Casey Laboratory

Abstract: Multicopper oxidases (MCOs) catalyze the oxidation of a diverse group of metal ions and organic substrates by successive single-electron transfers to O2 via four bound Cu ions. MnxG, which catalyzes MnO2 mineralization by oxidizing both Mn(II) and Mn(III), is unique among multicopper oxidases in that it carries out two energetically distinct electron transfers and is tightly bound to accessory proteins. There are two of these, MnxE and MnxF, both approximately 12 kDa. Although their sequences are similar to those found in the genomes of several Mn-oxidizing Bacillus species, they are dissimilar to those of proteins with known function. Here, MnxE and MnxF are co-expressed independent of MnxG and are found to oligomerize into a higher order stoichiometry, likely a hexamer. They bind copper and heme, which have been characterized by electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS), and UV–visible (UV–vis) spectrophotometry. Cu is found in two distinct type 2 (T2) copper centers, one of which appears to be novel; heme is bound as a low-spin species, implying coordination by two axial ligands. MnxE and MnxF do not oxidize Mn in the absence of MnxG and are the first accessory proteins to be required by an MCO. This may indicate that Cu and heme play roles in electron transfer and/or Cu trafficking.

Cristina N. Butterfield, Lizhi Tao, Kelly N. Chacón, Thomas G. Spiro, Ninian J. Blackburn, William H. Casey, R. David Britt, Bradley M. Tebo,Multicopper manganese oxidase accessory proteins bind Cu and heme, Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics, Volume 1854, Issue 12, December 2015, Pages 1853-1859.

Abstract: Two new rhodium-substituted polyoxoniobates, [H2RhNb9O28]6- (RhNb9) and [Rh2(OH)4Nb10O30]8- (Rh2Nb10) are reported. The two distinct RhIII-substituted niobate clusters behave differently when the pH is raised with TMAOH: the Rh2Nb10 is stable until pH ~ 12.7, but RhNb9 dissociates to form RhNb5 and RhNb10, similar to some of our other metal-substituted niobates, such as the MNb9 ions (M = Cr or Mn), which transform to MNb10 when the solution pH is raised.

Son, J. H.; Casey, W. H.Two RhIII-substituted polyoxoniobates and their base-induced transformation: [H2RhNb9O28]6- and [Rh2(OH)4Nb10O30]8- Dalton Trans., 2015, Advance Article

Abstract: The intercalation of lithium from solution into the six-membered µ2-oxo rings on the basal planes of gibbsite is well-constrained chemically. The product is a lithiated layered-double hydroxide solid that forms via in situ phase change. The reaction has well established kinetics and is associated with a distinct swelling of the gibbsite as counter ions enter the interlayer to balance the charge of lithiation. Lithium reacts to fill a fixed and well identifiable crystallographic site and has no solvation waters. Our lithium-isotope data shows that 6Li is favored during this intercalation and that the solid-solution fractionation depends on temperature, electrolyte concentration and counter ion identity (whether Cl-, NO3- or ClO4-). We find that the amount of isotopic fractionation between solid and solution (?Lisolid-solution) varies with the amount of lithium taken up into the gibbsite structure, which itself depends upon the extent of conversion and also varies with electrolyte concentration and in the counter ion in the order: ClO4- < NO3- < Cl-. Higher electrolyte concentrations cause more rapid expansion of the gibbsite interlayer and some counter ions, such as Cl-, are more easily taken up than others, probably because they ease diffusion. The relationship between lithium loading and ?Lisolid-solution indicates two stages: (1) uptake into the crystallographic sites that favors light lithium, in parallel with adsorption of solvated cations, and (2) continued uptake of solvated cations after all available octahedral vacancies are filled; this second stage has no isotopic preference. The two-step reaction progress is supported by solid-state NMR spectra that clearly resolve a second reservoir of lithium in addition to the expected layered double-hydroxide phase.

Josh Wimpenny, Christopher A. Colla, Ping Yu, Qing-Zhu Yin, James R. Rustad, William H. Casey, Lithium isotope fractionation during uptake by gibbsite, Geochimica et Cosmochimica Acta, Volume 168, 1 November 2015, Pages 133-150.

Abstract: Caps in a-Keggin-type polyoxometalates [PM2Nb12O40]9- (M: Nb=O or V=O) can be removed in basic condition to produce uncapped [PNb12O40]15-. Transmetalation or capping occurs from the reaction of [PNb14O42]9- or [PNb12O40]15- with either Sb2O3 or V2O5 to form [PSb2Nb12O40]9- or [PV2Nb12O42]9-, respectively.

Son, Jung-Ho; Casey, William H. Reversible capping/uncapping of phosphorous-centered Keggin-type polyoxoniobate clusters Chemical Communications (2015), 51(8), 1436-1438.

Abstract: Nuclear spin relaxation rates of 2H and 139La in LaCl3+2H2O and La(ClO4)3+2H2O solutions were determined as a function of pressure in order to demonstrate a new NMR probe designed for solution spectroscopy at geochemical pressures. The 2H longitudinal relaxation rates (T1) vary linearly to 1.6 GPa, consistent with previous work at lower pressures. The 139La T1 values vary both with solution chemistry and pressure, but converge with pressure, suggesting that the combined effects of increased viscosity and enhanced rates of ligand exchange control relaxation. This simple NMR probe design allows experiments on aqueous solutions to pressures corresponding roughly to those at the base of the Earth’s continental crust.

Ochoa, G.; Pilgrim, C. D.; Martin, M. N.; Colla, C. A.; Klavins, P.; Augustine, M. A.; Casey, W. H. (2015), 2H and 139La NMR Spectroscopy in Aqueous Solutions at Geochemical Pressures. Angew. Chem. Int. Ed.

Abstract: The dynamics of manganese solid formation (as MnOx) by the multicopper oxidase (MCO)-containing Mnx protein complex were examined by electron paramagnetic resonance (EPR) spectroscopy. Continuous-wave (CW) EPR spectra of samples of Mnx, prepared in atmosphere and then reacted with Mn(II) for times ranging from 7 to 600 s, indicate rapid oxidation of the substrate manganese (with two-phase pseudo-first-order kinetics modeled using rate coefficients of: k1obs = 0.205 ± 0.001 s–1 and k2obs = 0.019 ± 0.001 s–1). This process occurs on approximately the same time scale as in vitro solid MnOx formation when there is a large excess of Mn(II). We also found CW and pulse EPR spectroscopic evidence for at least three classes of Mn(II)-containing species in the reaction mixtures: (i) aqueous Mn(II), (ii) a specifically bound mononuclear Mn(II) ion coordinated to the Mnx complex by one nitrogenous ligand, and (iii) a weakly exchange-coupled dimeric Mn(II) species. These findings provide new insights into the molecular mechanism of manganese mineralization.

Tao, L.; Stich, T. A.; Butterfield, C. N.; Romano, C. A.; Spiro, T. G.; Tebo, B. M.; Casey, W. H.; Britt, R. D. Mn(II) Binding and Subsequent Oxidation by the Multicopper Oxidase MnxG Investigated by Electron Paramagnetic Resonance Spectroscopy J. Am. Chem. Soc., 2015, 137 (33), pp 10563–10575

Abstract: This Perspective article highlights some of the traditional and non-traditional analytical tools that are presently used to characterize aqueous inorganic nanoscale clusters and polyoxometalate ions. The techniques discussed in this article include nuclear magnetic resonance spectroscopy (NMR), small angle X-ray scattering (SAXS), dynamic and phase analysis light scattering (DLS and PALS), Raman spectroscopy, and quantum mechanical computations (QMC). For each method we briefly describe how it functions and illustrate how these techniques are used to study cluster species in the solid state and in solution through several representative case studies. In addition to highlighting the utility of these techniques, we also discuss limitations of each approach and measures that can be applied to circumvent such limits as it pertains to aqueous inorganic cluster characterization.

Jackson, M. N.; Kamunde-Devonish, M. K.; Hammann, B. A.; Wills, L. A.; Fullmer, L. B.; Hayes, S. E.; Cheong, P. H. Y.; Casey, W. H.; Nyman, M.; Johnson, D. W. An overview of selected current approaches to the characterization of aqueous inorganic clusters Dalton Trans., 2015,44, 16982-17006.

Abstract: The e-Keggin [AlO4Al12(OH)24(H2O)12]7+ ion (AlAl127+) is a metastable precursor in the formation of aluminum oxyhydroxide solids. It also serves as a useful model for the chemistry of aluminous mineral surfaces. Herein we calculate the enthalpies of formation for this aqueous ion and its heterometal-substituted forms, GaAl127+ and GeAl128+, using solution calorimetry. Rather than measuring the enthalpies of the MAl127/8+ ions directly from solution hydrolysis, we measured the metathesis reaction of the crystallized forms with barium chloride creating an aqueous aluminum solution monospecific in MAl127/8+. Then, the contributions to the heat of formation from the crystallized forms were subtracted using referenced states. When comparing the aqueous AlAl127+ ion to solid aluminum (oxy)-hydroxide phases, we found that this ion lies closer in energy to solid phases than to aqueous aluminum monomers, thus explaining its role as a precursor to amorphous aluminum hydroxide phases.

Reusser, D., Casey, W. H. and Navrotsky, A. (2015), Energetic Insight into the Formation of Solids from Aluminum Polyoxocations. Angew. Chem., 127: 9385–9388.

Abstract: Polyoxoniobate clusters that are stable in acidic solutions are rare and particularly useful in industrial processes. Here we report a new pentaphosphate niobate polyoxometalate cluster (TMA)9H3Nb9P5O41·28H2O (Nb9P5) that is stable over a wide pH range and that can be converted reversibly into the peroxo form.

Son, J.H.; Casey, W.H A new Keggin-like niobium-phosphate cluster that reacts reversibly with hydrogen peroxide Chem. Commun., 2015,51, 12744-12747.

Abstract: Environmental context Most chem. processes in water are either ligand- or electron-exchange reactions. Here the general reactivity trends for ligand-exchange reactions in aq. solns. are reviewed and it is shown that simple rules dominate the chem. These simple rules shed light on most mol. processes in water, including the uptake and degrdn. of pesticides, the sequestration of toxic metals and the corrosion of minerals. Abstr. It is through ligand-?exchange kinetics that environmental geochemists establish an understanding of mol. processes, particularly for insulating oxides where there are not explicit electron exchanges. The substitution of ligands for terminal functional groups is relatively insensitive to small changes in structure but are sensitive to bond strengths and acid-?base chem. Ligand exchanges involving chelating org. mols. are separable into two classes: (i) ligand substitutions that are enhanced by the presence of the chelating ligand, called a 'spectator' ligand and (ii) chelation reactions themselves, which are controlled by the Lewis basicity of the attacking functional group and the rates of ring closure. In contrast to this relatively simple chem. at terminal functional groups, substitutions at bridging oxygens are exquisitely sensitive to details of structure. Included in this class are oxygen-isotope exchange and mineral-dissoln. reactions. In large nanometer-sized ions, metastable structures form as intermediates by detachment of a surface metal atom, often from a underlying, highly coordinated oxygen, such as µ4-oxo, by solvation forces. A metastable equil. is then established by concerted motion of many atoms in the structure. The newly undercoordinated metal in the intermediate adds a water or ligand from soln., and protons transfer to other oxygens in the metastable structure, giving rise to a characteristic broad amphoteric chem. These metastable structures have an appreciable lifetime and require charge sepn., which is why counterions affect the rates. The no. and character of these intermediate structures reflect the symmetry of the starting structure.

Casey, William H. Ligand- and oxygen- isotope-exchange pathways of geochemical interest Environmental Chemistry (2015), 12(1), 1-19.

Abstract: Two new peroxoniobophosphate clusters were isolated as tetramethylammonium (TMA) salts having the stoichiometries: TMA5[HNb4P2O14(O2)4] 9H2O and TMA3[H7Nb6P4O24(O2)6] 7H2O. The former is stable over the pH range: 3-pH-12 and the latter is stable only below pH 3. These two molecules interconvert as a function of solution pH. The [H7Nb6P4O24(O2)6]3- cluster can be used to fabricate patterned niobium phosphate films by electron-beam lithography after solution deposition.

Son, Jung-Ho; Park, Deok-Hie; Keszler, Douglas A.; Casey, William H. Acid-Stable Peroxoniobophosphate Clusters To Make Patterned Films Chemistry - A European Journal (2015), 21(18), 6727-6731.

Abstract: A new 30-center Ga(III)-oxy-hydroxide cation cluster was synthesized by hydrolysis of an aqueous GaCl3 solution near pH = 2.5 and crystallized using 2,6-napthalene disulfonate (NDS). The cluster has 30 metal centers and a nominal stoichiometry: [Ga30(µ4-O)12(µ3-O)4(µ3-OH)4(µ2-OH)42(H2O)16](2,6-NDS)6, where 2,6-NDS = 2,6-napthalene disulfonate This cluster augments the very small library of Group 13 clusters that have been isolated from aqueous solution and closely resembles one other Ga(III) cluster with 32 metal centers that had been isolated using curcurbit ligands. These clusters have uncommon linked Ga(O)4 centers and sets of both protonated and unprotonated µ3-oxo.

Casey, W.H.; Olmstead, M.M.; Hazlett, C.R.; Lamar, C.; Forbes, T.Z. A New Nanometer-Sized Ga(III)-Oxyhydroxide Cation. Inorganics 2015, 3, 21-26.