Influence of Phospholipid Composition on Self-Assembly and Energy-Transfer Efficiency in Networks of Light-Harvesting 2 Complexes

In the photosynthetic membrane of purple bacteria networks of light-harvesting 2 (LH2) complexes capture the sunlight and transfer the excitation energy. In order to investigate the mutual relationship between the supramolecular organization of the pigment–protein complexes and their biological function, the LH2 complexes were reconstituted into three types of phospholipid membranes, consisting of l-α-phosphatidylglycerol (PG), l-α-phosphatidylcholine (PC), and l-α-phosphatidylethanolamine (PE)/PG/cardiolipin (CL). Atomic force microscopy (AFM) revealed that the type of phospholipids had a crucial influence on the clustering tendency of the LH2 complexes increased from PG over PC to PE/PG/CL, where the LH2 complexes formed large, densely packed clusters. Time-resolved spectroscopy uncovered a strong quenching of the LH2 fluorescence that is ascribed to singlet–singlet and singlet–triplet annihilation by an efficient energy transfer between the LH2 complexes in the artificial membrane systems. Quantitative analysis reveals that the intercomplex energy transfer efficiency varies strongly as a function of the morphology of the nanostructure, namely in the order PE/PG/CL > PC > PG, which is in line with the clustering tendency of LH2 observed by AFM. These results suggest a strong influence of the phospholipids on the self-assembly of LH2 complexes into networks and concomitantly on the intercomplex energy transfer efficiency.