Ultrafast excitation energy transfer dynamics in photosynthetic pigment–protein complexes

Abstract

Photosynthetically active membranes of certain bacteria and higher plants contain antenna systems which surround the reaction center to increase its absorption cross section for the incoming sun light. The excitation energy created in the antenna pigments is transferred via an exciton mechanism to the reaction center where charge separation takes place. Sub-picosecond laser spectroscopy makes it possible to follow the initial dynamic events of excitation energy (exciton) motion and exciton relaxation in real time. On the other hand, the success of structure resolution opened the door to the microscopic understanding of spectroscopic data and to an appreciation of the structure–function relationship realized in different systems. Here, it will be demonstrated how the combination of microscopically based theoretical models and numerical simulations pave the road from spectroscopic data to a deeper understanding of the functionality of photosynthetic antenna systems. The density matrix technique is introduced as the theoretical tool providing a unified description of the processes which follow ultrafast laser excitation. This includes in particular coherent exciton motion, vibrational coherences, exciton relaxation, and exciton localization. It can be considered as a major result of recent investigations that a theoretical model of intermediate complexity was shown to be suitable to explain a variety of experiments on different photosynthetic antenna systems. We start with introducing the structural components of antenna systems and discuss their general function. In the second part the formulation of the appropriate theoretical model as well as the simulation of optical spectra is reviewed in detail. Emphasis is put on the mapping of the complex protein structure and its hierarchy of dynamic phenomena onto models of static and dynamic disorder. In particular, it is shown that the protein spectral density plays a key role in characterizing excitation energy dissipation. The theoretical concepts are illustrated in the third part by results of numerical simulations of linear and nonlinear optical experiments for three types of antennae: the peripheral light-harvesting complex 2 of purple bacteria, the Fenna–Mathew–Olson complex of green bacteria, and the light-harvesting complex of photosystem II of green plants.