The relation of efficiency of excitation energy transfer and ring size of ring-shaped light harvesting complex

The existence of various ring sizes of peripheral light harvesting complex (LH2) in photosynthetic bacteria was reported recently. To investigate the dependence of LH2’s functions as an excitation energy transfer (EET) carrier on its structure, the relation of EET in the systems consisting of ring-shaped aggregates and their ring size is evaluated. Setting the square simulation area (70 nm × 70 nm), we defined the EET efficiency as the rate of population transferred from the area irradiated by the incident light to another area, which is separated by ∼20 nm from the former area, by the time corresponding to the dephasing time of pigments (∼4 ps) after the irradiation of the incident light pulse. As the result, the system composed of large ring with 28 pigments realizes the EET efficiency of 5.3%, 7.0%, and 8.8% in the randomly distributed system of rings when the ring number is 15, 20, and 25, respectively. These values are ∼1.5 and ∼3 times higher than the EET efficiency in the systems composed of rings with 18 pigments and 10 pigments, respectively, when the total numbers of pigments in the simulation area are the same. In the lattice structures, on the other hand, the EET efficiency of the three types of systems shows the similar value of ∼20%. The results suggest that forming a large ring as the unit of EET carrier could be effective to realize efficient EET in the systems containing randomness in distribution of units, whereas in the lattice structure, the EET efficiency may be not critically affected by the ring size of ring-shaped unit carrier.