18O isotope effect in the photosynthetic water splitting process.

In mass spectroscopic experiments of oxygen evolution in Photosystem II at 50% enrichment of H(2)18O, one expects equal signals of 18O(2) and 16O(2) unless one of the isotopes is favored by the oxygen evolving complex (OEC). We have observed a deviation from this expectation, being a clear indication of an isotope effect. We have measured the effect to be 1.14-1.30, which is higher than the theoretically predicted value of 1.014-1.06. This together with the strong temperature variation of the measured effect with a discontinuity at 11 degrees C observed for wild-type tobacco and at 9 degrees C for a yellow-green tobacco mutant suggest that an additional mechanism is responsible for the observed high isotope effect. The entry of a finite size of water clusters to the cleavage site of the OEC can explain the observation.