The Prominent Role of Charge Transfer in the Spectral Tuning of Photosynthetic Light-Harvesting I Complex

Purple bacteria possess two ring-shaped protein complexes, light-harvesting 1 (LH1) and 2 (LH2), which exhibit distinct absorption properties and function as antennas for solar energy utilization for photosynthesis. The amount of bacteriochlorophyll (BChl) a is different in the two antennas; however, their relationship with spectral tuning remains elusive. Herein, we report a high-precision evaluation of the physicochemical factors contributing to the difference in absorption maxima between LH1 and LH2 in the model purple bacterium Rhodospirillum rubrum in terms of BChl a structural distortion, protein electrostatic interaction, excitonic coupling, and charge transfer (CT) effects, derived from detailed spectral calculations using an extended version of the exciton model. Spectral analysis confirmed that the electronic structure of the excited state in LH1 extended to the BChl a 16-mer, and further analysis revealed that the CT effect due to the closer inter-BChl distance in LH1 than in LH2 predominantly contributed to the LH1-specific redshift (~61% in energy). Our analysis explains how LH1 and LH2, which possess chemically identical chromophores of BChl a, use distinct physicochemical effects to enable a progressive redshift from LH2 to LH1 for efficient energy transfer to the reaction center special pair.