Abstract: Hydrothermal synthesis of [Cr^III₂(OH)₄Nb₁₀O₃₀]^8- in gram-scale quantities leads to a new polyoxometalate ion composed of two CrNb₆O₁₉ Lindqvist-type units that are fused via shared faces. The two Cr^III atoms are located in the centre of the molecule and are bridged by two μ₄-O atoms. Electronic transitions are calculated using density functional theory and compare well with the measured UV-Vis spectra..

Son, Jungho;[1] Ohlin, C. André;[2] Casey, William H.;[1] A New Class of Soluble and Stable Transition-Metal-Substituted polyoxoniobates: [Cr₂(OH)₄Nb₁₀O₃₀]^8-. Dalton Trans. , 2012, 41, 12674-12677. Link.

1. Department of Chemistry, University of California, Davis, California 95616, USA.
2. School of Chemistry, Monash University, Vic 3800, Australia.

Abstract: Recently we demonstrated that the decavanadate (V₁₀) ion is a stronger Ca²⁺-ATPase inhibitor than other oxometalates, such as the isoelectronic and isostructural decaniobate ion, and the tungstate and molybdate monomer ions, and that it binds to this protein with a 1:1 stoichiometry. The V₁₀ interaction is not affected by any of the protein conformations that occur during the process of calcium translocation (i.e. E1, E1P, E2 and E2P) ( Fraqueza et al, J. Inorg. Biochem. 2012 ). In the present study, we further explore this subject, and we can now show that the decaniobate ion, [Nb₁₀O₂₈]^6-, is a useful tool in deducing the interaction and the non-competitive Ca²⁺-ATPase inhibition by decavanadate ion. Moreover, decavanadate and vanadate induce protein cysteine oxidation whereas no effects were detected for the decaniobate, tungstate or molybdate ions. The presence of the antioxidant quercetin prevents cysteine oxidation, but not ATPase inhibition, by vanadate or decavanadate. Typical V(IV) EPR spectra were observed for decavanadate in the presence of sarcoplasmic reticulum Ca²⁺-ATPase, indicating a vanadate reduction at some stage of the protein interaction. Raman spectroscopy clearly shows that the protein conformation changes that are induced by decavanadate, decaniobate and vanadate are different from the ones induced by molybdate and tungstate monomer ions. This was observable through an attenuation of specific bands in the presence of V and Nb, but with an increase of intensity of several peaks in the presence of Mo and W. Here, Mo and W cause changes similar to those by phosphate, yielding changes similar to the E1P protein conformation. The putative reduction of vanadium(V) to vanadium(IV) and the non-competitive binding of the [V₁₀O₂₈]^6- and [Nb₁₀O₂₈]^6- decametalates may explain the differences in the Raman spectra compared those seen in the presence of molybdate or tungstate. We suggest that the ability of decavanadate to inhibit the Ca2+-ATPase may be at least in part due to the process of vanadate reduction and associated protein cysteine oxidation. These results contribute to the understanding and application of these families of mono- and polyoxometalates as effective modulators of many biological processes, particularly those associated with calcium homeostasis.

Fraqueza, Gil;[1] Batista de Carvalho, Luis A.E.;[2] Marques, M. Paula M.;[3] Maia, Luisa;[4] Ohlin, C. André;[5] Casey, William H.;[6] Aureliano, Manuel [7] Decavanadate, decaniobate, tungstate and molybdate interactions with sarcoplasmic reticulum Ca²⁺-ATPase: Quercetin prevents cysteine oxidation by vanadate but does not revert ATPase inhibition Dalton Trans. , 2012, 41, 12749-12758. Link.

1. ISE and CCmar, University of Algarve, 8005-139 Faro, Portugal.
2. Molecular Physical Chemistry R&D Unit, University of Coimbra,  Portugal.
3. Department of Life Sciences, University of Coimbra, 3004-535 Coimbra, Portugal.
4. REQUIMTE, Centro de Química Fina e Biotecnologia, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Campus da Caparica, 2829-516 Caparica, Portugal.
5. School of Chemistry, Monash University, Vic 3800, Australia.
6. Department of Chemistry, University of California, Davis, California 95616, USA.
7. CCmar and FCT, University of Algarve, 8005-139 Faro, Portugal

Abstract: The ammonium salt of [Fe₄O(OH)(hpdta)₂­(H₂O)₄]- is soluble and makes a monospecific solution of [Fe₄O(OH)(hpdta)₂­(H₂O)₄]⁰(aq) in acidic solutions (hpdta= 2-Hydroxypropane-1,3-diamino-N,N,N’,N’-tetraacetate). This tetramer is a diprotic acid with pKa₁ estimated at 5.7±0.2 and pKa₂ = 8.8(5)±0.2. In the pH region below pKa₁, the molecule is stable in solution and ¹⁷O-NMR line widths can be interpreted using the Swift-Connick equations to acquire rates of ligand substitution at the four isolated bound water sites. Averaging five measurements at pH<5, where contribution from the less-reactive conjugate base are minimal, we estimate: k_ex²⁹⁸ = 8.1(±2.6)^.10⁵ s^-1, ΔH‡ = 46.(±4.6 ) kJ mol^-1, ΔS‡ = 22(±18) J mol-1 K-1 and ΔV‡ = +1.85 (±0.1) cm³ mol^-1 for waters bound to the fully protonated, neutral molecule. Regressing the experimental rate coefficients versus 1/[H⁺] to account for the small pH variation in rate yields a similar value of k_ex²⁹⁸ = 8.3(±0.8)^.10⁵ s^-1. These rate are ca 10⁴ times faster than those of the [Fe(OH₂)₆]³⁺ ion (k_ex²⁹⁸ =1.6^.10² s^-1) but are about an order of magnitude slower than other studied aminocarboxylate complexes, although these complexes have seven-coordinated Fe(III), not six as in the [[Fe₄O(OH)(hpdta)₂­(H₂O)₄]⁰(aq) molecule. As pH approaches pKa₁, the rates decrease and a compensatory relation is evident between the experimental ΔH‡ and ΔS‡ values. Such variation cannot be caused by enthalpy from the deprotonation reaction and is not well understood. A correlation between <FeIII-OH₂> bond lengths and the logarithm of k_ex²⁹⁸ is geochemically important because it could be used to estimate rate coefficients for geochemical materials for which only DFT calculations are possible. This molecule is the only neutral, oxo-bridged Fe(III) multimer for which rate data are available.

Panasci, Adele F.;[1,2] Ohlin, C. André;[1,2,3] Harley, Stephen J.;[1,2,4] Casey, Willian H. [1,2] Rates of water exchange on the [Fe₄O(OH)(hpdta)₂­(H₂O)₄]⁰ molecule and its implications for geochemistry Inorganic Chemistry , 2012, 51 (12), 6731-673. Link.

1. Department of Chemistry, University of California, Davis, California, USA.
2. Department of Geology, University of California, Davis, California, USA.
3. School of Chemistry, Monash University, Victoria 3800, Australia.
4. Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, California, USA.

Abstract: Mass spectrometry both complements other analytical techniques, and allows for types of analyses and experiments not possible with common analytical methods such as NMR, IR and UV/VIS. Electrospray constitutes one of the mildest forms of ionisation, making it the preferred method for the analysis of large fragile or reactive ions. There is particular promise for mass spectrometry in aiding the characterisation of polyoxometalates and their solutions, but caution must be taken in designing the experiments in order to yield reliable data, and to avoid the temptation of over-interpreting the relevance of gas-phase data to solution chemistry.

Ohlin, C. André[1] Reaction dynamics and solution chemistry of polyoxometalates by electrospray ionisation mass spectrometry Chem. - Asian J. , 2012, 7(2), 262-270. Link.Highlighted by Angewandte Chemie in vol. 51, issue 7, 2012.

1. School of Chemistry, Monash University, Vic 3800, Australia

Abstract: Over the last few decades there has been increasing interest in oxometalate and polyoxometalate applications to medicine and pharmacology. This interest arose, at least in part, due to the properties of these classes of compounds as anticancer, antidiabetic agents, and also for treatment of neurodegenerative diseases, among others. However, our understanding of the mechanism of action would be improved if biological models could be used to clarify potential toxicological effects in main cellular processes. Sarcoplasmic reticulum (SR) vesicles, containing a large amount of Ca²⁺-ATPase, an enzyme that accumulates calcium by active transport using ATP, have been suggested as a useful model to study the effects of oxometalates on calcium homeostasis. In the present article, it is shown that decaniobate, decavanadate, vanadate, tungstate and molybdate, all inhibited SR Ca²⁺-ATPase, with the following IC₅₀ values: 12, 15, 50, 400 μM and 45 mM, respectively. Decaniobate (Nb₁₀), is the strongest P-type enzyme inhibitor, closely followed by decavanadate (V₁₀). Atomic-absorption spectroscopy (AAS) analysis, indicates that decavanadate binds to the protein with a 1:1 decavanadate:Ca²⁺-ATPase stoichiometry. Furthermore, V₁₀ binds with similar extension to all the protein conformations, which occur during calcium translocation by active transport, namely E1, E1P, E2 and E2P, as analysed by AAS. In contrast, it was confirmed that the binding of monomeric vanadate (H₂VO₄^2-; V₁) to the calcium pump is favoured only for the E2 and E2P conformations of the ATPase, whereas no significant amount of vanadate is bound to the E1 and E1P conformations. Scatchard plot analysis, confirmed a 1:1 ratio for decavanadate-Ca²⁺-ATPase, with a dissociation constant, kd of 1 μM^-1. The interaction of decavanadate V₁₀O₂₈^6- (V₁₀) with Ca²⁺-ATPase is prevented by the isostructural and isoelectronic decaniobate Nb₁₀O₂₈^6- (Nb₁₀), whereas no significant effects were detected with ATP or with heparin, a known competitive ATP binding molecule, suggesting that V₁₀ binds non-competitively, with respect to ATP, to the protein. Finally, it was shown that decaniobate inhibits SR Ca²⁺-ATPase activity in a non competitive type of inhibition, with respect to ATP. Taken together, these data demonstrate that decameric niobate and vanadate species are stronger inhibitors of the SR calcium ATPase than simple monomeric vanadate, tungstate and molybdate oxometalates, thus affecting calcium homeostasis, cell signalling and cell bioenergetics, as well many other cellular processes. The ability of these oxometalates to act either as phosphate analogues, as a transition-state analogue in enzyme-catalysed phosphoryl group transfer processes and as potentially nucleotide-dependent enzymes modulators or inhibitors, suggests that different oxometalates may reveal different mechanistic preferences in these classes of enzymes.

Fraqueza, Gil;[1] Ohlin, C. André[2,3] Casey, William H.;[2] Aureliano, Manuel [4] Sarcoplasmic reticulum calcium ATPase interactions with decaniobate, decavanadate, vanadate, tungstate and molybdate J. Inorg. Biochem. 2012, 107(1), 82-89. Link

1. Department of Food Engineering, ISE, University of Algarve, 8005-139 Faro, Portugal.
2. Department of Chemistry, and Department of Geology, University of California, Davis, CA.
3. School of Chemistry, Monash University, Vic 3800, Australia.
4. CCMar and DCBB, FCT, University of Algarve, 8005-139 Faro, Portugal.

Abstract: Water molecules bound to mineral oxides differ from aqueous ions in that they are usually attached to different metal centers and thus separated from one another. In contrast, for monomeric ions, such as octahedral aquo ions [e.g., Al(H₂O)₆³⁺], the bound waters are closely packed and ions like these have been used to establish reactivity trends for ligand substitutions. Thus, the existing literature about ligand substitution in monomer ions may be a poor guide to the reactions of geochemical interest. To understand how coordination of the reactive functional groups might affect the rates of simple water exchange reactions, we synthesized two structurally similar Rh(III) complexes, [Rh(phen)₂(H₂O)₂]³⁺ [1] and [Rh(phen)₂(H₂O)Cl]²⁺ [2] where (phen)=1,10-phenanthroline. Complex [1] has two adjacent, geminal, water molecules and [2] has a single water adjacent to a bound chloride ion. We employed Rh(III) as a trivalent metal rather than a more geochemically relevant metal like Fe(III) or Al(III) to slow the rate of reaction and dissociation of the molecules, which makes possible measurement of the rates of isotopic substitution by mass spectrometry over weeks. We prepared isotopically unique versions of the molecules, dissolved them in isotopically dissimilar solvent, and measured the rates of exchange from the extents of ¹⁸O and ¹⁶O exchange at the bound waters. The pH dependency of rates differ enormously between the two complexes. Psuedo-first-order rates coefficients at 298 K for water exchanges from the fully protonated molecules are close: k_0,298 = 5^.10^-8(± 0.5^.10^-8) s^-1 for [1] and k_0,298 = 2.510^-9(± 1^.10^-9) for [2]. Enthalpy and entropy activation parameters (ΔH^‡ and ΔS^‡) were measured to be 121.1(± 3) kJ mol^-1, and 22(±10) J mol^-1 K^-1, respectively for [1]. The corresponding parameters for the mono-aquo complex, [2], are 134.6(±3) kJ mol^-1 and 35.6(±1.3) J mol^-1 K^-1. These are comparable to values for other metals and nanometer-size clusters, indicating that this elementary process is not profoundly affected by small changes in structure and size. Rates increase, however, by many orders of magnitude upon deprotonation of one of the bound waters in complex [1]. Key at these conditions is the close proximity of a transferable proton that 2 can convert the bound hydroxyl to a bound water. This interconversion allows the oxygen to exchange as a bound water for a bulk water, rather than as a bound hydroxyl exchanging for a bulk-water molecule, which is slow.

Adele Panasci[1,2], J. Gregory McAlpin[1,2], C. André Ohlin [1,2,4], Shauna Christensen[3]; James C. Fettinger[1], R. David Britt[1], James R. Rustad[2,4], William H. Casey[1,2] Cooperation between bound waters and hydroxyls in controlling isotope-exchange rates Geochim. Cosmochim. Acta 2012, 78, 18-27. Link.

1. Department of Chemistry, University of California, Davis, CA
2. Department of Geology, University of California, Davis, CA
3. Department of Chemistry, Brigham Young University, Rexburg, Idaho
4. Present address: School of Chemistry, Monash University, Vic 3800, Australia (CAO). Sullivan Park Research Center, Corning Inc., One Science Center Drive, Corning, NY 14831 (JRR).