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

In the antenna system of photosynthetic purple bacteria, bacteriochlorophyl ls (Bchls) are arranged in beautiful nanoscale rings called LH1 and LH2. In two low-lying transition dipoles of each Bchl, the lower-energy larger-mag nitude 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) b etween adjacent BChls. Electronic excitations therein are excitons with a c oherence length over at least several Bchls. Excitation-energy transfer (EE T) takes place rapidly from the outer to the inner ring within LH2, from LH 2 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: rational ized 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 structura l strategies of the antenna system for efficiently capturing, storing and t ransferring solar-radiation energies with the use of quantum-mechanical coh erence among pigments. (C) 2000 Elsevier Science B.V. All rights reserved.