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

In time-dependent measurements of oxygen evolution in tobacco thylakoid mem branes ne varied the fraction of (H2O)-O-18 and the temperature and measure d water splitting as O-18(2), (OO)-O-16-O-18, and O-16(2) by mass spectrome try, We show that the approach to the equilibrium of the system after (H2O) -O-18 addition can be very well understood in terms of the diffusion of wat er molecules. The equilibrium states of O-16(2), (OO)-O-16-O-18, and O-18(2 ) evolution differ from the theoretical binomial distributions, which are e xpected under the prerequisite of ideal mixing of the water molecules and t hat of the chemical equivalence of (H2O)-O-18 and (H2O)-O-16 for an infinit e cluster. The presence of tills deviation means that there is a typical si ze of water clusters having access to cleavage by the water splitting enzym e. We estimated that this cluster contains about 12+/-2 water molecules, (C ) 2001 Federation of European Biochemical Societies, Published by Elsevier Science B.V, All rights reserved.