Electronic and spin structures of manganese clusters in the photosynthesis II system

Electronic and spin structures of manganese clusters, Mn₄O ₄ (1), CaMn₃O₄ (2), Mn₃O₄ (3), MnX (4), and CaMn₄O₄ (5), in the photosynthesis II system are investigated using the classical and quantum Heisenberg models. The molecular orbital calculations by the use of general spin orbitals (GSO) are performed for cubane-type calcium manganese cluster 2 with noncollinear spin alignment, which has also been concluded in our previous studies of 1 and 3. The calculated results, together with available experiments, enable us to propose possible electronic states (from S₀ to S₄) of tetranuclear manganese cluster 5, which is the active site of oxygen evolution center (OEC). The low-spin (LS) ground states of 2-5 are consistent with the ESR and other magnetic observations. A new reaction scheme for oxygen evolution from water is resulted from both theoretical and experimental results for manganese clusters 1-5 in OEC on the basis of the newly determined X-ray structure by Ferreira et al. [Science 303 (2004) 1831]. In this mechanism, both calcium and manganese ions play important roles for formation of peroxide anion bridge, which is the precursor of molecular oxygen. Implications of computational results are discussed in relation to a key role of the high-valent manganese oxo (Mn(V)O) species in OEC for oxygen evolution from water. Our previous and present computations conclude that Mn(V)O plays a crucial role in both native and artificial OEC systems.