Photosynthetic Energy Transfer and Charge Separation in Higher Plants

Abstract

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

Charge transfer states in phycobilisomes

“…Furthermore, the mixing strength determines the polarity of the hybrid state [39,40], which is very sensitive to the charged protein microenvironment of the embedded pigments. The spectroscopic properties of the hybrid state, especially its energy, are therefore very sensitive to the mixing strength and consequently also to dynamic and static disorder [44,47]. An enhanced sensitivity to dynamic disorder explains well the broad spectral widths of the far-red states (Fig.…”

“…This mixing generates a CT state with a weak emissive character as well as one or more excited states with some CT character [39][40][41], the latter of which are sufficiently low in energy to be populated. Furthermore, CT states are known to couple more strongly to fast environmental vibrations (phonons) than the excited states of the same pigments [42][43][44], giving rise to significant homogeneous line broadening [40,42,45] and increased optical reorganization energy, leading to an enhanced Stokes shift [39][40][41][42]46]. The far-red states of individual PBs clearly show evidence of large Stokes shifts (Fig.…”

How reduced excitonic coupling enhances light harvesting in the main photosynthetic antennae of diatoms

“…Such disorder-induced spectroscopic variations are averaged out by standard bulk spectroscopy approaches but are accessible by single-molecule spectroscopy (SMS). In addition, SMS enables one to distinguish between complex-to-complex heterogeneity and spectroscopic changes occurring within a sin-gle complex on a millisecond-to-minute timescale, the timescale of slow protein dynamics (28,29).…”