Rationally designed inhibitors as tools for comparing the mechanism of squalene-hopene cyclase with oxidosqualene cyclase

The inhibition of squalene-hopene cyclase (SHC) (E.C. 5.4.99.-), an enzyme of bacterial membranes catalyzing the formation of pentacyclic sterol-like triterpenes, was studied by using different classes of compounds originally developed as inhibitors of oxidosqualene cyclase (OCS) (E.C. 5.4.99.7), the enzyme of eukaryotes responsible for the formation of tetracyclic precursors of sterols. The mechanism of cyclization of squalene by SHC, beginning with a protonation of the 2,3 double bond by an acidic residue of the enzyme, followed by a series of electrophilic additions of the carbocationic intermediates to the double bonds, is similar to the mechanism of cyclization of 2,3-oxidosqualene by OSC. The inhibitors studied included: (i) analogs of the carbocationic intermediates formed during cyclization, such as aza-analogs of squalene and 2,3-oxidosqualene; (ii) affinity-labeling inhibitors bearing a methylidene reactive group; and (iii) vinyldioxidosqualenes and vinylsulfide derivatives of the substrates. Comparison of the results obtained with the two enzymes, SHC and OSC, showed that many of the most effective inhibitors of OSC were also able to inhibit SHC, while some derivatives acted as specific inhibitors. Differences could be easily explained on the basis of the different substrate specificity of the two enzymes.     

Elastomeric properties of ethylene/propylene copolymers prepared by zirconocene/methylaluminoxane catalysts

Ethylene/propylene copolymers (EPM) have been prepared by a liquid propylene suspension process, using homogeneous catalysts based on nonbridged zirconocenes and methylaluminoxane (MAO). When bis(η⁵-cyclopentadienyl)zirconium dichloride (Cp₂ZrCl₂), bis(η⁵-cyclopentadienyl) dimethyl zirconium (Cp₂ZrMe₂) and bis(η⁵-cyclopentadienyl), dibenzyl zirconium (Cp₂ZrBz₂) were used as catalyst components, EPM having low average molecular weight (MW) were in general obtained in low yield. On the contrary, a very large improvement of productivity and MW was observed when bis(η⁵-indenyl)zirconium dichloride (Ind₂ZrCl₂) and bis(η⁵-indenyl) dimethyl zirconium (Ind₂ZrMe₂) as well as bis(η⁵-tetrahydroindenyl)zirconium dichloride [(IndH₄)₂ZrCI₂] were employed in combination with MAO. In particular, this last catalytic system allowed one to obtain EPM with a narrower distribution of MW in addition to the above characteristics. Better performances, in terms of rheological and elastomeric properties, were also found for the crude and vulcanized samples prepared by (IndH₄)₂ZrCl₂/MAO catalyst.     

How changes affect software entropy: an empirical study

Context Software systems continuously change for various reasons, such as adding new features, fixing bugs, or refactoring. Changes may either increase the source code complexity and disorganization, or help to reducing it. Aim This paper empirically investigates the relationship of source code complexity and disorganization—measured using source code change entropy—with four factors, namely the presence of refactoring activities, the number of developers working on a source code file, the participation of classes in design patterns, and the different kinds of changes occurring on the system, classified in terms of their topics extracted from commit notes. Method We carried out an exploratory study on an interval of the life-time span of four open source systems, namely ArgoUML, Eclipse-JDT, Mozilla, and Samba, with the aim of analyzing the relationship between the source code change entropy and four factors: refactoring activities, number of contributors for a file, participation of classes in design patterns, and change topics. Results The study shows that (i) the change entropy decreases after refactoring, (ii) files changed by a higher number of developers tend to exhibit a higher change entropy than others, (iii) classes participating in certain design patterns exhibit a higher change entropy than others, and (iv) changes related to different topics exhibit different change entropy, for example bug fixings exhibit a limited change entropy while changes introducing new features exhibit a high change entropy. Conclusions Results provided in this paper indicate that the nature of changes (in particular changes related to refactorings), the software design, and the number of active developers are factors related to change entropy. Our findings contribute to understand the software aging phenomenon and are preliminary to identifying better ways to contrast it.     

Alder ene functionalization of polyisobutene oligomer and styrene-butadiene-styrene triblock copolymer

The Alder ene functionalization reaction of double bonds containing macromolecules such as polyisobutene oligomer (PIB) and a styrene-butadiene-styrene triblock copolymer (SBS) samples with maleic anhydride (MAH) or diethyl maleate (DEM) as enophiles is described. The analysis of the products by means of different techniques assesses the addition of the polar molecules to the reactive vinylidene units of the polymer with functional degrees (FD) depending on the type of enophile and polymer reactivity. The role of the reaction conditions and the use as Lewis acids as catalysts are discussed in terms of their influence on the addition reaction extent and on the polymer molecular weight.     

Electrochemical reduction of benzyl halides at a silver electrode

The electrochemical reduction of benzyl halides PhCH₂X (X = Cl, Br and I) has been investigated at Ag and glassy carbon (GC) electrodes in CH₃CN + 0.1 M Et₄NClO₄. At both electrodes reduction of PhCH₂X involves irreversible electron transfer concerted with breaking of the carbon-halogen bond. All three halides exhibit a single 2e⁻ reduction peak at GC, whereas up to three peaks can be observed at the Ag electrode. Silver exhibits remarkable catalytic properties for the reduction process, which is positively shifted by 0.45-0.72 V with respect to GC. The mechanism of reduction of the organic halides at Ag involves adsorption of both the starting reagents and their reduction products. Adsorption of PhCH₂Cl and PhCH₂Br is weak and slow, whereas PhCH₂I is more rapidly and strongly adsorbed, so that two distinct peaks can be observed for the reduction of the dissolved and adsorbed molecules. Controlled-potential electrolyses at Ag have shown that the process may be directed to the production of bibenzyl or toluene, depending on the applied potential.     

Enzymatic hydrolysis of cotton wastes. Effect of γ-ray pretreatments

Textile cotton wastes were irradiated with γ-rays at different doses and solubilized with 10% NaOH solutions. The regenerated products underwent enzymatic hydrolysis and their degradation pattern was compared with that of the cotton celluloses untreated or treated only with γ-rays. The different samples were characterized by measurements of X-ray diffraction, degree of polymerization, moisture regain and water retention. The structural and morphological modifications induced by the pretreatments were correlated with the variations of kinetic parameters, particularly the V_max/K_m ratio, defined as specificity constant. Only the combination of irradiation with dissolution pretreatments gave a considerable increase of hydrolysis reaction effectiveness.     

The KlaperSuite framework for model-driven reliability analysis of component-based systems

Automatic prediction tools play a key role in enabling the application of non-functional requirements analysis, to simplify the selection and the assembly of components for component-based software systems, and in reducing the need for strong mathematical skills for software designers. By exploiting the paradigm of Model-Driven Engineering (MDE), it is possible to automatically transform design models into analytical models, thus enabling formal property verification. MDE is the core paradigm of the KlaperSuite framework presented in this paper, which exploits the KLAPER pivot language to fill the gap between design and analysis of component-based systems for reliability properties. KlaperSuite is a family of tools empowering designers with the ability to capture and analyze quality of service views of their systems, by building a one-click bridge towards a number of established verification instruments. In this article, we concentrate on the reliability-prediction capabilities of KlaperSuite and we evaluate them with respect to several case studies from literature and industry.     

Effective molarities in supramolecular catalysis of two-substrate reactions

Reactivity data related to processes in which molecular receptors promote the reaction of two simultaneously complexed reactants have been surveyed and analyzed in terms of effective molarity (EM). Methods and criteria for the calculation of reliable EM's have been highlighted. Extension of a previous extrathermodynamic treatment of intramolecular reactions of bifunctional chains to the intracomplex reactions of the ternary complexes involved in two-substrate catalyzed reactions has provided a sound framework for a comparative analysis of reactivity and catalytic efficiency in structurally diverse and apparently unrelated systems.