Structural strategies in the antenna system of photosynthesis on the basis of quantum-mechanical coherence among pigments

In the antenna system of photosynthetic purple bacteria, bacteriochlorophylls (Bchls) are arranged in beautiful nanoscale rings called LH1 and LH2. In two low-lying transition dipoles of each Bchl, the lower-energy larger-magnitude dipole is arranged nearly tangentially to the ring circumference in both LH1 and the inner ring of LH2, with small distances (similar to 9 A) between adjacent BChls. Electronic excitations therein are excitons with a coherence length over at least several Bchls. Excitation-energy transfer (EET) takes place rapidly from the outer to the inner ring within LH2, from LH2 to LH1, and from LH1 to the reaction center. Since these EETs take place to and/or from optically forbidden exciton states, they cannot be rationalized by Forster's formula. A recently proposed new formula for such EETs [H. Sumi, J. Phys. Chem. B 103 (1999) 252] enables us to understand structural strategies of the antenna system for efficiently capturing, storing and transferring solar-radiation energies with the use of quantum-mechanical coherence among pigments. (C) 2000 Elsevier Science B.V. All rights reserved.