Pigment-pigment Interactions and Energy Transfer in the Antenna Complex of the Photosynthetic Bacterium Rhodopseudomonas Acidophila

Background: Photosynthesis starts with the absorption of solar radiation by antenna pigment molecules. In purple bacteria these chromophores, (bacteriochlorophyll a and carotenoid) are embedded in the membrane; they are non-covalently bound to apoproteins which have the ability to modulate the chromophores' absorbing characteristics. The first structure of the bacterial antenna complex from Rhodopseudomonas acidophila, strain 10050, shows a ring of nonameric symmetry. Two concentric cylinders of apoproteins enclose the pigment molecules. The current resolution of the structure, to 2.5 A, allows us to begin to explore the mechanism of energy transfer among these pigments.

Results: The mechanism of energy transfer, from the short- to long-wavelength-absorbing pigments, is largely determined by the relative distances and orientations of the chromophores. In this paper we provide evidence that energy transfer between the B800 and B850 bacteriochlorophylls is largely via Förster induced dipole-dipole resonance. Strong Coulombic (exciton) coupling among the 18 short distanced chromophores in the B850 macrocycle is promoted by good alignment of the Qy dipoles. Singlet-singlet energy transfer from carotenoid to the B800 macrocycle appears to be minimal, with most of the energy transfer going to B850. The higher energy state of both chromophores dominates in more complex situations.

Conclusions: The structure of the antenna complex not only shows Nature at its most aesthetic but also illustrates how clever and efficient the energy transfer mechanism has become, with singlet-singlet excitation being passed smoothly down the spectral gradient to the reaction centre.