THE INHIBITION OF PHOTOSYNTHETIC OXYGEN EVOLUTION BY AMMONIA PROBED BY EPR

EPR was used to study the binding of NH3 to the photosynthetic O2-evolving center. NH3-treated, Ca2+-depleted Photosystem II (PS II) membranes exposed to continuous light at 250 K showed a 10 mT-wide asymmetric EPR signal, centered around g = 2. When dark-adapted material was illuminated with a sequence of laser flashes the same signal appeared after the second flash, indicating that the g = 2 signal arises from a modified S3 state. The signal is different from the 15-16.5 mT-wide EPR signal at g = 2 ascribed to the S3' state in Ca2+-depleted material in the absence of NH3, indicating that NH3 perturbs the structure of the O2-evolving complex in the S3' state. Illumination of native NH3-treated PS II membranes with continuous light results in the appearance of an EPR signal at g = 2 with a width similar to that in Ca2+-depleted, NH3-treated membranes. The conditions for the formation of the signal and its properties suggest that it also arises from a perturbed S3 State with NH3 in close association with the manganese.