Supramolecular structure in extramembraneous antennae of green photosynthetic bacteria

Supramolecular structures in extramembraneous antennae of green photosynthetic bacteria (so-called ''chlorosomes'') are reviewed. In chlorosomes, bacteriochlorophylls-c, d and e (magnesium complexes of chlorins with a 1-hydroxyethyl group) self-aggregate to form the main light-harvesting components. This arrangement is unique and different from that in other antennae where the pigments bond with some proteins. Model studies of artificial aggregates and comparison of in-vitro aggregates with in-vivo aggregates provide useful insights in the elucidation of the supramolecular structures. The pigments in natural chlorosomes self-aggregate with the assistance of a special hydrogen-bond, C=O ... H(X)O ... Mg and pi-pi interaction of the cyclic tetrapyrroles. Magnesium chlorins thus form a two-dimensional sheet and subsequent rolling of the sheet makes a rod of the main components in chlorosomes.