Nickel(II)-Mediated Reversible Thiolate/Disulfide Conversion as a Mimic for a Key Step of the Catalytic Cycle of Methyl-Coenzyme M Reductase

According to the well-accepted mechanism, methyl-coenzyme M reductase (MCR) involves Ni-mediated thiolate-to-disulfide conversion that sustains its catalytic cycle of methane formation in the energy saving pathways of methanotrophic microbes. Model complexes that illustrate Ni-ion mediated reversible thiolate/disulfide transformation are unknown. In this paper we report the synthesis, crystal structure, spectroscopic properties and redox interconversions of a set of Ni^II complexes comprising a tridentate N₂S donor thiol and its analogous N₄S₂ donor disulfide ligands. These complexes demonstrate reversible Ni^II-thiolate/Ni^II-disulfide (both bound and unbound disulfide-S to Ni^II) transformations via thiyl and disulfide monoradical anions that resemble a primary step of MCR's catalytic cycle.     

Systematic mutational analysis of an ubiquitin ligase (MDM2)-binding peptide: computational studies

To understand the importance of amino acids that comprise the peptide PMI (p53-MDM2/MDMX inhibitor), a p53-mimicking peptide with high affinity for the ubiquitin ligase MDM2, computational alanine scanning has been carried out using various protocols. This approach is very useful for identifying regions of a peptide that can be mutated to yield peptides that bind to their targets with higher affinities. Computational alanine scanning is a very useful technique that involves mutating each amino acid of the peptide in its complex with its target (MDM2 in the current study) to alanine, running short simulations on the mutated complex and computing the difference in interaction energies between the mutant peptides and the target protein (MDM2 in the current study) relative to the interaction energy of the original (wild-type) peptide and the target protein (MDM2 in the current study). We find that running multiple short simulations yield values of computed binding affinities (enthalpies) that are similar to those obtained from a long simulation and are well correlated with the trends in the data available from experiments that used Surface Plasmon Resonance to obtain dissociation constants. The p53-mimicking peptides contain three amino acids (F19, W23 and L26) that are major determinants of the interactions between the peptides and MDM2 and form an essential motif. We find in the current study that the trends amongst the contributions to experimental binding affinities of the hydrophobic residues F19, W23 and L26 are the best reproduced in all the computational protocols examined here. This study suggests that running such short simulations may provide a rapid method to redesign peptides to obtain high-affinity variants against a target protein. We further observe that modelling an extended conformation at the C-terminus of the helical PMI peptides, in accord with the conformation of the p53-peptide complexed to MDM2, reproduces the trends seen amongst the experimental affinities of the peptides that carry the alanine mutations at their C-termini. This suggests that some of the mutant peptides possibly interconvert between helical and extended states and can bind to MDM2 in either conformation. This novel feature, not obvious from the crystallographic data, if factored into modelling protocols, may yield novel high-affinity peptides. Our findings suggest that such protocols may enable rapid investigations of at least certain types of amino acid mutations, notably from large to small amino acids.     

Synthesis, characterization and ion transport properties of hot-pressed solid polymer electrolytes (1−x) PEO:x KI

Synthesis and ion transport properties of hot-pressed solid polymer electrolytes (SPEs), (1-x) PEO: x KI, where x is the content of KI in wt%, are reported. A hot-press technique has been used for the formation of the polymeric membranes in place of the usual solution cast method. The composition (80PEO:20KI) was identified as the highest conducting polymer electrolyte on the basis of compositional dependent conductivity studies of PEO:KI films. A conductivity enhancement of more than two orders of magnitude from that of the pure PEO was achieved. Materials characterization and ion transport mechanism were explained by using various experimental techniques.     

Rhenium,part XIV. Studies on the action of hydrogen halides on [ReO2en2]+

Summary The products from the reaction oftrans-dioxobis(ethylenediamine)rhenium(V) halides with hydrogen halides have been investigated. Dilute (2M) and concentrated hydrochloric acid react with (ReO₂en₂)Cl in the cold to give ReO(OH)enCl₂ and IRe(OH)₂enCl₂]Cl respectively, while with hot 6M HCl [ReOCl₅]^2– is formed. Dilute solutions of HX protonate [ReO₂en₂]X (X = Br and I) giving [ReO(OH)en₂]X₂ which are converted slowly into ReO(OH)enBr₂ and [ReO(OH)en2](I₃)₂. Hot and concentrated solutions of HX reduce Re^v in (ReO₂en2 )X (X = Br and I) giving enH₂(ReBr₆) and ReI₄en. The thermal decomposition of (ReO₂en₂)X (X = Cl, Br and I) has been studied by thermogravimetry and a polymeric compound, Re₂O₇en₂, has been isolated by heating (ReO₂en₂)X at 200°. The compounds have been characterised by molecular conductivities, magnetic susceptibility and i.r. spectra.     

Biligand metal chelates of 1,10-phenanthroline and several salicyclic acid derivatives in solution

Summary Mixed ligand complexes of copper(II), zinc(II), nickel(II) and cobalt(II) ions involving 1,10-phenanthroline (phen) as primary and 3,5-dinitrosalicylic acid (dnsa), 5-nitrosalicylic acid (nsa), 5-chlorosalicylic acid (csa) and 3,5-dibromosalicylic acid (dbsa) as secondary ligands in solution have been investigated potentiometrically [25°, µ = 0.1 M [NaClO₄], medium 50% v: v aqueous ethanol]. The stability order of mixed ligand complexes with respect to the metal ions obeys the natural order: cobalt(II) < nickel(II) < copper(II) > zinc(II). The stabilities of the heterometal chelates have been compared with the corresponding homometal chelates of the secondary ligands and have been interpreted in terms of metal-ligand effects and coulombic interactions between various ligand anion species present.     

Ternary complexes of copper(II), nickel(II) and zinc(II) with nitrilotriacetic acid as a primary ligand and some phenolic acids as secondary ligands

Summary The formation of ternary complexes of the MAL^3– type [where M = Cu^II, Ni^II and Zn^II ; A = nitrilotriacetic acid (NTA); L = 1-hydroxy-2-naphthoic acid (1,2 HNA) and 2-hydroxy-1-naphthoic acid (2,1 HNA)] have been studied potentiometrically in 50% v/v aqueous — ethanol (25° and µ = 0.1). Under identical conditions the binary complexes of the 1,2- and 2,1-HNA ligands have also been examined. The values of mixed ligand formation constants K_MAL have been found to be lower than K_ML (first step formation constant of binary complexes) and even less than (second step formation constant of binary complexes).     

Chemical Composition and Antioxidant Activity of the Leaf Essential Oil of Schefflera venulosa

Chemistry of Natural Compounds -     

Differential scanning calorimetry studies on aged iron(III)-chromium(III)-hydroxide coprecipitates

DSC measurements in air from 20 to 800° are reported on aged iron(III)-chromium(III) coprecipitated hydroxides (with Fe³⁺Cr³⁺ wt% ratios of 91, 73, 11, 37 and 19). A comparison of the DSC profiles of aged coprecipitates with the thermal behaviour of freshly precipitated and well characterized samples provided information on the modification occurring in the structure and reactivity of the coprecipitates on prolonged aging.

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Zusammenfassung Es werden DSC-Messungen an gealterten kogefällten Eisen(III)Chrom(III)-hydroxiden (mit Fe³⁺Cr³⁺ Gewichtsanteilen von 91, 73, 11, 37 und 19) im Temperaturbereich von 20 bis 800° beschrieben. Ein Vergleich der DSC-Profile von gealterten Kofällungen mit dem thermischen Verhalten frisch gefällter und gut definierbarer Proben lieferte Informationen über die Veränderungen in Struktur und Reaktivität der Koniederschläge bei längerem Altern.

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Dedicated to Professor Wahid U. Malik, Founder-President, Indian Council of Chemists, India, with profound regards and fond memories of his youthful enthusiasm and ceaseless and pioneering efforts towards the development of chemical education and research activities at the University of Roorkee, Roorkee, India, on the occasion of his 70th birthday.

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Paper presented at the World Conference on Thermal Analysis, Capri (Italy), 1989.

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We thank Professor J. Pescia, Dr.G. Ablart and N. Favillier of Laboratoire de Magnétisme et d'Electronique Quantique for their interest in this work. A. C. Vajpei expresses his gratefulness to C.N.R.S. and C.N.O.U.S., Paris, for financial support of this work.     

Photometric determination of trivalent gallium,indium and thallium with Xylenol orange

Summary Xylenol orange reacts very sensitively with gallium(III), indium (III) and thallium(III) to form reddish violet colored chelates having _max 560 nm in case of Ga and In and _max 590 nm in case of Ti at P_H 4.0. The molar ratio for all the chelates is 1 1 (metal reagent). Optimum conditions including the range for adherence to Beer's law, effect of P_H on the color intensity, effect of excess reagent, and sensitivity are reported for the photometric determination of these metal ions using Xylenol orange.

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Zusammenfassung Xylenolorange reagiert sehr empfindlich mit Gallium(III), Indium(III) und Thallium(III) unter Bildung rötlich-violetter Chelate mit einem Absorptionsmaximum bei 560 nm im Falle von Ga und In bzw. 590 nm für Tl bei p_H 4,0. Das Molverhältnis ist in jedem Fall 11. Die besten Arbeitsbedingungen, der Gültigkeitsbereich des Beerschen Gesetzes, der Einfluß des P_H auf die Farbintensität und des Reagensüberschusses sowie die Empfindlichkeit werden für die photometrische Bestimmung der genannten Ionen angegeben.