Structure-based calculation of pigment-protein and excitonic pigment-pigment coupling in photosynthetic light-harvesting complexes

Abstract

In photosynthesis, specialized pigment-protein assemblies, termed light-harvesting complexes (LHCs) or antenna proteins, absorb solar photons and deliver the excitation energy by exciton transfer (XT) to the photochemically active centers. To understand this process, it has to be linked to molecular structures and spectroscopic properties of LHCs. In this chapter, we show how this link is provided by theoretical modeling. We first describe what excitons are and how the mechanism of XT is influenced by the coupling of excited states to molecular vibrations. This description defines key parameters that are important for a modeling of XT: (1) site energies, (2) excitonic couplings, and (3) exciton-vibrational coupling constants. Next, we discuss how these parameters can be calculated from a crystal structure of the LHC within the framework of an electrostatic model of intermolecular interactions. Finally, we show applications to the Fenna-Matthews-Olson (FMO) protein of green sulfur bacteria and the major LHC of higher plants (LHCII) to illustrate how this approach works and what has already been learned from it.