Photosynthetic antennae. Photosynthetic light-harvesting

Abstract

The peripheral light-harvesting complex of the photosynthetic bacterium Rhodopseudomonas acidophila (LH2) and the major plant light-harvesting complex LHCII have a very similar function: to absorb solar photons and to transfer the electronic excitation to the pigments surrounding the reaction center, the so-called ‘core’. Nevertheless, their structures exhibit a dramatically different arrangement of the pigments. In LH2 the bacteriochlorophyll molecules are arranged in a highly symmetric ring, while in LHCII the positioning of the chlorophylls is very irregular. In both complexes the average distance between the pigments is 1 nm or less and, as a consequence, the electronic interaction between the pigments is strong ( > 100 cm−1). Therefore, the excitation transport in these photosynthetic light-harvesting systems can not be described by a simple Förster type transfer mechanism, but new or other transfer mechanisms may be operative, for instance a mechanism in which the excitation is to some extent delocalized. Crucial parameters are the strength of the electronic coupling, the amount of energetic disorder and/or heterogeneity and the nature and strength of the interactions of the pigments with the protein. Here we will discuss the current status of the field of photosynthetic energy transfer in particular for LH2. We will evaluate a few simple models that contain some of the essential ingredients to describe the process of energy transfer and finally we will discuss some of the perspectives in this scientific field.