Single-molecule study of the electronic couplings in a circular array of molecules: light-harvesting-2 complex from Rhodospirillum molischianum. 
Applying single-molecule spectroscopic techniques allowed us to determine the mutual angles between the transition-dipole moments associated with optical transitions of the eight bacteriochlorophyll a molecules which form the so-called B800 ring of the light-harvesting-2 complex from Rhodospirillum molischianum.  The orientation of the transition-dipole moment is a sensitive probe for the strength of the local intermolecular interactions because of the well-defined arrangement of the individual molecules within the B800 ring.  Our data reveal that the strength of the electronic coupling between individual molecules in the ring is subjected to spatial as well as temporal variations.