PHOTOSYSTEM I (PSI) is the photochemical reaction complex involved in the generation of a low potential reductant in oxygen-evolving photosynthetic organisms. The primary photochemical reaction in PSI has been identified as the photo-oxidation of a reaction centre chlorophyll complex P700 (ref. 1). This photo-oxidation occurs both at room temperature and in the frozen state at temperatures as low as 4.2 K. At cryogenic temperatures this photo-oxidation has generally been found to be irreversible2. Malkin and Bearden3 showed that the electron acceptor for this photo-oxidation reaction is a bound ferredoxin. This ferredoxin has two iron–sulphur centres with very low redox potentials (Em10 = −550 and −590 mV)4–6. It has been proposed that these centres are the primary electron acceptor of PSI3,4. More recent evidence suggests it may not be7–11, and we have shown11 that when the bound ferredoxin is chemically reduced, illumination at low temperature results in the photo-oxidation of P700 without any change in the bound ferredoxin, this oxidation is reversible. In photosynthetic bacteria a component with an EPR signal at g = 1.82 has been identified as the primary electron acceptor of the photochemical system12,13 and it is possible that a similar component might be involved in chloroplast photochemical reactions. McIntosh et al.14 obtained kinetic evidence for the presence in PSI particles of a component with an EPR signal at g = 2.06 and g = 1.76 which showed a reversible redox change on illumination of the particles at low temperature parallel to that of P700. The changes observed were small and it was not possible to obtain a spectrum of the component. Using a more highly purified PSI preparation we have now obtained strong evidence that this component is the primary electron acceptor of PSI.
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Applied Magnetic Resonance (2010)
Photosynthesis Research (2008)