Photoaccumulation in Photosystem I Does Not Produce a Phylloquinone Radical

A characteristic EPR spectrum is observed from Photosystem I (PS I) particles that are illuminated under reducing conditions. This signal is thought to arise from the secondary electron acceptor of PS I, A1. This EPR spectrum, along with data from other spectroscopic methods, has been taken as evidence that A1is phylloquinone. In the work reported here, we have used a methionine auxotroph ofAnabaenato deuterate specifically the 2‐methyl group on the phylloquinone ring. Mass spectrometry of the isolated quinone shows 65% yield in the labeling procedure. The photoaccumulated A1EPR spectra in protonated and deuterium‐labeled cells are indistinguishable, from which we conclude that the photoaccumulated radical is not phylloquinone.