Sol-gel entrapped light harvesting antennas: immobilization and stabilization of chlorosomes for energy harvesting

The chlorosome is a highly specialized supramolecular light-harvesting antenna complex found in green photosynthetic bacteria and is composed of self-assembled bacteriochlorophyll (BChl) pigments entrapped in a lipid vesicle. These organelles are of interest for development of synthetic devices for solar harvesting and conversion because the organization and packing of BChls in the chlorosome provides a highly efficient light collection and energy funneling mechanism with properties that are superior to similar artificial systems based on self-assembled BChl pigment analogues. In this study, we investigated sol-gel chemistry as an approach to entrap and stabilize chlorosomes isolated from Chloroflexus aurantiacus. Two distinct synthesis approaches that differed in the H2O/Si ratio in the gels were investigated. Spectrophotometric analysis showed that the chlorosomes were intact when encapsulated in sol-gels and did not suffer any deleterious effects during the entrapment process. In addition, the integrity of the chlorosomes was unaffected by methanol levels that can result during the formation of sol-gels. Using small-angle neutron scattering it was not only possible to characterize the properties of the sol-gel matrix but also the size, shape and aggregation state of the entrapped chlorosomes. The sol-gels formed at a higher H2O/Si ratio (FH gels) resulted in a more branched gel structure with a larger pore size compared to the gels formed at lower H2O/Si ratio (PH gels). The chlorosomes entrapped in FH gels had dimensions of similar to 16.0 x 51.1 x 180.1 nm which agrees well with the size of chlorosomes previously determined using cryo-transmission electron microscopy, while the chlorosomes in the PH gels appear to be aggregated. The approach described here offers new possibilities for the development of artificial solar-harvesting and energy conversion devices based on naturally occurring photosynthetic systems.