Excitonic Charge Transfer in the Red Chlorophylls of Photosystem I

Photosystem I (PSI) is a natural light harvesting complex that uses precisely arranged chlorophyll molecules to absorb photons, transfer energy toward the PSI reaction center core, and create a charge separated state with high quantum efficiency. The light absorption of the chlorophylls is tuned by their local protein environment as well as by interactions with other nearby chlorophylls. Of particular interest to light absorption are the ‘Red Chlorophyll’ sites within PSI, which expand the spectral range of photosynthesis by capturing lower energy (700-745 nm) photons. While they serve an important function within PSI, the full functionality and characteristics of the Red Chlorophyll sites remain unknown. Here we apply multispectral multidimensional spectroscopy in the visible and mid-infrared regions to PSI, correlating the visible and vibrational spectra of the Red Chlorophyll sites. The spectra reveal that the red sites exhibit mixed Frenkel and charge transfer excitonic features. Understanding the nature of the Red Chlorophyll excited states and the energy flow pathways connecting them to the rest of the photosystem is critical in developing our understanding of solar energy capture. These results provide insight into the biological mechanism of light harvesting at long wavelengths – a vital aspect in designing artificial photosynthetic devices or improving crop yields through genetic engineering.