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The Design of Natural Photosynthetic Antenna Systems

The nanoscale dimensions of the photosynthetic complexes are of critical importance for their effective function. Ultrafast spectroscopic studies by ourselves and by other groups, advances in obtaining high-resolution structural models, and more realistic light-harvesting models have revealed that the electronic interactions between the light-absorbing components in photosynthetic antenna complexes and reaction centers (RCs) are both qualitatively and quantitatively different than in chemical model systems where chromophores are spaced by distances that are large compared with their size. As a result, there are only a few cases in which the energy transfer within natural photosynthetic antenna can be correctly characterized by conventional Forster theory. In order to obtain a detailed understanding of the interactions between photosynthetic pigments embedded in protein complexes, two additional photosynthetic design features must be taken into account. One is that they utilize confined molecular geometries in which antenna molecules are spaced by distances that are small compared with their size. Such intermolecular distances enable transitions and energy levels that would be ineffective or even inoperative in systems with widely spaced chromophores to play crucial roles in light harvesting and energy transfer. The second is that they exploit energetic disorder in order to improve spectral coverage, reduce energy mismatches, and broaden excitonic manifolds to make the system exceedingly robust with respect to temperature variation and fluctuations in site energy and electronic coupling. Paradoxically, the same pigment interactions that can be used to optimize photosynthesis can also generate efficient photoprotection. Each of these points will be described and exemplified by theoretical and experimental work done in our laboratory on bacteria, cyanobacteria, and plants. 67