Planar arrangement of bacteriochlorophyll c aggregates in chlorosomes of Chloroflexus aurantiacus.

Chlorosomes are main light-harvesting complexes of green photosynthetic bacteria. Pigments are organized in self-assembling aggregates within chlorosomes rather than being coordinated by proteins as seen for other photosynthetic complexes. Chlorosomes isolated from Chloroflexus aurantiacus were subjected to cryoelectron tomography, which revealed the long-range organization of bacteriochlorophyll c within the chlorosome interior. The resulting electrostatic scattering potential map is well described by a planar lamellar model. Molecular dynamics simulations substantiated an important role of van der Waals interactions in the aggregate formation. van der Waals interactions were shown to be involved also in development of curved geometries that are readily formed in single layers due to lateral interactions between esterifying alcohols. In contrast, the planar arrangement is favored due to interactions with esterifying alcohols from neighboring layers. This provides a plausible mechanistic explanation for the formation of both planar and curved lamellar arrangements which were observed in chlorosomes from different species.