No abstract

We report powder and single crystal EPR measurements of [Cu(tda)(phen)](2)·H(2)tda (tda = thiodiacetate, phen = phenanthroline) at 9.7 GHz. This compound consists of centrosymmetric copper(II) ion dimers, weakly ferromagnetically exchange-coupled (J = +3.2 cm(-1)), in which the dimeric units are linked by hydrophobic chemical paths involving the phen molecules. EPR revealed that the triplet spectra are collapsed by interdimeric exchange interactions mediated by that chemical path. Analysis and simulation of the single crystal EPR spectra were performed using Anderson's exchange narrowing model, together with statistical arguments. This approach allowed us to interpret the spectra modulated by the interdimeric interactions in situations of weak, intermediate, and strong exchange. We evaluated an interdimeric exchange constant J' = 0.0070(3) cm(-1), indicating that hydrophobic paths can transmit weak exchange interactions between centers at relatively long distances of the order of ∼10 Å.

We report the synthesis and X-ray structure of the dimeric zinc (II) compound [Zn(tda)(phen)] 2 •H 2 tda, (tda = thiodiacetic acid, phen = 1,10-phenanthroline) hereafter Zn(tda)(phen), and a single crystal EPR study of Zn(tda)(phen) doped with Cu(II)ions. Zn(tda)(phen) is isomorphous to its Cu(II) analogue. The EPR spectra show a central signal composed of one to four resonances assigned to Cu-Zn heterodimers, flanked by less intense satellite signals assigned to Cu-Cu homodimers. Analysis of single crystal EPR data allowed us to determine the g-and A-matrices of the Cu(II) ion and the anisotropic ZFS parameters of the homodimer. Within the experimental error, the Cu(II) g-matrix obtained for the diluted compound was identical to that previously determined by us in pure Cu(tda)(phen). The ZFS is shown to be dominated by magnetic dipolar coupling between the unpaired electrons mainly centered around the Cu(II) ions, although partially delocalized over the equatorial copper ligands. DFT calculations yielded spin population values compatible with those determined from the analysis of the anisotropic ZFS assuming a distributed dipole model. The information obtained from the diluted compound was used to evaluate the interdimeric exchange interaction between dimeric units in pure Cu(tda)(phen). The comparison between both point and distributed dipole approximations is discussed with reference to the analysis of the EPR data.

Aldehyde oxidoreductase from Desulfovibrio gigas (DgAOR) is a member of the xanthine oxidase (XO) family of mononuclear Mo-enzymes that catalyzes the oxidation of aldehydes to carboxylic acids. The molybdenum site in the enzymes of the XO family shows a distorted square pyramidal geometry in which two ligands, a hydroxyl/water molecule (the catalytic labile site) and a sulfido ligand, have been shown to be essential for catalysis. We report here steady-state kinetic studies of DgAOR with the inhibitors cyanide, ethylene glycol, glycerol, and arsenite, together with crystallographic and EPR studies of the enzyme after reaction with the two alcohols. In contrast to what has been observed in other members of the XO family, cyanide, ethylene glycol, and glycerol are reversible inhibitors of DgAOR. Kinetic data with both cyanide and samples prepared from single crystals confirm that DgAOR does not need a sulfido ligand for catalysis and confirm the absence of this ligand in the coordination sphere of the molybdenum atom in the active enzyme. Addition of ethylene glycol and glycerol to dithionite-reduced DgAOR yields rhombic Mo(V) EPR signals, suggesting that the nearly square pyramidal coordination of the active enzyme is distorted upon alcohol inhibition. This is in agreement with the X-ray structure of the ethylene glycol and glycerolinhibited enzyme, where the catalytically labile OH/OH 2 ligand is lost and both alcohols coordinate the Mo site in a η 2 fashion. The two adducts present a direct interaction between the molybdenum and one of the carbon atoms of the alcohol moiety, which constitutes the first structural evidence for such a bond in a biological system.