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Role of Oxido Incorporation and Ligand Lability in Expanding Redox Accessibility of Structurally Related Mn4 Clusters.

ABSTRACT: Photosystem II supports four manganese centers through nine oxidation states from manganese(II) during assembly through to the most oxidized state before O2 formation and release. The protein-based carboxylate and imidazole ligands allow for significant changes of the coordination environment during the incorporation of hydroxido and oxido ligands upon oxidation of the metal centers. We report the synthesis and characterization of a series of tetramanganese complexes in four of the six oxidation states from MnII3MnIII to MnIII2 MnIV2 with the same ligand framework (L) by incorporating four oxido ligands. A 1,3,5-triarylbenzene framework appended with six pyridyl and three alkoxy groups was utilized along with three acetate anions to access tetramanganese complexes, Mn4O x , with x = 1, 2, 3, and 4. Alongside two previously reported complexes, four new clusters in various states were isolated and characterized by crystallography, and four were observed electrochemically, thus accessing the eight oxidation states from MnII4 to MnIIIMnIV3. This structurally related series of compounds was characterized by EXAFS, XANES, EPR, magnetism, and cyclic voltammetry. Similar to the ligands in the active site of the protein, the ancillary ligand (L) is preserved throughout the series and changes its binding mode between the low and high oxido-content clusters. Implications for the rational assembly and properties of high oxidation state metal-oxido clusters are presented.

SUBMITTER: Kanady JS

PROVIDER: S-EPMC3806640 | biostudies-literature | 2013 Oct

REPOSITORIES: biostudies-literature

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Role of Oxido Incorporation and Ligand Lability in Expanding Redox Accessibility of Structurally Related Mn<sub>4</sub> Clusters.

Kanady Jacob S JS Tran Rosalie R Stull Jamie A JA Lu Luo L Stich Troy A TA Day Michael W MW Yano Junko J Britt R David RD Agapie Theodor T

Chemical science 20131001 10

Photosystem II supports four manganese centers through nine oxidation states from manganese(II) during assembly through to the most oxidized state before O<sub>2</sub> formation and release. The protein-based carboxylate and imidazole ligands allow for significant changes of the coordination environment during the incorporation of hydroxido and oxido ligands upon oxidation of the metal centers. We report the synthesis and characterization of a series of tetramanganese complexes in four of the si ...[more]

PMID: 24163730

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Project description:A central issue in understanding redox properties of iron-sulfur proteins is determining the factors that tune the reduction potentials of the Fe-S clusters. Recently, Solomon and coworkers have shown that the Fe-S bond covalency of protein analogs measured by %L, the percent ligand character of the Fe 3d orbitals, from ligand K-edge X-ray absorption spectroscopy (XAS) correlates with the electrochemical redox potentials. Also, Wang and coworkers have measured electron detachment energies for iron-sulfur clusters without environmental perturbations by gas-phase photoelectron spectroscopy (PES). Here the correlations of the ligand character with redox energy and %L character are examined in [Fe(4)S(4)L(4)](2-) clusters with different ligands by broken symmetry density functional theory (BS-DFT) calculations using the B3LYP functional together with PES and XAS experimental results. These gas-phase studies assess ligand effects independently of environmental perturbations and thus provide essential information for computational studies of iron-sulfur proteins. The B3LYP oxidation energies agree well with PES data, and the %L character obtained from natural bond orbital analysis correlates with XAS values, although it systematically underestimates them because of basis set effects. The results show that stronger electron-donating terminal ligands increase %L(t), the percent ligand character from terminal ligands, but decrease %S(b), the percent ligand character from the bridging sulfurs. Because the oxidized orbital has significant Fe-L(t) antibonding character, the oxidation energy correlates well with %L(t). However, because the reduced orbital has varying contributions of both Fe-L(t) and Fe-S(b) antibonding character, the reduction energy does not correlate with either %L(t) or %S(b). Overall, BS-DFT calculations together with XAS and PES experiments can unravel the complex underlying factors in the redox energy and chemical bonding of the [4Fe-4S] clusters in iron-sulfur proteins.

Dataset Information

Role of Oxido Incorporation and Ligand Lability in Expanding Redox Accessibility of Structurally Related Mn4 Clusters.

Publications

Role of Oxido Incorporation and Ligand Lability in Expanding Redox Accessibility of Structurally Related Mn<sub>4</sub> Clusters.

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