall trapping time in the core of Photosystem II is much slower than in Photosystem I. In the peripheral light-harvesting complexes of Photosystem I (LHC I) and Photosystem II (LHC II, CP24, CP26, CP29) the energy transfer is characterized by a multitude of timescales, largely due to the spread in distances and orientations (at least in LHC II). The energy transfer in LHC II must be considered as a mixture of 'excitonic relaxation' and 'Forster hopping.' The intercomplex rates of energy transfer from LHC II monomer to monomer, between LHC II trimers and probably towards and away from the minor complexes CP24, CP26 and CP29 are slow, several tens of ps. Consequently the migration of the excitation through the Photosystem II supercomplex contributes heavily to the observed slow overall trapping time in intact Photosystem II.