Abstract: | Optical measurements were made during low temperature photoreduction of photosystem I acceptors, A1 and A0. In the presence of a significant amount ofA1 (detected by EPR), no absorbance changes occurred between 750-350 nm, indicating that this species is not a chlorophyll or pheophytin molecule. Spectral changes in this region that may be correlated with the appearance of A−0, suggest that this component is a chlorophyll a anion monomer. The species is present in reaction centres in a ratio of 0.94 Ao/P700. Photosystem I Primary acceptor Optical difference spectrum Chlorophyll a monomer |