Manganese in Natural and Artificial Photosynthesis 27 Chapter 2 MANGANESE IN NATURAL AND ARTIFICIAL PHOTOSYNTHESIS Harvey J.M. Hou ∗ Department of Physical Sciences, Alabama State University, Montgomery, Alabama 36104. ABSTRACT Manganese is presented in the form of a Mn 4 CaO 5 cluster in photosystem II and is one of the most important cofactors in oxygenic photosynthesis. The Mn 4 CaO 5 cluster of photosystem II located in the thylakoid membranes catalyzes a light-driven water- splitting reaction to achieve energy storage on the large scale at room temperature and neutral pH in green plants, algae, and cyanobacteria. The Mn-mediated water splitting reaction uses the chemical bond rearrangement to transform the energy-deficient water molecule to energy-rich oxygen and hydrogen molecules for energy storage. Great progress and breakthroughs in illustrating the structure and mechanism of water oxidation in photosystem II have been made using the combination of modern molecular genetics and sophiscated biophysical techniques in the past decade. In particular, the three- dimensional structure of photosystem II with oxygen-evolving activity has been determined at an atomic level in 2011, which provides a complete picture with the specific position of each atom in the Mn 4 CaO 5 cluster and interaction between the each of atoms with its own amino acid ligand in the protein complexes. These progresses have significantly enhanced our understanding of the mechanisms of water splitting in natural photosynthesis and offered a unique opportunity for transforming solar energy into our energy system to solve the global energy crisis. To mimic the water oxidation of photosystem II oxygen evolving complex, appealing Mn-containing catalytic materials were discovered. In this chapter, the structural and mechanistic models of manganese in natural photosynthetic systems and the applications of manganese-containing catalytic materials via artificial photosynthesis in green renewable energy production will be summarized and evaluated. The manganese-based systems include Mn-oxo mix valence compound, Mn-oxo cubic compound, calcium manganese materials, Mn-oxo ∗ E-mail address: hhou@umassd.edu