Unraveling the electronic structure of individual photosynthetic pigment-protein complexes

Low-temperature single-molecule spectroscopic techniques were applied to a Light-harvesting pigment-protein complex (LH2) from purple photosynthetic b acteria. The properties of the electronically excited states of the two cir cular assemblies (B800 and B850) of bacteriochlorophyll a (BChl a) pigment molecules in the individual complexes were revealed, without ensemble avera ging. The results show that the excited states of the B800 ring of pigments are mainly Localized on individual BChl a molecules. In contrast, the abso rption of a photon by the B850 ring can be consistently described in terms of an excitation that is completely delocalized over the ring. This propert y may contribute to the high efficiency of energy transfer in these photosy nthetic complexes.