FTIR studies of metal ligands, networks of hydrogen bonds, and water molecules near the active site Mn4CaO5 cluster in Photosystem II

The photosynthetic conversion of water to molecular oxygen is catalyzed by the Mn4CaO5 cluster in Photosystem II and provides nearly our entire supply of atmospheric oxygen. The Mn4CaO5 cluster accumulates oxidizing equivalents in response to light-driven photochemical events within Photosystem II and then oxidizes two molecules of water to oxygen. The Mn4CaO5 cluster converts water to oxygen much more efficiently than any synthetic catalyst because its protein environment carefully controls the cluster's reactivity at each step in its catalytic cycle. This control is achieved by precise choreography of the proton and electron transfer reactions associated with water oxidation and by careful management of substrate (water) access and proton egress. This review describes the FTIR studies undertaken over the past two decades to identify the amino acid residues that are responsible for this control and to determine the role of each. In particular, this review describes the FTIR studies undertaken to characterize the influence of the cluster's metal ligands on its activity, to delineate the proton egress pathways that link the Mn4CaO5 cluster with the thylakoid lumen, and to characterize the influence of specific residues on the water molecules that serve as substrate or as participants in the networks of hydrogen bonds that make up the water access and proton egress pathways. This information will improve our understanding of water oxidation by the Mn4CaO5 catalyst in Photosystem II and will provide insight into the design of new generations of synthetic catalysts that convert sunlight into useful forms of storable energy. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.