ENDOR and special TRIPLE resonance spectroscopy of photoaccumulated semiquinone electron acceptors in the reaction centers of green sulfur bacteria and heliobacteria

A study of the oxygen-sensitive reaction centers of anaerobic, photosynthetic organisms, namely, green sulfur bacteria and heliobacteria, was conducted to determine whether EPR signals in these organisms originate from a semiquinone electron acceptor in bacterial type I reaction centers. Magnetic resonance techniques were used to conclusively assign observed asymmetric EPR signals in anaerobically prepared membranes of Chlorobium limicola and Heliobacterium chlorum following photoaccumulation at low temperatures under reducing conditions. Results are discussed.