School of Chemistry - Research Publications

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    The structure and activity of the glutathione reductase from Streptococcus pneumoniae
    Sikanyika, M ; Aragao, D ; McDevitt, CA ; Maher, MJ (INT UNION CRYSTALLOGRAPHY, 2019-01)
    The glutathione reductase (GR) from Streptococcus pneumoniae is a flavoenzyme that catalyzes the reduction of oxidized glutathione (GSSG) to its reduced form (GSH) in the cytoplasm of this bacterium. The maintenance of an intracellular pool of GSH is critical for the detoxification of reactive oxygen and nitrogen species and for intracellular metal tolerance to ions such as zinc. Here, S. pneumoniae GR (SpGR) was overexpressed and purified and its crystal structure determined at 2.56 Å resolution. SpGR shows overall structural similarity to other characterized GRs, with a dimeric structure that includes an antiparallel β-sheet at the dimer interface. This observation, in conjunction with comparisons with the interface structures of other GR enzymes, allows the classification of these enzymes into three classes. Analyses of the kinetic properties of SpGR revealed a significantly higher value for Km(GSSG) (231.2 ± 24.7 µM) in comparison to other characterized GR enzymes.
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    A luminescent solar concentrator ray tracing simulator with a graphical user interface: features and applications
    Zhang, B ; Yang, H ; Warner, T ; Mulvaney, P ; Rosengarten, G ; Wong, WWH ; Ghiggino, KP (IOP PUBLISHING LTD, 2020-07)
    A Monte-Carlo ray tracing simulator with a graphical user interface (MCRTS-GUI) has been developed to provide a quantitative description, performance evaluation and photon loss analysis of luminescent solar concentrators (LSCs). The algorithm is applied to several practical LSC device structures including multiple dyes in the same waveguiding layer, and structures where a dye layer is sandwiched between clear substrates. The effect of the host matrix absorption and the influence of the neighboring layers are investigated. Validations demonstrate that the MCRTS-GUI developed provides a reliable and accurate description of LSC performance. Code for the mixed-dye single layer configuration is converted into a ray-tracing package with a user-friendly interface and is made available as open source software.
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    Incorporating whey protein aggregates produced with heat and ultrasound treatment into rennet gels and model non-fat cheese systems
    Gamlath, CJ ; Leong, TSH ; Ashokkumar, M ; Martin, GJO (Elsevier, 2020-12-01)
    Native whey proteins (WP) are expulsed from cheese coagulation during syneresis. Although incorporating denatured WP aggregates into cheese gels has been previously proposed to improve the overall cheese yield, the effects of WP aggregate properties on gelation kinetics and protein retention are not yet fully understood. In this study, heat and power ultrasound were used to produce denatured whey protein aggregates with a wide range of sizes. The effects of size and hydrophobicity differences in WP aggregates produced by heat and heat coupled with ultrasound were investigated in relation to the kinetics of rennet gelation and protein retention in model non-fat cheddar cheeses. Rheological measurements showed that sufficiently large, denatured WP aggregates could avoid impairment of rennet gelation caused by native WP, irrespective of changes in the soluble calcium concentration or the surface hydrophobicity of the aggregates. WP aggregates formed by the combined heat and ultrasound treatment were more hydrophobic than the larger heat-treated aggregates and were better retained in the cheese. However, inclusion of sufficiently large aggregates in cheese milk conferred an openness to the cheese microstructure and showed promise in improving the otherwise rigid non-fat cheese.
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    Element 25-Manganese
    Boskovic, C (CSIRO Publishing, 2019)
    This essay in a series being published in the Australian Journal of Chemistry concerns this author’s favourite element, manganese. I was introduced to the chemistry of manganese when undertaking postdoctoral work with George Christou at Indiana University. My first reaction involved the disproportionation of manganese(VII) and manganese(II) to give a trinuclear manganese( III) complex. This one reaction encompasses several of the most appealing aspects of inorganic chemistry – the redox chemistry of metals that exist in multiple oxidation states, the change in a metal ion’s properties upon changing the number of valence electrons, the beautifully symmetric structures of polynuclear metal complexes and, of course, the colours!
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    DFT Prediction and Experimental Investigation of Valence Tautomerism in Cobalt-Dioxolene Complexes
    Gransbury, GK ; Boulon, M-E ; Petrie, S ; Gable, RW ; Mulder, RJ ; Sorace, L ; Stranger, R ; Boskovic, C (AMER CHEMICAL SOC, 2019-04-01)
    The family of complexes of general formula [Co(Me ntpa)(Xdiox)]+ (tpa = tris(2-pyridylmethyl)amine, n = 0-3 corresponds to successive methylation of the 6-position of the pyridine rings; X = Br4, Cl4, H4, 3,5-Me2, 3,5- tBu2; diox = dioxolene) was investigated by density functional theory (DFT) calculations to predict the likelihood of valence tautomerism (VT). The OPBE functional with relativistic and solvent corrections allowed accurate reproduction of trends in spin-state energetics, affording the prediction of VT in complex [Co(Me3tpa)(Br4diox)]+ (1+). One-electron oxidation of neutral precursor [CoII(Me3tpa)(Br4cat)] (1) enabled isolation of target compounds 1(PF6) and 1(BPh4). Solution variable-temperature UV-vis absorption and Evans method magnetic susceptibility data confirm DFT predictions that 1+ exists in a temperature-dependent valence tautomeric equilibrium between low-spin Co(III)-catecholate and high-spin Co(II)-semiquinonate forms. The solution VT transition temperature of 1+ is solvent-tunable with critical temperatures in the range of 291-359 K for the solvents measured. Solid-state magnetic susceptibility measurements of 1(PF6) and 1(BPh4) reveal the onset of VT transitions above room temperature.
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    Valence tautomerism and spin crossover in pyridinophane-cobalt-dioxolene complexes: an experimental and computational study
    Tezgerevska, T ; Rousset, E ; Gable, RW ; Jameson, GNL ; Carolina Sanudo, E ; Starikova, A ; Boskovic, C (ROYAL SOC CHEMISTRY, 2019-08-21)
    Compounds [Co(L-N4R2)(dbdiox)](BPh4) (L-N4R2 = N,N'-di-alkyl-2,11-diaza[3.3]-(2,6)pyridinophane, R = iPr (1a), Et (2a); dbdiox = 3,5-di-tert-butyldioxolene) and [M(L-N4iPr2)(dbdiox)](BPh4) (M = Mn (3a), Fe (4a)) have been synthesized and investigated with a view to possible valence tautomeric (VT) or spin crossover (SCO) interconversions. Single crystal X-ray diffraction data for all compounds at 100 or 130 K indicate trivalent metal cations and di-tert-butylcatecholate (dbcat2-) dioxolene ligands. Variable temperature magnetic susceptibility data for all species between 2 and 340 K are consistent with these redox states, with low spin configurations for the cobalt(iii) ions and high spin for the manganese(iii) and iron(iii) ions. Above 340 K, compound 1a exhibits an increase in magnetic susceptibility, suggesting the onset of a VT interconversion from low spin Co(iii)-dbcat to high spin Co(ii)-dbsq (dbsq- = di-tert-butylsemiquinonate) that is incomplete up to 400 K. In solution, variable temperature electronic absorption spectra and Evans NMR method magnetic susceptibility data indicate reversible VT interconversions for 1a in several solvents, with the transition temperature varying with solvent. Variable temperature electronic absorption spectra are temperature-invariant for 3a and 4a, while spectra for 2a in 1,2-dichloroethane suggest the onset of a VT transition at the highest temperatures measured. Density functional theory calculations on all four compounds and literature analogues provide key insights into the relative energies of the different electromeric forms and the possibilities for VT versus SCO interconversions in this family of compounds.
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    Tetraoxolene-bridged rare-earth complexes: a radical-bridged dinuclear Dy single-molecule magnet
    Reed, WR ; Dunstan, MA ; Gable, RW ; Phonsri, W ; Murray, KS ; Mole, RA ; Boskovic, C (ROYAL SOC CHEMISTRY, 2019-11-07)
    Two families of neutral tetraoxolene-bridged dinuclear rare earth complexes of general formula [((HBpz3)2RE)2(μ-tetraoxolene)] (RE = Y and Dy; HBpz3- = hydrotris(pyrazolyl)borate; tetraoxolene = fluoranilate (fa2-; 1-RE) or bromanilate (ba2-; 2-RE)) have been synthesised and characterised. In each case, the bridging tetraoxolene ligand is in the diamagnetic dianionic form and each rare earth metal centre has two HBpz3- ligands completing the coordination. Electrochemical studies on the soluble 2-RE family reveal a tetraoxolene-based reversible one-electron reduction. Bulk chemical reduction with cobaltocene affords the cobaltocenium (CoCp+) salt of the 1e-reduced analogue: [CoCp][((HBpz3)2RE)2(μ-ba˙)] (3-RE) that incorporates a radical trianionic form of the bromanilate bridging ligand. Alternating current (ac) magnetic susceptibility studies of 2-Dy reveal slow magnetic relaxation only in the presence of an applied magnetic field, but reduction to radical-bridged 3-Dy affords frequency-dependent peaks in the out-of-phase ac susceptibility in zero applied field. Exchange coupling between the Dy(iii) ions and the radical bridging ligand thus reduces zero-field magnetisation quantum tunnelling and confers single-molecule magnet status on the complex. Comprehensive analysis of the magnetic relaxation data indicates that a combination of Orbach, Raman and direct relaxation processes are required to fit the data for both dysprosium bromanilate complexes.
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    Incorporation of Vanadium and Molybdenum into Yttrium-Arsenotungstates Supported by Amino Acid Ligands
    Bagherjeri, FA ; Ritchie, C ; Gable, RW ; Bryant, G ; Boskovic, C (CSIRO PUBLISHING, 2020)
    The preference for incorporation of molybdenum over tungsten into specific sites of a family of yttrium-arsenotungstates with amino acid ligands prompted exploration of the incorporation of other metals, affording three new vanadium-containing (V/W and V/Mo/W) analogues: K2(GlyH)10[As4(V2W2)W44Y4O160(Gly)8(H2O)12]·11Gly (1), (MBAH)9(L-NleH)3[As4(V2W2)W44Y4O160(L-Nle)8(H2O)12] (2), and (MBAH)9(L-NleH)3[As4(V2W2)Mo2W42Y4O160(L-Nle)8(H2O)12] (3) (Gly=glycine and L-Nle=l-norleucine, MBAH=4-methylbenzylammonium). These hybrid polyoxometalates all possess a tetrametallic oxo-bridged {VIV2WVI2} central core surrounded by an amino acid-ligated cyclic metal-oxo framework. X-Ray photoelectron, UV-visible reflectance, and electron paramagnetic resonance spectroscopy, together with metal analysis, confirm the incorporation of vanadium into the polyoxometalates, while single crystal X-ray diffraction analysis supports the location of the vanadium atoms in the central core.
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    Tetrahalocatecholate Rare Earth Complexes: Dinuclear Motifs with Intramolecular RE•••XC(Ar) Interactions
    Rousset, E ; Gable, RW ; Starikova, A ; Boskovic, C (AMER CHEMICAL SOC, 2020-05-06)
    The reaction of yttrium or cerium nitrate and tetrachloro- or tetrabromocatecholate (X4Cat2- with X = Cl or Br) has afforded the compounds (Et3NH)2[{Y(Cl4Cat)(H2O)2}2(μ-Cl4Cat)2]·2MeOH·2H2O (1-Cl), (Et3NH)2[{Y(Br4Cat)(H2O)2}2(μ-Br4Cat)2]·1.5MeCN (1-Br), (Et3NH)4[(Cl4Cat)(H2O)2Y(μ-Cl4Cat)2Y(Cl4Cat)2]·2.5MeOH·3.5H2O (2-Cl), (Et3NH)4[{Y(Cl4Cat)(Cl4CatH)(H2O)}2(μ-Cl4Cat)2]·4H2O (3-Cl), (Et3NH)7[{CeIV(Cl4CatH)(NO3)(μ2-Cl4Cat)3}2CeIII](NO3)2 (4-Cl), and (Et3NH)4[CeIV(X4Cat)4] (5-X with X = Cl, Br). Small variations of the reaction stoichiometry and crystallization methods allow the isolation of dinuclear yttrium complexes with four, five, and six tetrachlorocatecholate ligands in 1-Cl, 2-Cl, and 3-Cl, respectively. Single crystal X-ray diffraction studies of these compounds reveal a conserved tetrachlorocatecholate-bridged dinuclear yttrium core in each case, but with different peripheral ligation. A key feature of the core unit is Y···ClC(Ar) intramolecular interactions with a catecholate chloro substituent ortho to one of the coordinating oxygen atoms. The tetrabromocatecholate analogue 1-Br has also been obtained. Applying similar methods to redox-active cerium, rather than yttrium, instead affords an unusual mixed-valence trinuclear {CeIIICeIV2} complex in 4-Cl, as well as two mononuclear cerium(IV) complexes in 5-Cl and 5-Br. Density functional theory calculations confirm the [CeIV(Cl4Cat)4]4- charge distribution for 5-Cl.
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    Understanding the Origin of One- or Two-Step Valence Tautomeric Transitions in Bis(dioxolene)-Bridged Dinuclear Cobalt Complexes
    Gransbury, GK ; Livesay, BN ; Janetzki, JT ; Hay, MA ; Gable, RW ; Shores, MP ; Starikova, A ; Boskovic, C (AMER CHEMICAL SOC, 2020-06-17)
    Valence tautomerism (VT) involves a reversible stimulated intramolecular electron transfer between a redox-active ligand and redox-active metal. Bis(dioxolene)-bridged dinuclear cobalt compounds provide an avenue toward controlled two-step VT interconversions of the form {CoIII-cat-cat-CoIII} ⇌ {CoIII-cat-SQ-CoII}⇌{CoII-SQ-SQ-CoII} (cat2- = catecholate, SQ•- = semiquinonate). Design flexibility for dinuclear VT complexes confers an advantage over two-step spin crossover complexes for future applications in devices or materials. The four dinuclear cobalt complexes in this study are bridged by deprotonated 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-5,5',6,6'-tetraol (spiroH4) or 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-4,4',7,7'-tetrabromo-5,5',6,6'-tetraol (Br4spiroH4) with Mentpa ancillary ligands (tpa = tris(2-pyridylmethyl)amine, n = 0-3 corresponds to methylation of the 6-position of the pyridine rings). Complementary structural, magnetic, spectroscopic, and density functional theory (DFT) computational studies reveal different electronic structures and VT behavior for the four cobalt complexes; one-step one-electron partial VT, two-step VT, incomplete VT, and temperature-invariant {CoIII-cat-cat-CoIII} states are observed. Electrochemistry, DFT calculations, and the study of a mixed-valence {ZnII-cat-SQ-ZnII} analog have allowed elucidation of thermodynamic parameters governing the one- and two-step VT behavior. The VT transition profile is rationalized by (1) the degree of electronic communication within the bis(dioxolene) ligand and (2) the matching of cobalt and dioxolene redox potentials. This work establishes a clear path to the next generation of two-step VT complexes through incorporation of mixed-valence class II and class II-III bis(dioxolene) bridging ligands with sufficiently weak intramolecular coupling.