PHOTOACCUMULATION IN PHOTOSYSTEM-I DOES PRODUCE A PHYLLOQUINONE (A(1)(CENTER-DOT-)) RADICAL

Previous work has challenged the assignment of a photoaccumulated EPR signal to the phylloquinone electron acceptor in photosystem I, A(1)(. -). Biosynthetic deuteration of the phylloquinone in the cyanobacteriu m Anabaena variabilis has been shown to narrow this photoaccumulated s ignal, demonstrating that the signal arises from A(1)(.-). The ESP sig nal attributed to P700(.+)A(1)(.-) is also narrowed by this deuteratio n, showing that the photoaccumulated EPR signal and the ESP signal are monitoring the same redox component. Confirmation that the photoaccum ulated EPR signal comes from deuterated phylloquinone was obtained by exchanging the deuterated for protonated phylloquinone, which broadene d the photoaccumulated EPR signal.