Theoretical modeling of the mechanism of dioxygen activation and evolution by tetranuclear manganese complexes

Theoretical investigation on peripheral ligands of oxygen-evolving center in photosystem II

The interaction between the Mn-cluster and its peripheral ligands in oxygen-evolving center is still unclear. Theoretical investigation on the coordination of histidine, H(2)O, and CI to Mn2O2 units in OEC is conducted. The following conclusions are obtained: (i) both histidine and H(2)O molecule, bound to the two Mn ions, respectively, are vertical to the Mn2O2 plane, and maintain a large distance; (ii) the two H(2)O molecules cannot bind to the same Mn2O2 unit. Based on Mn-cluster structure in OEC, we theoretically predict that two H(2)O molecules bind to the two Mn ions at the "C"-shaped open end in S(0) state, while two His residues at the closed end. Cl ion can only terminally ligate at the open end. Individual valence for the four Mn ions in S(0) state is assigned.

Manganese citrate chemistry: syntheses, spectroscopic studies, and structural characterizations of novel mononuclear, water-soluble manganese citrate complexes

The first two mononuclear manganese citrate complexes, (NH4)4[MnII(C6H5O7)2] (1) and (NH4)5[MnIII(C6H4O7)2].2H2O (2) were synthesized in aqueous solutions near physiological pH values. They were isolated in their pure crystalline forms and characterized by elemental analyses and spectroscopic techniques, including UV/visible, electron paramagnetic resonance, Fourier transformed infrared, and magnetic susceptibility measurements. Compound 1 crystallizes in the monoclinic space group P2(1)/c, with a = 8.777(1) A, b = 13.656(3) A, c = 9.162(2) A, beta = 113.62(2) degrees, V = 1006.2(6) A3, and Z = 2. Compound 2 crystallizes in the triclinic space group P1, with a = 9.606(3) A, b = 9.914(3) A, c = 7.247(3) A, alpha = 91.05(1) degrees, beta = 105.60(1) degrees, gamma = 119.16(1) degrees, V = 571.3(3) A3, and Z = 1. The X-ray crystal structures of 1 and 2 revealed that, in both cases, the manganese ion is six-coordinate and is bound by two citrate ligands in a distorted octahedral fashion. In the case of complex 1, the citrate ion binds to Mn2+ as a triply deprotonated ligand, retaining the central carbon hydroxyl hydrogen, whereas, in the case of compound 2, the citrate ligand coordinates to Mn3+ as a fully deprotonated entity. Compound 2 contains water molecules of crystallization in the unit cell which, through extensive hydrogen-bonding interactions, bestow considerable stability upon the Mn(3+)-citrate assembly. There are significant contributions to the stabilities of the assembled lattices in 1 and 2 arising from the ammonium counterions neutralizing the high anionic charges of the complexes. The EPR spectra attest to the presence of paramagnetic Mn2+ and Mn3+ species in the solid state. Corroborative evidence is obtained from the magnetic susceptibility measurements in the range 5-300 K. Complexes 1 and 2 present clear cases of mononuclear manganese citrate species relevant to manganese speciation in biological media and potentially related to the beneficial as well as toxic effects of manganese on humans.

Redox-Induced Formation and Cleavage of O-O sigma and pi Bonds in a Peroxo-Bridged Manganese Dimer: A Density Functional Study

The structural and electronic consequences of reduction and oxidation of a peroxo-bridged Mn(2)(IV/IV) dimer, Mn(2)(&mgr;-O)(2)(&mgr;-O(2))(NH(3))(6)(2+), are examined using approximate density functional theory. In both cases, the initial electron-transfer step is localized on the metal centers, but subsequent structural rearrangement results in transfer of the excess charge to the &mgr;-O(2) unit, with concomitant regeneration of the Mn(2)(IV/IV) core. Two-electron reduction results in population of the O-O sigma orbital and complete cleavage of the O-O bond, whereas two-electron oxidation depopulates the O-O pi orbital, forming molecular oxygen. The coupling between the metal centers (antiferromagnetic or ferromagnetic) affects the stability of the intermediate species, in which the redox process is metal based, and hence influences the kinetic barrier to bond formation or cleavage. Reductive cleavage of the O-O sigma bond is favored when the metal centers are antiferromagnetically coupled, whereas oxidative formation of the pi component of the O-O bond is favored by ferromagnetic coupling. The possible implications for the relationship between structure and function in the oxygen-evolving complex found in photosynthetic organisms are discussed.