Reversible precipitation of bovine serum albumin by metal ions and synthesis, structure and reactivity of new tetrathiometallate chelating agents.

Independent research is an important component of any undergraduate chemistry program. This article reports the findings of two of many undergraduate research projects directed by Ed Stiefel in the hopes that the results will be inspiring and useful to the scientific community. The neurological disorders associated with insufficient copper in Menkes disease and an excess of copper in Wilson's disease are well established; however, recent evidence suggests that copper may also be involved in other disorders, such as Alzheimer's, angiogenesis, and prion diseases. The exact role of copper, however, is uncertain. This study examines the role of copper and zinc in the formation of protein deposits and the chelation and removal of the metal ions to reverse the process. The bovine serum albumin (BSA) protein forms a precipitate after the addition of approximately 6 copper(II) atoms or 8 zinc(II) atoms. Other metal ions, such as Ca(II), Al(III), Ni(II), and Co(II), did not precipitate the BSA even when the metal ion to BSA ratios were in excess of 1000. The copper and zinc protein precipitates returned to solution after addition of the chelating agents, ethylenediaminetetraacetic acid (EDTA) or tetrathiometallates [(MS(4)(2-)), where M=Mo, W]. Two new choline and acetylcholine tetrathiomolybdate and tetrathiotungstate chelating agents have been synthesized and characterized. The infrared (IR) and X-ray crystal structures of the complexes revealed that the (MS(4)(2-)) cores had approximate T(d) symmetry in the choline (Ch) salts and C(2v) symmetry in the acetylcholine (AcCh) salts. The AcCh salts hydrolyzed more slowly than the ammonium or Ch salts and the tetrathiotungstate salts hydrolyzed approximately two orders of magnitude more slowly than the tetrathiomolybdate salts. The slower hydrolysis of tetrathiotungstate may make it more useful as an inorganic reagent and therapeutic agent.

Comparison of the kinetics of iron release from a marine (Trichodesmium erythraeum) Dps protein and mammalian ferritin in the presence and absence of ligands.

The ferritin superfamily of iron storage proteins includes ferritin proper and Dps (DNA binding protein from starved cells) along with bacterioferritin. We examined the release of Fe from the Dps of Trichodesmium erythraeum (Dps(tery)) and compared it to the release of Fe from horse spleen ferritin (HoSF) under various conditions. Both desferrioxamine B (DFB), a Fe(III) chelator, and ascorbic acid were able to mobilize Fe from Dps(tery) at rates comparable to those observed for HoSF. The initial Fe release rate from both proteins increased linearly with the concentration of DFB, suggesting that the chelator binds to Fe in the protein. A small but significant rate obtained by extrapolation to zero concentration of DFB implies that Dps(tery) and HoSF might release Fe(III) spontaneously. A similar result was observed for HoSF in the presence of sulfoxine. In a different experiment, Fe(III) was transferred from holoferritin to apotransferrin across a dialysis membrane in the absence of chelator or reducing agent. The apparent spontaneous release of Fe from HoSF and Dps(tery) brings forth the hypothesis that the Fe core in Fe storage proteins might be continuously dissolving and re-precipitating in vivo, thus maintaining it in a highly reactive and bioavailable form.

Theoretical studies of [FeFe]-hydrogenase: structure and infrared spectra of synthetic models.

First-principles density functional theory calculations of synthetic models of [FeFe]-hydrogenase are used to show that the theoretical methods reproduce observed structures and infrared spectra to high accuracy. The accuracy is demonstrated for synthetic Fe(I)Fe(I) models ([(mu-PDT)Fe2(CO)6] and [(CN)(CO)2(mu-PDT)Fe2(CO)2(CN)]2-), for which we show that their infrared spectra are sensitive to the geometric arrangement of their CO/CN ligands and can be used in conjunction with quantum-mechanical total energies to predict the correct ligand geometry. We then analyze and predict the structure of mixed-valence Fe(II)Fe(I) models ([(mu-MeSCH2C(Me)(CH2S)2)Fe2(CO)4(CN)2]x-). These capabilities promise to distinguish among the various structural isomers of the enzyme's active site which are consistent with the limited accuracy of the X-ray observations.

Theoretical studies of [FeFe]-hydrogenase: infrared fingerprints of the dithiol-bridging ligand in the active site.

An unresolved structural issue for [FeFe]-hydrogenases is the nature of the dithiol-bridging ligand in the diiron subcluster of the active site. The two most probable candidates are 1,3-dithiopropane (propane dithiol, PDT) and di-(thiomethyl)-amine (DTN). In the latter case, the dithiol-bridging ligand is assumed to play a major role in the reaction cycle. We report density-functional theory studies of the differing roles of these dithiol-bridging ligands in the infrared spectra of synthetic models and of computational representations of the diiron cluster of the active site. Our analysis shows distinct spectral features associated with the dithiol-bridging NH mode for compounds having a DTN bridge, which, however, would have been obscured by the H2O vibrations in existing measurements. However, if indeed nitrogen is present in the dithiol-bridging ligand, a combination of selective deuteration and chemical inactivation with CO would create a unique signature in an accessible region of the infrared spectrum, whose position and intensity are predicted.

Complexation of oxoanions and cationic metals by the biscatecholate siderophore azotochelin.

Azotochelin is a biscatecholate siderophore produced by the nitrogen-fixing soil bacterium Azotobacter vinelandii. The complexation properties of azotochelin with a series of oxoanions [Mo(VI), W(VI) and V(V)] and divalent cations [Cu(II), Zn(II), Co(II) and Mn(II)] were investigated by potentiometry, UV-vis and X-ray spectroscopy. Azotochelin forms a strong 1:1 complex with molybdate (log K=7.6+/-0.4) and with tungstate and vanadate; the stability of the complexes increases in the order Mo

Resolving the CO/CN ligand arrangement in CO-inactivated [FeFe] hydrogenase by first principles density functional theory calculations.

The currently presumed assignment of CO/CN ligands in the structure of the active cluster in CO-inactivated [FeFe] hydrogenase is shown to be inconsistent with the available IR data in the enzyme from Clostridium pasteurianum I. A different arrangement has the correct qualitative and quantitative features, reproducing the observed line spacing and intensities and the observed line shift consequent to inactivation with labeled 13CO instead of 12CO. The new assignment is also consistent with the observed change from rhombic to axial symmetry of the electron paramagnetic resonance g tensor upon inactivation.

[PdS2C2(COOMe)2](6n) (n = 0, -1, -2, -3, -4): hexanuclear homoleptic palladium dithiolene complexes.

Reaction of a stoichiometric equivalent of the zinc-dithiolene complex, (tmeda)ZnS2C2(COOMe)2 (tmeda = tetramethylethylenediamine), with (MeCN)2PdCl2 results in a 1:1 homoleptic dithiolene that forms the hexanuclear cluster [PdS2C2(COOMe)2]6 (1). X-ray structure analysis of 1 indicates a Pd6S12 core comprised of six face-centered palladium atoms and 12 edge-centered sulfur atoms situated on an imaginary approximate cube. Complex 1 undergoes four distinct and reversible one-electron redox steps in dichloromethane at -186, -484, -1174, and -1524 mV versus a standard calomel electrode (ferrocenium+/ferrocene redox couple 409 mV). The two-electron reduction product of 1, [Bu4N]2[(PdS2C2(COOMe)2)6] (2), has been chemically isolated and characterized.