transition. In addition to the dioxygen product for which the OEC is named, the other products are four hydrogen ions. These are delivered to the enclosed interior of the thylakoid membrane where they contribute to the driving force for ADP → ATP phosphorylation. Several channels containing water molecules and hydrophilic amino acid site chains have been identified in the protein which may enable the protons to escape from the active site. Research The details of how the substrate protons are separated from the substrate oxygens bound to the Mn4O5Ca cluster have not been established. One point of view holds that the protons are shed from the cluster in response to its oxidation by YZ• where this redox reaction is thought of as electron transfer from the cluster to YZ•. An alternative proposal is that the protons are given up by YZ as it is oxidized by P680+and that YZ• extracts both a proton and an electron from the cluster on each transition. Because protons are so numerous in the samples and are not observable in crystal studies, this question may eventually be decided by computational methods. Another question that may be decided by computational methods is the nature of oxygen-oxygen bond forming reaction. This question is of great interest because the reaction is unique in biology and because the answer may help in designing artificial catalysts for water splitting. From experimental work on the S3 to S4 to S0 transition, it is known that the time course of YZ• reduction roughly matches the time course of O2 release. This means that any potential intermediates in the process can have only very short lifetimes, making them undetectable. Extensive computational investigations using density functional theory have been carried out to map out the entire S-state cycle and especially to probe the S3 to S0 transition; for example, see. Hybrid methods melding quantum mechanics with molecular mechanics calculations are also being used. The results of such efforts are not always in agreement. There is still much to be learned about the mechanism of photosynthetic water oxidation. References Further reading - outlines a possible route for the evolution of the OEC. Photosynthesis Light reactions