Paracrystalline aggregates of bacteriochlorophyll protein from green photosynthetic bacteria

Paracrystalline aggregates of bacteriochlorophyll-protein are observed in preparations of the mother liquor from which crystals have been obtained. These aggregates are confocal in texture but are rigid and fragile as opposed to the classic examples of confocal smectic “liquid crystals.” Observations of birefringence and dichroism indicate that bacteriochlorophyll is molecularly oriented in a filamentous fine texture that constitutes each ordered domain of the aggregates. Electron microscopy of these aggregates shows that this texture is composed of tubules. These tubules are more or less parallel to each other within the small confines of each domain but their lateral spacing is variable and random. Cross sections of the tubules show a hexagonal array of electron-dense elements surrounding an electron-transparent channel. This array seems congruous but not identical to hexagonal units in the lattice observed in cross sections of bacteriochlorophyll-protein crystals. The macromolecular arrangement is considered in terms of the lateral bonding sites in bacteriochlorophyll-protein paracrystals.