Long-range molecular order as an efficient strategy for light harvesting in photosynthesis

Abstract

Photosynthesis is an extremely efficient converter of light into chemical energy, with an observed quantum yield for primary photochemistry in the region of 90%. To achieve this the photosynthetic apparatus must be highly optimized¹, and some of the design principles that may be involved have been suggested². A search for the realization of these principles in natural photosynthetic units led to the discovery of predicted strong orientational ordering of the long-wave (Q_y) transition moments of light-harvesting bacteriochlorophyll c in the green bacterium Chlorobium limicola^3,4. These transition moments have been shown by linear dichroism to be parallel to each other and ideally oriented along the long axis of the chlorosome. A conclusion of such importance needs to be tested in an independent experiment with intact cells, and this we report here. We have directly measured the picosecond polarized fluorescence kinetics for bacteriochlorophyll emissions upon selective polarized light excitation of the Q_y transition of bacteriochlorophyll c fluorescence polarization value was found to be Chlorobiaceae and Chloroflexaceae. The measured bacteriochlorophyll c fluorescence polarization value was found to be constant during the lifetime of the bacteriochlorophyll c excited state, reaching a limiting value for monomeric bacteriochlorophyll of 0.42. These results demonstrate that in living cells of green bacteria excitation energy transfer within the bacteriochlorophyll c antenna occurs between chromophores (or their associates) with parallel transition moments.