STUDIES OF THE PHOTOSYNTHETIC OXYGEN-EVOLVING SYSTEM BY ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY

The EPR was used to study the effect of ammonia on the formation and s pectroscopic properties of formal S3 states in the photosynthetic wate r-splitting reaction. The PS II membranes were used as EPR samples. Th e data indicating the close interaction of NH3 with the paramagnetic c enter of Ca2+-depleted PS II membranes were obtained. The elimination of the 15 mT wide S3' EPR signal in Ca2+-depleted membranes with the h istidine modifier DEPC supports the involvement of histidine as the or ganic entity interacting with the manganese center in S3'.