Relation between Structure and Functionality in Photosynthetic Antenna Complex of Green Sulfur Bacteria: Efficiency...

Large-scale simulations of light-matter interaction in natural photosynthetic antenna complexes of the Chlorobium tepidum green sulfur bacteria (GSB), containing more than one hundred thousand chlorophyll molecules, comparable with natural size, have been performed. Here, we have modeled the entire process of exciton energy transfer, from sunlight absorption to exciton trapping in the reaction centers (RCs), in the presence of a thermal bath. The energy transfer has been analyzed using the radiative non-Hermitian Hamiltonian and by solving the rate equations for the populations. Sunlight pumping has been modeled as blackbody radiation at T = 5800 K, with an attenuation factor that takes the Sun-Earth distance into account. Cylindrical structures typical of GSB antenna complexes and the dimeric baseplate have been considered. The maximal antenna size, comparable to natural size, includes three adjacent, 4-walled concentric cylinders 1485.7 Å long, arranged over a dimeric baseplate with dimensions of 3075 Å × 1148 Å, for an overall number of molecules greater than 10⁵. Our analysis shows that under natural sunlight, in photosynthetic antennae of GSB, the number of excitations reaching the RC per unit time matches the RC closure rate, and the internal efficiency shows values close to ∼80%. We also considered cylindrical structures where the orientation of the dipoles does not reflect the natural one. Specifically, we vary continuously the angle of the transition dipole with respect to the cylinder's main axis, focusing on the case where all dipoles are parallel to the cylinder axis. We also consider the important case where the dipoles are randomly oriented. In all cases, the light-harvesting efficiency is lower than in the natural structure, showing the high sensitivity of light harvesting to the specific orientation of the dipole moments. Our results provide a better understanding of the relationship between structure and functionality in natural photosynthetic antennae of green sulfur bacteria and could drive the design of efficient light-harvesting devices.