Photoaccumulation in photosystem I does produce a phylloquinone (A₁^•-) radical

Previous work has challenged the assignment of a photoaccumulated EPR signal to the phylloquinone electron acceptor in photosystem I, A₁^•-. Biosynthetic deuteration of the phylloquinone in the cyanobacterium Anabaena variabilis has been shown to narrow this photoaccumulated signal, demonstrating that the signal arises from A₁^•-. The ESP signal attributed to P700^•+A₁^•- is also narrowed by this deuteration, showing that the photoaccumulated EPR signal and the ESP signal are monitoring the same redox component. Confirmation that the photoaccumulated EPR signal comes from deuterated phylloquinone was obtained by exchanging the deuterated for protonated phylloquinone, which broadened the photoaccumulated EPR signal. † This work was supported by grants from the U.K. Biotechnology and Biological Sciences Research Council (BBSRC) and the University of London Intercollegiate Research Service (ULIRS).