Single-Molecule Magnetism Properties of the First Strontium-Manganese Cluster [SrMn14O11(OMe)3(O2CPh)18(MeCN)2]

The preparation and properties of the first strontium-manganese molecular complex are described. The reaction of (NBun4)[Mn4O2(O2CPh)9(H2O)] (4MnIII) with Sr(ClO4)2 in MeCN/MeOH led to the isolation of [SrMn14O11-(OMe)3(O2CPh)18(MeCN)2] (1; 13MnIII, MnII). The structure of 1 consists of two [Mn4O3(OMe)] cubane units attached to a central, near-planar, trinuclear [Mn3O4] unit, to which are also attached a Mn and a Sr above the plane and a [Mn2O(OMe)] rhomb below the plane. Peripheral ligation is provided by 18 bridging benzoate and two terminal MeCN groups. Variable-temperature and -field dc magnetization (M) data were collected in the 1.8-10 K and 0.1-4.0 T ranges and fit by matrix diagonalization methods to give S ) 9/2, D ) -0.50(5) cm-1, and g ) 1.88(10), where S is the ground-state spin and D is the axial zero-field splitting parameter. Magnetization versus dc field sweeps at various temperatures and scan rates exhibited hysteresis loops, confirming 1 to be a new single-molecule magnet. Because complex 1 is the initial molecular example of intimately associated Mn and Sr atoms, Sr EXAFS studies have been performed for the first time on a synthetic Sr-containing molecule. This has also allowed comparisons with the EXAFS data on the Sr-substituted water oxidizing complex (WOC) of Photosystem II (PS II), which contains a SrMn4 complex

THE STATE OF MANGANESE IN THE PHOTOSYNTHETIC APPARATUS DETERMINED BY X-RAY ABSORPTION SPECTROSCOPY

We present our results of X-ray edge and EXAFS studies of Mn in intermediate states (S0-S4) of the photosynthetic O2 evolving complex prepared from spinach and the thermophilic cyanobacterium Synechococcus. We observed a shift to higher energy in the X-ray absorption K-edge energy of Mn upon advancement from the dark adapted S1 state to the light-induced S2 state in spinach and in Synechococcus. We have recently anaiysed the Mn K-edges of samples poised in the S0 and S3 states. The K-edge inflection energy of the S0 samples is lower than that of samples poised in the S1 state, indicating a reduction in the effective positive charge on Mn in S0 relative to S1. The K-edge inflection energy of the S3 state is similar to that of the S2 state, suggesting an invariance in the oxidation state of Mn on advancing from the S2 to the S3 state. We have examined the Mn K-edge spectra of preparations depleted of essential peptides to determine the changes produced in the structure of the Mn complex. Mn EXAFS results for spinach S1 and S2 samples and Synechococcus S1 all show a Mn neighbor at ~ 2.7 Å and two N or O shells at ~ 1.8 and 2.0 Å indicating a µ-oxo bridged Mn complex. We conclude from the edge and EXAFS studies that the light-induced S1 to S2 transition involves a change in the oxidation state of Mn with no change in the coordination of Mn in the O2-evolving complex. The similarity of the edges and EXAFS results from spinach and Synechococcus suggest that the basic structure of the Mn center in the O2-evolving complex is conserved over a period of two billion years. Finally, an analysis of a composite of the S1 and S2 spectra has revealed a fourth shell in the Fourier transform of the EXAFS spectra. Simulations have shown that it is compatible with an additional Mn shell at ~ 3.3 Å or possibly second shell contributions from histidine ligands

Glutamate-354 of the CP43 polypeptide interacts with the oxygen-evolving Mn4Ca cluster of photosystem II: a preliminary characterization of the Glu354Gln mutant

In the recent X-ray crystallographic structural models of photosystem II, Glu354 of the CP43 polypeptide is assigned as a ligand of the O2-evolving Mn4Ca cluster. In this communication, a preliminary characterization of the CP43-Glu354Gln mutant of the cyanobacterium Synechocystis sp. PCC 6803 is presented. The steady-state rate of O2 evolution in the mutant cells is only approximately 20% compared with the wild-type, but the kinetics of O2 release are essentially unchanged and the O2-flash yields show normal period-four oscillations, albeit with lower overall intensity. Purified PSII particles exhibit an essentially normal S2 state multiline electron paramagnetic resonance (EPR) signal, but exhibit a substantially altered S2-minus-S1 Fourier transform infrared (FTIR) difference spectrum. The intensities of the mutant EPR and FTIR difference spectra (above 75% compared with wild-type) are much greater than the O2 signals and suggest that CP43-Glu354Gln PSII reaction centres are heterogeneous, with a minority fraction able to evolve O2 with normal O2 release kinetics and a majority fraction unable to advance beyond the S2 or S3 states. The S2-minus-S1 FTIR difference spectrum of CP43-Glu354Gln PSII particles is altered in both the symmetric and asymmetric carboxylate stretching regions, implying either that CP43-Glu354 is exquisitely sensitive to the increased charge that develops on the Mn4Ca cluster during the S1→S2 transition or that the CP43-Glu354Gln mutation changes the distribution of Mn(III) and Mn(IV) oxidation states within the Mn4Ca cluster in the S1 and/or S2 states