An estimation of the size of the water cluster present at the cleavage site of the water splitting enzyme

Abstract In time-dependent measurements of oxygen evolution in tobacco thylakoid membranes we varied the fraction of H 2 18 O and the temperature and measured water splitting as 18 O 2 , 16 O 18 O, and 16 O 2 by mass spectrometry. We show that the approach to the equilibrium of the system after H 2 18 O addition can be very well understood in terms of the diffusion of water molecules. The equilibrium states of 16 O 2 , 16 O 18 O, and 18 O 2 evolution differ from the theoretical binomial distributions, which are expected under the prerequisite of ideal mixing of the water molecules and that of the chemical equivalence of H 2 18 O and H 2 16 O for an infinite cluster. The presence of this deviation means that there is a typical size of water clusters having access to cleavage by the water splitting enzyme. We estimated that this cluster contains about 12±2 water molecules.