Photosynthetic energy transfer and charge separation in higher plants

In this chapter we introduce the physical models at the basis of photosynthetic light harvesting and energy conversion (charge separation). We discuss experiments that demonstrate the processes of light harvesting in the major plant light-harvesting complex (LHCII) and charge separation in the photosystem II reaction center (PSII RC) and how these processes can be modeled at a quantitative level. This is only possible by taking into account the exciton structure of the chromophores in the pigment-protein complexes, static (conformational) disorder, and coupling of electronic excitations and charge-transfer (CT) states to fast nuclear motions. We give examples of simultaneous fitting of linear and nonlinear (timedependent) spectral responses based on modified Redfield theory that resulted in a consistent physical picture of the energy- and electron-transfer reactions. This picture, which includes the time scales and pathways of energy and charge transfer, allows for a visualization of the excitation dynamics, thus leading to a deeper understanding of how photosynthetic pigment-proteins perform their function in the harvesting and efficient conversion of solar energy. We show that LHCII has the intrinsic capacity to switch between different light-harvesting and energydissipating (quenched) states. We introduce the conformational "switching" model for the LHCII protein to explain its role both in light harvesting and in photoprotection. This model explains how the local environment of the protein controls its intrinsic conformational disorder to serve a functional role. Finally, we demonstrate that the PSII RC performs charge separation via two competing pathways of which the selection depends on the conformational disorder induced by slow protein motions. Therefore, we show that the pigment-protein interactions play a decisive role in controlling the functionality of the pigment-protein complexes at work in photosynthesis.