Orientation of Photosynthetic Pigments in vivo: Structural and Functional Aspects

Abstract

One of the main clues to the very high efficiency of the primary processes of photosynthesis resides in a precise organization of the pigments and redox carriers. Two main levels can be recognized for this organization: the first one concerns the chlorophyll-protein complexes, the building blocks of the photosynthetic membrane where relative orientations and distances between chromophores are precisely maintained by well-defined binding interactions between selected chemical groups of the pigments and some amino acid residues of the protein chain. Several links, which usually involve the conjugated carbonyls of the various chlorophylls (Lutz M. 1983), insure that the pigments are locked into position. The second main level of organization occurs within the membrane and is due to the amphiphilic nature of the chlorophyll protein complexes which keeps them in a fixed position with respect to the normal to the membrane. However this important restriction (as compared to randomness) still allows the complexes to undergo various other types of motion (rotational and lateral diffusion). Furthermore another type of organization has to be considered in the photosynthetic membrane: it deals with the relative orientations and distances between two adjacent complexes. This last type of organization is probably important in most of the primary processes of photosynthesis: The relative geometry between two antenna complexes plays a role in the process of energy transfer, the relative geometry between an antenna and a reaction center (RC) complex is important for the trapping of the exciton, the arrangement of the redox carriers must be strictly involved in the sequence of charge transfer.