Casey Laboratory

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, J.-H., Ohlin, C. A., Casey, W. H. A new class of soluble and stable transition-metal-substituted polyoxoniobate: [Cr₂(OH)₄Nb₁₀O₃₀]^8-. Dalton Trans., 2012, 41, 12674-12677.

Abstract: Recently the authors 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, the authors further explore this subject, and the authors can now show that the decaniobate ion, [Nb10 = Nb10O28]6-, is a useful tool in deducing the interaction and the non-competitive Ca2+- ATPase inhibition by the decavanadate ion [V10 = V10O28]6-. Moreover, decavanadate and vanadate induce protein cysteine oxidn. whereas no effects were detected for the decaniobate, tungstate or molybdate ions. The presence of the antioxidant quercetin prevents cysteine oxidn., but not ATPase inhibition, by vanadate or decavanadate. Definitive V(iv) EPR spectra were obsd. for decavanadate in the presence of sarcoplasmic reticulum Ca2+-ATPase, indicating a vanadate redn. at some stage of the protein interaction. Raman spectroscopy clearly shows that the protein conformation changes that are induced by V10, Nb10 and vanadate are different from the ones induced by molybdate and tungstate monomer ions. Here, Mo and W cause changes similar to those by phosphate, yielding changes similar to the E1P protein conformation. The putative redn. of vanadium(v) to vanadium(iv) and the non-competitive binding of the V10 and Nb10 decametalates may explain the differences in the Raman spectra compared to those seen in the presence of molybdate or tungstate. Putting it all together, the authors suggest that the ability of V10 to inhibit the Ca2+-ATPase may be at least in part due to the process of vanadate redn. and assocd. protein cysteine oxidn. These results contribute to the understanding and application of these families of mono- and polyoxometalates as effective modulators of many biol. processes, particularly those assocd. with calcium homeostasis.

Fraqueza, Gil; Batista de Carvalho, Luis A. E.; Marques, M. Paula M.; Maia, Luisa; Ohlin, C. Andre; Casey, William H.; Aureliano, Manuel Decavanadate, decaniobate, tungstate and molybdate interactions with sarcoplasmic reticulum Ca2+-ATPase: quercetin prevents cysteine oxidation by vanadate but does not reverse ATPase inhibition Dalton Transactions, 2012, 41(41), 12749-12758.

Abstract: Aluminum possesses significant and diverse chem. Numerous compds. have been defined, and the elucidation of their chem. is of significant geochem. interest. In this paper, a brucite-like, eight-aluminum aq. cluster is modeled with d. functional theory to identify its primary site of deprotonation and the assocd. pKa const. using both explicit (a full first solvent shell) and implicit solvent. Two methods for calcg. the pKa are compared. We found that a bond d. approach is better than a direct energy calcn. for ions with large charge and high symmetry. The terminal aluminum atoms have equatorial ligated waters that in solvent have one long O-H bond. This site is more reactive than any of the other protons on the particle. Insights into the exptl. crystal structure and Bader's Atoms in Mols. d. anal. are presented as routes to reduce the computational time required for the identification of protonation sites.

Wander, Matthew C. F.; Shuford, Kevin L.; Rustad, James R.; Casey, William H. Ab initio calculation of the deprotonation constants of an atomistically defined nanometer-sized, aluminium hydroxide oligomer Molecular Simulation, 2012, 39(3), 220-227.

Abstract: 31P NMR investigation of the CoPi water-oxidn. catalyst is discussed.

Harley, Stephen J.; Mason, Harris E.; McAlpin, J. Gregory; Britt, R. David; Casey, William H. A 31P NMR Investigation of the CoPi Water-Oxidation Catalyst Chemistry - A European Journal, 2012, 18(34), 10476-10479.

Abstract: The ammonium salt of [Fe₄O(OH)(hpdta)₂(H₂O)₄]^- is soluble and makes a monospecific solution of [Fe₄(OH)₂(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 pK_a1 estimated at 5.7±0.2, pK_a2 = 8.8(5)±0.2. In the pH region below pK_a1, the molecule is persistent 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 waters. Averaging five measurements at pH<5, where contribution from the less-reactive conjugate base are minimal, we estimate: k_ex²⁹⁸ = 8.±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 pH variation of the experimental rate coefficient versus (1/[H⁺]) yields a similar value of k_ex²⁹⁸ = 8.3(±0.8)·10⁵ s^-1. These rates are ~10⁴ times faster than those of the [Fe(OH₂)₆]³⁺ ion (k_ex²⁹⁸ =1.6·10² s^{-1 3}) 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 [Fe₄(OH)₂(hpdta)₂(H₂O)₄]⁰(aq) molecule. As pH approaches pK_a1, the rates decrease and a compensatory relation becomes 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 <Fe^III-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, A. F., Ohlin, C. A., Harley, S. J. and Casey, W. H. Rates of Water Exchange on the [Fe₄(OH)₂(hpdta)₂(H₂O)₄]⁰ Molecule and Its Implicatiopns for Geochemistry. Inorg. Chem., 2012, 51, 6731-6738.

Abstract: A review; recently, Nocera and coworkers claim that an "artificial leaf" - able to harvest up to 4.7% of incident photonic energy to split water and produce fuels using only Earth abundant elements - has been produced with much of its design inspired by photosynthetic elements. In this short review, we compare the structure and function of the cobalt-based water oxidn. center of the artificial leaf with what is known about the manganese-based oxygen-evolving complex (OEC) of photosystem II (PSII). The geometric structure, proposed oxidn. state topol., and putative mechanism of water oxidn. for the resp. catalytic sites are discussed.

McAlpin, J. Gregory; Stich, Troy A.; Casey, William H.; Britt, R. David Comparison of cobalt and manganese in the chemistry of water oxidation Coordination Chemistry Reviews, 2012, 256(21-22), 2245-2452.

Abstract: [ThB5O6(OH)6][BO(OH)2]·2.5H2O (Notre Dame Th Borate-1, NDTB-1) is an inorg. supertetrahedral cationic framework material that is derived from boric acid flux reactions. NDTB-1 exhibits facile single crystal to single crystal anion exchange with a variety of common anions such as Cl-, Br-, NO3 -, IO3 -, ClO4 -, MnO4 -, and CrO4 2-. More importantly, NDTB-1 is selective for the removal of TcO4 - from nuclear waste streams even though there are large excesses of competing anions such as Cl-, NO3 -, and NO2 -. Competing anion exchange expts. and magic-angle spinning (MAS)- NMR spectroscopy of anion-exchanged NDTB-1 demonstrate that this unprecedented selectivity originates from the ability of NDTB-1 to trap TcO4 - within cavities, whereas others remain mobile within channels in the material. The exchange kinetics of TcO4 - in NDTB-1 are 2nd-order with the rate const. k2 of 0.059 s-1 M-1. The anion exchange capacity of NDTB-1 for TcO4 - is 162.2 mg g-1 (0.5421 mol mol-1) with a max. distribution coeff. Kd of 1.0534 × 104 mL g- 1. Finally, the exchange for TcO4 - in NDTB-1 is reversible. TcO4 - trapped in NDTB-1 can be exchanged out using higher-charged anions with a similar size such as PO4 3- and SeO4 2-, and therefore the material can be easily recycled and reused.

Wang, Shuao; Yu, Ping; Purse, Bryant A.; Orta, Matthew J.; Diwu, Juan; Casey, William H.; Phillips, Brian L.; Alekseev, Evgeny V.; Depmeier, Wulf; Hobbs, David T.; Albrecht-Schmitt, Thomas E. Selectivity, Kinetics, and Efficiency of Reversible Anion Exchange with TcO4- in a Supertetrahedral Cationic Framework Advanced Functional Materials, 2012, 22(11), 2241-2250.

Abstract: N/A

James R. Rustad and William H. Casey Metastable structures and isotope exchange reactions in polyoxometalate ions provide a molecular view of oxide dissolution. Nature Mater., 2012, 11, 223-226.

Abstract: N/A

Fraqueza, Gil; Ohlin, C. Andre; Casey, William H.; Aureliano, Manuel Sarcoplasmic reticulum calcium ATPase interactions with decaniobate, decavanadate, vanadate, tungstate and molybdate Journal of Inorganic Biochemistry , 2012, 107/1, 82-89.