Electron spin resonance spectrum of species "X" which may function as the primary electron acceptor in photosystem I of green plant photosynthesis.

Purified Photosystem I particles from spinach when reduced with 10 mM dithionite at pH 9 exhibited a 50% light reversible-ESR Signall (P-700+) at about 10 K. It was possible to show by signal-averaging techniques that a light-reversible ESR spectrum concomitant with the reversible Single 1 can be observed with approximate principal g factors at g = 2.07, g = 1.86 and g = 1.75.