A Theoretical Model for Excitation Energy Transfer in Chlorosomes: Lamellar and Rod-Shaped Antenna Structures

Abstract

A model based on exciton theory is presented for description of excitation energy transfer in chlorosomes. Three models to describe the internal organization of the pigments inside the chlorosome were considered, a stack of single-wall rods, a stack of double-wall rods and a stack of lamellae directed along the long axis of the chlorosome. Simulated absorption, circular dichroism and linear dichroism spectra of single-wall rod and the lamella structures turned out to be practically identical. It was shown that rod—rod interactions may localize the exciton states in the regions of a rod facing a neighboring rod. Such localized states provide a fast excitation energy transfer mechanism in perpendicular direction with respect to the long axis of the chlorosome, a mechanism very different from Forster picture. Calculated intra chlorosome energy transfer rates for the lamella structure are very fast, an order of magnitude faster than the calculated intra chlorosome rates for the rod structures with the same inter-antenna distance. Since experimental energy transfer kinetics of chlorosomes is multiexponential with long lived components, then collinear uniform lamellae organization of pigments along the whole length of chlorosome seems unlikely.