PHOTOSYNTHETIC LIGHT-HARVESTING

The peripheral light-harvesting complex of the photosynthetic bacteriu m Rhodopseudomonas acidophila (LH2) and the major plant light-harvesti ng 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 struct ures exhibit a dramatically different arrangement of the pigments. In LH2 the bacteriochlorophyll molecules are arranged in a highly symmetr ic ring, while in LHCII the positioning of the chlorophylls is very ir regular. In both complexes the average distance between the pigments i s 1 nm or less and, as a consequence, the electronic interaction betwe en the pigments is strong (>100 cm(-1)). Therefore, the excitation tra nsport in these photosynthetic light-harvesting systems can not be des cribed by a simple Forster type transfer mechanism, but new or other t ransfer 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 disor der and/or heterogeneity and the nature and strength of the interactio ns of the pigments with the protein. Here we will discuss the current status of the field of photosynthetic energy transfer in particular fo r LH2. We will evaluate a few simple models that contain some of the e ssential ingredients to describe the process of energy transfer and fi nally we will discuss some of the perspectives in this scientific fiel d.