Arrested Substrate Binding Resolves Catalytic Intermediates in Higher‐Plant Water Oxidation

Among the intermediate catalytic steps of the water‐oxidizing Mn4CaO5 cluster of photosystem II (PSII), the final metastable S3 state is critically important because it binds one substrate and precedes O2 evolution. Herein, we combine X‐ and Q‐band EPR experiments on native and methanol‐treated PSII of Spinacia oleracea and show that methanol‐treated PSII preparations of the S3 state correspond to a previously uncharacterized high‐spin (S=6) species. This is confirmed as a major component also in intact photosynthetic membranes, coexisting with the previously known intermediate‐spin conformation (S=3). The high‐spin intermediate is assigned to a water‐unbound form, with a MnIV3 subunit interacting ferromagnetically via anisotropic exchange with a coordinatively unsaturated MnIV ion. These results resolve and define the structural heterogeneity of the S3 state, providing constraints on the S3 to S4 transition, on substrate identity and delivery pathways, and on the mechanism of O−O bond formation

Adsorption and radical stabilization of humic-acid analogues and Pb ²⁺ on restricted phyllomorphous clay

Humic acids have stable radicals that are indigenous to their structure. Hydroxybenzoic acid derivatives such as gallic acid (GA) and protocatechuic acid are appropriate models for the radical properties of humic acids. Here we show that the adsorption or intercalation of gallic acid in Laponite clay results in a significant thermodynamic stabilization of gallic acid radicals. Moreover, the formed organoclay shows enhanced stability against acid dissolution. The structural details of the association of gallic acid with Laponite depend on the GA/Laponite loading. At low GA/Laponite ratios (∼10-6 M of gallic acid per gram of clay), gallic acid is adsorbed at the variable charge sites of Laponite. This adsorption can be adequately described by surface complexation modeling. At higher GA/Laponite ratios (∼10-3 M of gallic acid per gram of clay), X-ray diffraction data show that gallic acid is intercalated at the interlamellar sites of Laponite. In the presence of Pb 2+ ions, the formed GA/Pb complex is associated with Laponite in an analogous structural manner, that is, adsorption at variable charge sites or intercalation at the interlamellar sites of Laponite, depending on the loading. Laponite stabilizes the GA/Pb radicals. At prolonged exposure to ambient O 2, Laponite promotes the formation of stable oligomeric GA/Pb radical species, which are intercalated into interlamellar sites. © 2006 American Chemical Society

Metal complexes with the quinolone antibacterial agent N-propyl-norfloxacin: Synthesis, structure and bioactivity

Nine new metal complexes of the quinolone antibacterial agent N-propyl-norfloxacin, pr-norfloxacin, with VO2+, Mn2+, Fe3+, Co2+, Ni2+, Zn2+, MoO22 +, Cd2+ and UO22 + have been prepared and characterized with physicochemical and spectroscopic techniques while molecular mechanics calculations for Fe3+, VO2+ and MoO22 + complexes have been performed. In all complexes, pr-norfloxacin acts as a bidentate deprotonated ligand bound to the metal through the pyridone and one carboxylate oxygen atoms. All complexes are six-coordinate with slightly distorted octahedral geometry. For the complex VO(N-propyl-norfloxacinato)2(H2O) the axial position, trans to the vanadyl oxygen, is occupied by one pyridone oxygen atom. The investigation of the interaction of the complexes with calf-thymus DNA has been performed with diverse spectroscopic techniques and has shown that the complexes can be bound to calf-thymus DNA resulting to a B → A DNA transition. The antimicrobial activity of the complexes has been tested on three different microorganisms. The complexes show equal or decreased biological activity in comparison to the free pr-norfloxacin except UO2(pr-norf)2 which shows better inhibition against S. aureus. © 2006 Elsevier Inc. All rights reserved

Electronic and magnetic properties of the binuclear [Mn 2(OPPh2)2N4] complex, as revealed by magnetometry, EPR and density functional broken-symmetry studies

In this work, magnetometry and EPR spectroscopy studies on the binuclear Mn(II) complex [Mn2(OPPh2)2N4] (1) established antiferromagnetic interactions between the two S = 5/2 Mn(II) centers being at 3.378 Å from each other. The magnitude of the J AB coupling constant, in the framework of the ĤHDvV= -2JABŜAŜB formalism, was determined to be -4.5 ± 0.5 cm-1, by the temperature dependence of the magnetic susceptibility of 1. This value is comparable with the one of the previously studied [Mn2(OPPh2)[OP(OEt) 2]N4(H2O)2] complex (-3.5 cm -1), which bears similar type of ligands. The antiferromagnetic nature of the coupling within the binuclear Mn2O4 core of 1 was unequivocally confirmed also by EPR studies on frozen CH 2Cl2 solutions at various temperatures. Density functional broken-symmetry calculations confirmed the above experimental findings and provided JAB values in close agreement with the experiment. Comparisons with literature binuclear Co(II) or Mn(II) complexes were made