Significant Effects of Excitonic Coupling and Charge Transfer on the Circular Dichroism Spectrum of Photosynthetic...

Spectral properties of light-harvesting (LH) complexes are key to understanding excitation-energy transfer in photosynthesis. This study examines the physicochemical factors that shape the circular dichroism (CD) spectrum of LH1 in Rhodospirillum rubrum. Calculations using four computational models reveal that intermolecular electronic interactions, particularly excitonic coupling, primarily determine the LH1 CD spectrum shape, while charge transfer effects contribute significantly to the redshift of the peaks. Another finding is that the LH1-specific spectrum shape arises from excitonic coupling extending over 12 bacteriochlorophyll (BChl) a molecules. Further computational analysis additionally identified an anticlockwise rotation of the transition dipole moments of the BChl a assembly as a contributing factor. Our results demonstrate how slight variations in intermolecular orientation influence CD spectrum shape, offering insight into the mechanisms governing the optical properties of light-harvesting antennas. Our computational approach advances the systematic study of CD spectrum shapes associated with chromophore aggregation.