Casey Laboratory

Abstract Rates of oxygen-isotope exchange at all structural sites in two isostructural polyoxometalates, [H_xNb₁₀O₂₈]^(6−x)− and [H_xTi₂ Nb₈O₂₈]^(8−x)−, show that small changes in structure have surprising and profound effects: a single-site substitution of Ti(IV) for Nb(V) inverts the pH dependencies for rates throughout the structures. Within a given structure, all oxygens exhibit similar pH dependencies although they react over a range greater than 10⁴, indicating that pathways involve concerted motions of the entire lattices. Profound sensitivity to changes in structure and composition suggests reaction pathways in polyoxometalate ions will be highly variable even within structural classes. The results also require new thinking about how ab initio simulations are used to understand reaction pathways involving extended structures, like the mineral−water interface. Our data indicate that reactions proceed via metastable intermediates and that the simulations must be structurally faithful or will miss the essential chemistry.

Villa, Eric M.; Ohlin, C. André; Rustad, James R., Casey, William H. "Isotope-Exchange Dynamics in Isostructural Decametalates with Profound Differences in Reactivity", J. Am. Chem. Soc., 2009 , 131(45), 16488-16492. Issue cover.

Abstract The rate and activation parameters for oxygen exchange of the mu₃-oxo bridges in a manganese tetranuclear cluster with H₂¹⁸O have been measured by ESI-MS; the observed activation entropy of -146 ± 22 J K^-1.mol^-1 is consistent with the expected associative mechanism of substitution.

Ohlin, C. André; Brimblecombe, Robin; Spiccia, Leone; Casey, William H. "Oxygen isotopic exchange in a Mn^IIIMn₃^IV-oxo cubane", Dalton Trans. , 2009, 5278-5280.

Abstract The electrochemical behavior of a trinuclear ruthenium(II)-containing complex, [((phen) Ru(dpp))₂RhCl₂]⁵⁺ (where phen = 1,10-phenanthroline, dpp = 2,3-bis(2-pyridyl )pyrazine), was studied in acetonitrile (MeCN) and aqueous solutions. In MeCN containing 0.10 M tetra-n -butylammonium perchlorate (TBAP), the complex displayed a reversible, overlapping RuII/III redox process with E_1/2 = +1.21 V vs Ag/Ag+ (10 mM), an irreversible reduction of RhIII/I at −0.73 V vs Ag/Ag+, and two quasi-reversible dpp/dpp− couples with E1/2 = −1.11 and −1.36 V vs Ag/Ag+ at a Pt electrode with a scan rate of 50 mV s−1. In 0.20 M Tris buffer solution (pH 7.4), an irreversible, overlapping RuII/III oxidation at +1.48 V vs Ag /AgCl (3 M KCl), and an irreversible reduction of RhIII/II at −0.78 V vs Ag/AgCl were observed at a glassy carbon electrode with a scan rate of 50 mV/s. Investigations on the electrogenerated chemiluminescence (ECL) of the complex revealed that 2-(dibutylamino) ethanol (DBAE) was superior to tri-n-propylamine (TPrA) as an ECL coreactant within their entire concentration range of 10−100 mM in MeCN, and in aqueous media, as low as 1.0 nM of the complex can be detected using TPrA coreactant ECL. A maximum ECL emission of 640 nm, which is about 55 nm blue shift to its fluorescence, was observed in MeCN with DBAE as a coreactant. Interactions of the complex with calf thymus DNA (ctDNA) were conducted with a flow-cell based quartz-crystal microbalance, and a binding constant of 2.5 × 105 M−1 was calculated on the basis of the Langmuir isotherm equation.

Wang, Shijun; Milam, Jenifer; Ohlin, C. Andr%eacute;; Rambaran, Varma; Clark, Eva; Ward, Woodrow; Seymour, Luke; Casey, William H.; Holder, Alvin A.; Miao, Wujian "Electrochemical and Electrogenerated Chemiluminescent Studies of a Trinuclear Complex, [((phen)Ru(dpp))₂RhCl₂]⁵⁺, and Its Interactions with Calf Thymus DNA", Analytical Chem. 2009, 81(10), 4068-4075

Abstract The aluminum Keggin polycation (Al₁₃) has been identified as an effective specie for neutralization and coagulation of anionic contaminants in water. In this study, we compare efficacy of the aluminum Keggin-ion to the analogues containing a single Ga-atom or single Ge-atom (GaAl₁₂ and GeAl₁₂, respectively) substituted into the center of the polycation in water-treatment studies. We investigated removal of bacteriophage (model viruses), Cryptosporidium, dissolved organic carbon (DOC), and turbidity. In every study, the order of contaminant removal efficacy trends GaAl₁₂ > Al₁₃ > GeAl₁₂. By ESI MS (electrospray ionization mass spectrometry), we noted the GaAl₁₂ deprotonates least of the three aluminum polycations, and thus probably carries the highest charge, and also optimal contaminant-neutralization ability. The ESI MS studies of the aluminum polycation solutions, as well as solid-state characterization of their resulting precipitates both reveal some conversion of Al₁₃ to larger polycations, Al₃₀ for instance. The GaAl12 does not show any evidence for this alteration that is responsible for poor shelf life of commercial prehydrolyzed aluminum coagulants such as polyaluminum chloride. Based on these studies, we conclude that substitution of a single Ga-atom in the center of the aluminum Keggin polycation produces an optimal water-treatment product due to enhanced shelf life and efficacy in neutralization of anionic contaminants.

Stewart, Tom; Trudell, Daniel; Alam, Todd ; Ohlin, André; Lawler, Christian; Casey, William; Jett, Stephen; Nyman, May Enhanced water purification: a single atom makes a difference, Environ. Sci. Techn. 2009, 43(14), 5416-5422.

This was featured in C&EN 2009, 87(25),35. Link

Abstract The growth of calcium oxalate monohydrate in the presence of Tamm-Horsfall protein (THP), osteopontin, and the 27-residue synthetic peptides (DDDS)6DDD and (DDDG)6DDD (D = aspartic acid, S = serine, and G = glycine) was investigated via in situ atomic force microscopy. The results show that these four growth modulators create extensive deposits on the crystal faces. Depending on the modulator and crystal face, these deposits can occur as discrete aggregates, filamentary structures, or uniform coatings. These proteinaceous films can lead to either the inhibition of or an increase in the step speeds (with respect to the impurity-free system), depending on a range of factors that include peptide or protein concentration, supersaturation, and ionic strength. While THP and the linear peptides act, respectively, to exclusively increase and inhibit growth on the (101) face, both exhibit dual functionality on the (010) face, inhibiting growth at low supersaturation or high modulator concentration and accelerating growth at high supersaturation or low modulator concentration. Based on analyses of growth morphologies and dependencies of step speeds on supersaturation and protein or peptide concentration, we propose a picture of growth modulation that accounts for the observations in terms of the strength of binding to the surfaces and steps and the interplay of electrostatic and solvent-induced forces at the crystal surface.

Weaver, M. L., Qiu, S. R., Hoyer, J. R., Casey, W. H., Nancollas, G. H., De Yoreo, J.J. Surface Aggregation of Urinary Proteins and Aspartic Acid-Rich Peptidees on the Faces of Calcium Oxalate Monohydrate Investigated by In Situ Force Microscopy, Calcif. Tissue Int., 2009, 84(6) , 462-473.

Abstract A new water-soluble polyoxometalate, [TiNb₉O₂₈]^7- , completes an isostructural and homologous decametalate series from the decaniobate to the dititano-octaniobate ion.

Ohlin, C. A., Villa, E. M., Fettinger, J. C., Casey, W.H. "A new titanoniobate ion - Completing the series [Nb₁₀O₂₈]^6-,[TiNb₉O₂₈]^7- and [Ti₂ Nb₈O₂₈]^8-" Dalton Trans., 2009 , 2677-2678.

Abstract Minerals dissolve by repeated ligand exchange reactions and geochemists use polyoxometalate ions to establish structure-reactivity relations for environmentally important functional groups. Here, for example, are plotted the dissolution rates of two classes of minerals against rates of solvent exchanges around the corresponding aquo ions. Geochemists and environmental chemists make predictions about the fate of chemicals in the shallow earth over enormously long times. Key to these predictions is an understanding of the hydrolytic and complexation reactions at oxide mineral surfaces that are difficult to probe spectroscopically. These minerals are usually oxides with repeated structural motifs, like silicate or aluminosilicate polymers, and they expose a relatively simple set of functional groups to solution. The geochemical community is at the forefront of efforts to describe the surface reactivities of these interfacial functional groups and some insights are being acquired by using small oligomeric oxide molecules as experimental models. These small nanometer-size clusters are not minerals, but their solution structures and properties are better resolved than for minerals and calculations are relatively well constrained. The primary experimental data are simple rates of steady oxygen-isotope exchanges into the structures as a function of solution composition that can be related to theoretical results. There are only a few classes of large oxide ions for which data have been acquired and here we review examples and illustrate the general approach, which also derives directly from the use of model clusters to understand for the active core of metalloenzymes in biochemistry.

Casey, W.H., Rustad, J. R., Spiccia, L. Minerals as molecules - Use of aqueous oxide and hydroxide to understand geochemical reactions, Eur. Chem. J., 2009, 15(18), 4496-4515.

Selected as issue Cover

Abstract Rates of oxygen-isotope exchange at all structural sites in two isostructural polyoxometalates, [H_xNb₁₀O₂₈]^(6−x)− and [H_xTi₂ Nb₈O₂₈]^(8−x)−, show that small changes in structure have surprising and profound effects: a single-site substitution of Ti(IV) for Nb(V) inverts the pH dependencies for rates throughout the structures. Within a given structure, all oxygens exhibit similar pH dependencies although they react over a range greater than 10⁴, indicating that pathways involve concerted motions of the entire lattices. Profound sensitivity to changes in structure and composition suggests reaction pathways in polyoxometalate ions will be highly variable even within structural classes. The results also require new thinking about how ab initio simulations are used to understand reaction pathways involving extended structures, like the mineral−water interface. Our data indicate that reactions proceed via metastable intermediates and that the simulations must be structurally faithful or will miss the essential chemistry.

Ohlin, C.A., Villa, E.M., Casey, W. H. One-pot synthesis of the decaniobate salt [N(CH₃)₄]₆[Nb₁₀O₂₈]^.6[H₂O] from hydrous niobium oxide., Inorg. Chim. Acta, 2009, 362, 1391-1392.