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  • 1
    Language: English
    In: Chemistry – A European Journal, 04 November 2013, Vol.19(45), pp.15166-15174
    Description: Cobalt(diimine‐dioxime) complexes catalyze hydrogen evolution with low overpotentials and remarkable stability. In this study, DFT calculations were used to investigate their catalytic mechanism, to demonstrate that the initial active state was a Co complex and that H was evolved in a heterolytic manner through the protonation of a Cohydride intermediate. In addition, these catalysts were shown to adjust their electrocatalytic potential for hydrogen evolution to the pH value of the solution and such a property was assigned to the presence of a H‐exchange site on the oxime bridge. It was possible to establish that protonation of the bridge was directly involved in the H‐evolution mechanism through proton‐coupled electron‐transfer steps. A consistent mechanistic scheme is proposed that fits the experimentally determined electrocatalytic and electrochemical potentials of cobalt(diimine‐dioxime) complexes and reproduces the observed positive shift of the electrocatalytic potential with increasing acidity of the proton source. : Oxime‐bridge protonation, possibly through proton‐coupled electron transfer (PCET), is directly involved in H evolution catalyzed by cobalt(diimine‐dioxime) complexes (see scheme).
    Keywords: Bridging Ligands ; Cobalt ; Density Functional Calculations ; Hydrogen Evolution ; Proton Coupled Electron Transfer
    ISSN: 0947-6539
    E-ISSN: 1521-3765
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  • 2
    Language: English
    In: Inorganic Chemistry, July 2, 2012, Vol.51(13), p.7087-7093
    Keywords: Amines -- Chemical Properties ; Chemical Bonds -- Analysis ; Cobalt Compounds -- Chemical Properties ; Methyl Compounds -- Chemical Properties ; Nuclear Magnetic Resonance Spectroscopy -- Usage
    ISSN: 0020-1669
    Source: Cengage Learning, Inc.
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  • 3
    In: Chemical Communications, 2013, Vol.49(44), pp.5004-5006
    Description: A combined electrochemical and theoretical study suggests that hydrogen evolution from weak acids catalyzed by a structural mimic of the active site of NiFe hydrogenases [Ni(xbsms)Ru(C 6 Me 6 )Cl] + proceeds through proton-coupled electron transfer steps.
    Keywords: Biomimetic Materials -- Metabolism ; Hydrogen -- Chemistry ; Hydrogenase -- Chemistry;
    ISSN: 1359-7345
    E-ISSN: 1364-548X
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  • 4
    In: Dalton Transactions, Journal of the Chemical Society, Dalton Transactions, 2010, Vol.39(12), pp.3043-3049
    Description: DFT modeling has been used to investigate a previously proposed mechanism of proton reduction catalyzed by [Ni(xbsms)Ru(CO) 2 Cl 2 ] (H 2 xbsms = 1,2-bis(4-mercapto-3,3-dimethyl-2-thiabutyl)benzene), a bio-inspired mimic of NiFe hydrogenases based on a Ni–Ru framework. Protonation of the 2e − -reduced compound, from which a chloride anion has been eliminated, results in the formation of a semi-bridging hydride derivative with structural features comparable to those of the Ni–C state catalytic intermediate of native hydrogenases. The present study thus provides structural and functional insights into the enzymatic mechanism including the possible involvement of a bridging hydride derivative and heterolytic formation of a dihydrogen molecule on a {Ni(μ-S) 2 M} framework.
    Keywords: Chemical Sciences/Inorganic Chemistry ; Chimie/Chimie Inorganique;
    ISSN: 1477-9233
    ISSN: 14779226
    E-ISSN: 1477-9241
    E-ISSN: 14779234
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  • 5
    Language: English
    In: Technometrics, 01 February 2006, Vol.48(1), pp.104-111
    Description: In this article we consider a generalization of the univariate g-and-h distribution to the multivariate situation with the aim of providing a flexible family of multivariate distributions that incorporate skewness and kurtosis. The approach is to modify the underlying random variables and their quantiles, directly giving rise to a family of distributions in which the quantiles rather than the densities are the foci of attention. Using the ideas of multivariate quantiles, we show how to fit multivariate data to our multivariate g-and-h distribution. This provides a more flexible family than the skew-normal and skew-elliptical distributions when quantiles are of principal interest. Unlike those families, the distribution of quadratic forms from the multivariate g-and-h distribution depends on the underlying skewness. We illustrate our methods on Australian athletes data, as well as on some wind speed data from the northwest Pacific.
    Keywords: Kurtosis ; Multivariate ; Quantiles ; Shape ; Skewness ; Transformation ; Engineering ; Statistics ; Mathematics
    ISSN: 0040-1706
    E-ISSN: 1537-2723
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  • 6
    Language: English
    In: Fisheries Research, 2006, Vol.77(3), pp.312-325
    Description: were sampled from two disjunct regions of abundance off the South African east coast – the central Agulhas Bank ( = 227) and Algoa Bay ( = 550) – from March 2001 through February 2003. Marginal increment analyses supported annual deposition of growth zones in sagittal otoliths. Ages recorded using thin otolith sections were 2–23 years and 1–27 years for fish sampled on the central Agulhas Bank and in Algoa Bay, respectively. Maximum ages for males and females were 25 and 27 years, respectively. Although females were larger than males at most ages, comparisons of growth between sexes – based on -tests of mean lengths-at-age, likelihood ratio tests of fitted curves and parameters and comparisons of 95% parameter space – were not unequivocal and are not thought to be biologically significant. Growth rates (sexes pooled) did, however, differ between regions with central Agulhas Bank attaining a greater asymptotic length, but at a lower rate than those in Algoa Bay. The fitted von Bertalanffy growth equations for combined sexes are: = 397 (1 − e ) for the central Agulhas Bank and = 352 (1 − e ) for Algoa Bay. Histological examination of ovary sections confirmed that is a serial batch spawner. Trends in monthly proportions of macroscopic maturity stages in mature females and average monthly gonadosomatic indices suggested an extended spring–summer spawning season from August to December in both regions. Females dominated sex ratios over soft strata (i.e., trawl grounds) during the spawning season indicating that they may return to feeding grounds between spawning events. Estimated size at 50% maturity ( ) did not differ significantly (likelihood ratio tests) between regions or between sexes. was 22 cm for males and 23 cm for females (regions combined). Although female did not differ between regions, differences in growth rate resulted in a higher corresponding age at for Algoa Bay (3.2 years) than on the central Agulhas Bank (2.2 years).
    Keywords: Umbrina Canariensis ; Life-History ; Age and Growth ; Reproduction ; Agriculture
    ISSN: 0165-7836
    E-ISSN: 1872-6763
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  • 7
    In: Energy & Environmental Science, 2011, Vol.4(7), pp.2417-2427
    Description: The dinuclear nickel–manganese complex [Ni(xbsms)Mn(CO) 3 (H 2 O)] + (H 2 xbsms = 1,2-bis(4-mercapto-3,3-dimethyl-2-thiabutyl)benzene) is reported as a bio-inspired mimic of the active site of NiFe hydrogenases catalyzing hydrogen evolution from trifluoroacetic acid in DMF with an overpotential requirement of 860 mV. This is higher than that displayed by Ni–Ru systems [Canaguier et al. , Chem.–Eur. J. , 2009, 15 , 9350–9364] but similar to that found for related noble metal free Ni–Fe mimics [Canaguier et al. , Chem. Commun. 2010, 46 , 5876–5878]. A combined electrochemical and theoretical (DFT) study suggests a heterolytic mechanism for hydrogen evolution from a hydride derivative. The structure of the active intermediate, with a bridging hydride ligand between Ni and Mn, resembles that of the Ni–C active state of NiFe hydrogenases.
    ISSN: 1754-5692
    E-ISSN: 1754-5706
    Source: Royal Society of Chemistry
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  • 8
    Language: English
    In: Radiation oncology (London, England), 20 February 2013, Vol.8, pp.38
    Description: To determine the safety and efficacy of hypofractionated intensity modulated radiation therapy (Hypo-IMRT) using helical tomotherapy (HT) with concurrent low dose temozolomide (TMZ) followed by adjuvant TMZ in patients with glioblastoma multiforme (GBM). Adult patients with GBM and KPS 〉 70 were prospectively enrolled between 2005 and 2007 in this phase I study. The Fibonacci dose escalation protocol was implemented to establish a safe radiation fractionation regimen. The protocol defined radiation therapy (RT) dose level I as 54.4 Gy in 20 fractions over 4 weeks and dose level II as 60 Gy in 22 fractions over 4.5 weeks. Concurrent TMZ followed by adjuvant TMZ was given according to the Stupp regimen. The primary endpoints were feasibility and safety of Hypo-IMRT with concurrent TMZ. Secondary endpoints included progression free survival (PFS), pattern of failure, overall survival (OS) and incidence of pseudoprogression. The latter was defined as clinical or radiological suggestion of tumour progression within three months of radiation completion followed by spontaneous recovery of the patient. A total of 25 patients were prospectively enrolled with a median follow-up of 12.4 months. The median age at diagnosis was 53 years. Based on recursive partitioning analysis (RPA) criteria, 16%, 52% and 32% of the patients were RPA class III, class IV and class V, respectively. All patients completed concurrent RT and TMZ, and 19 patients (76.0%) received adjuvant TMZ. The median OS was 15.67 months (95% CI 11.56 - 20.04) and the median PFS was 6.7 months (95% CI 4.0 - 14.0). The median time between surgery and start of RT was 44 days (range of 28 to 77 days). Delaying radiation therapy by more than 6 weeks after surgery was an independent prognostic factor associated with a worse OS (4.0 vs. 16.1 months, P = 0.027). All recurrences occurred within 2 cm of the original gross tumour volume (GTV). No cases of pseudoprogression were identified in our cohort of patients. Three patients tolerated dose level I with no dose limiting toxicity and hence the remainder of the patients were treated with dose level II according to the dose escalation protocol. Grade 3-4 hematological toxicity was limited to two patients and one patient developed Grade 4 Pneumocystis jiroveci pneumonia. Hypo-IMRT using HT given with concurrent TMZ is feasible and safe. The median OS and PFS are comparable to those observed with conventional fractionation. Hypofractionated radiation therapy offers the advantage of a shorter treatment period which is imperative in this group of patients with limited life expectancy.
    Keywords: Brachytherapy ; Chemoradiotherapy ; Radiotherapy Planning, Computer-Assisted ; Radiotherapy, Intensity-Modulated ; Antineoplastic Agents, Alkylating -- Therapeutic Use ; Brain Neoplasms -- Therapy ; Dacarbazine -- Analogs & Derivatives ; Glioblastoma -- Therapy
    E-ISSN: 1748-717X
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  • 9
    Language: English
    In: Biochemistry, 10 December 2013, Vol.52(49), pp.8949-56
    Description: TrmFO is a tRNA methyltransferase that uses methylenetetrahydrofolate (CH2THF) and flavin adenine dinucleotide hydroquinone as cofactors. We have recently shown that TrmFO from Bacillus subtilis stabilizes a TrmFO-CH2-FADH adduct and an ill-defined neutral flavin radical. The adduct contains a unique N-CH2-S moiety, with a methylene group bridging N5 of the isoalloxazine ring and the sulfur of an active-site cysteine (Cys53). In the absence of tRNA substrate, this species is remarkably stable but becomes catalytically competent for tRNA methylation following tRNA addition using the methylene group as the source of methyl. Here, we demonstrate that this dormant methylating agent can be activated at low pH, and we propose that this process is triggered upon tRNA addition. The reaction proceeds via protonation of Cys53, cleavage of the C-S bond, and generation of a highly reactive [FADH(N5)═CH2]+ iminium intermediate, which is proposed to be the actual tRNA-methylating agent. This mechanism is fully supported by DFT calculations. The radical present in TrmFO is characterized here by optical and EPR/ENDOR spectroscopy approaches together with DFT calculations and is shown to be the one-electron oxidized product of the TrmFO-CH2-FADH adduct. It is also relatively stable, and its decomposition is facilitated by high pH. These results provide new insights into the structure and reactivity of the unique flavin-dependent methylating agent used by this class of enzymes.
    Keywords: Bacillus Subtilis -- Enzymology ; Bacterial Proteins -- Chemistry ; Flavin-Adenine Dinucleotide -- Chemistry ; Trna Methyltransferases -- Chemistry
    ISSN: 00062960
    E-ISSN: 1520-4995
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  • 10
    Language: English
    In: The Chronicle of Higher Education, July 14, 2013, Vol.59(42)
    Keywords: Higher Education -- Finance ; Educational Reform -- Analysis ; Education Policy -- Analysis ; Academia -- Beliefs, Opinions And Attitudes ; Student Financial Aid -- Analysis
    ISSN: 0009-5982
    Source: Cengage Learning, Inc.
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