The Long Wavelength Chlorophylls of Photosystem I

Abstract

In cyanobacteria, longwavelength chlorophylls (LWC) are located in the Photosystem I (PS I) core complex,whereas in plants and algae they are distributed between the PS I core and the light-harvesting complexes (LHC I). LWC are most probably aggregates of (excitonically) coupled chlorophylls, mainly dimers or trimers. The total number of LWC is rather small (≤10% of the total chlorophylls). Depending on their location in the PS I antenna and their distance from P700, they can play a crucial role in the kinetics of energy transfer and in the trapping of the excitation energy by charge separation. Energy absorbed by LWC is transferred uphill to P700 with high efficiency at room temperature, thereby increasing the cross-section for the absorption of red light. LWC are involved also in the dissipation of excess energy, thus protecting the reaction center. Under physiological conditions, the excitations within the PS I antenna are nearly thermally equilibrated over the different spectral forms and the excitation energy is efficiently trapped via charge separation in the reaction center. When the photochemistry in the reaction center is blocked, the excitations migrate to the LWC and are quenched either by P700+ or by the P700 triplet state depending on the state of P700.