Photosystem I particles containing I P700 per 32 chlorophyll molecules were illuminated at cryogenic temperatures in the presence of sodium dithionite. Under conditions which specifically led to reduction of acceptor a1 (as shown by its characteristic EPR spectrum) optical absorbance changes were detected between 240 and 325 nm. The appearance of these changes correlated closely with the increase in amplitude of the ai EPR signal. The possibility that a quinone‐like species is associated with, or directly involved in intermediary PS I electron flow is discussed.

Photosystem I Primary acceptor Optical difference spectrum Chlorophyll a monomer

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