Synthesis of a Val-Pro diaminodiol dipeptide isostere by epoxyamine cyclization

[reaction: see text] The stereoselective synthesis of a novel proline-containing dipeptide isostere is described. Starting from l-valine, three new contiguous stereocenters are generated by asymmetric induction and epoxide chemistry, while the pyrrolidine ring of proline is introduced in the final step via intramolecular ring opening of the amino acid derived epoxyamine. Proline-containing peptidomimetics are potentially attractive as selective inhibitors of proline-specific enzymes, such as PPIases and retroviral proteases, and as analogues of bioactive peptides.     

Electrochemistry of organometallic compounds: Part III. Oxidations of methyl derivatives of organocobalt(III) complexes with delocalized electronic structure in dimethylformamide

The influence of the equatorial ligand on the electrochemical oxidation of the compounds [H₃CCo(chel)B], where chel is bis (dimethylglyoximato), (DH)₂; bis(salicylaldehyde)ethylenediimine, salen; bis(salicylaldehyde) o-phenylenediimine, salophen; bis(salicylaldehyde)cyclohexylenediimine, salcn; bis(acetylacetone) ethylenediimine, bae; and where B is pyridine when chel is (DH₂), and dimethylformamide (DMF) when chel represents a Schiff base (salen, salcn, salophen and bae), was studied by means of cyclic voltammetry in DMF, 0.2 M in tetraethylammonium perchlorate, between 25 and ?25°C, with a platinum disk working electrode. Absorption spectra in the visible and near ultraviolet regions for these compounds in DMF at 25°C were obtained. The complexes exhibit a reversible one-electron oxidation, at ?20°C with scan rates >0.5 V s^?; chemical reactions following electron transfer are not detected under these conditions. At slower potential or higher temperatures, the oxidized product decomposes chemically in a solvent-assisted (or nucleophile-assisted) reaction, yielding products which are electroactive in the applied potential range. The behavior of the [H₃CCo (DH₂)py] derivative is better described as a quasi-reversible charge transfer followed by an irreversible chemical reaction. Experimental evidence suggests that in the case of the [H₃CCo(bae)] derivative at ?20°C, the reactive -species is pentacoordinated and weakly adsorbed at the electrode surface. The value of E${}_1\text{2}$ and the energies of the first two absorption bands in the visible spectra reveal the ability of the studied complexes to donate and to delocalize electronic charge.     

Geometrical non-linear analysis of structures by finite elements

Meccanica - The paper shows a comprehensive analysis of geometrically non linear structural problems by the finite element method. The theoretical approach is based on a variational principle...     

Preparation and characterization of MgO stepped surfaces

We present a study of the preparation procedure for stepped MgO surfaces which can be used as templates for the deposition of metallic nanostructures. A cleaved sample of MgO(0 0 1) was mechanically polished to reach the desired miscut angle along the [1 1 0] direction. Then a thermal annealing was performed. The effect of an intermediate chemical etching has been also studied. The surface was analyzed by means of contact AFM in air, LEED and XPS in UHV. The role of the chemical etching and the dependence of the final morphology on the annealing time and temperature were investigated. The influence of the miscut angle on the final surface topography is also briefly discussed.     

Quantum gravity and matter: counting graphs on causal dynamical triangulations

An outstanding challenge for models of non-perturbative quantum gravity is the consistent formulation and quantitative evaluation of physical phenomena in a regime where geometry and matter are strongly coupled. After developing appropriate technical tools, one is interested in measuring and classifying how the quantum fluctuations of geometry alter the behaviour of matter, compared with that on a fixed background geometry. In the simplified context of two dimensions, we show how a method invented to analyze the critical behaviour of spin systems on flat lattices can be adapted to the fluctuating ensemble of curved spacetimes underlying the causal dynamical triangulations (CDT) approach to quantum gravity. We develop a systematic counting of embedded graphs to evaluate the thermodynamic functions of the gravity-matter models in a high- and low-temperature expansion. For the case of the Ising model, we compute the series expansions for the magnetic susceptibility on CDT lattices and their duals up to orders 6 and 12, and analyze them by ratio method, Dlog Padé and differential approximants. Apart from providing evidence for a simplification of the model’s analytic structure due to the dynamical nature of the geometry, the technique introduced can shed further light on criteria à la Harris and Luck for the influence of random geometry on the critical properties of matter systems.     

Sulla caratterizzazione dei grafi domistabili

Riassunto Viene presentato un algoritmo atto a verificare se un grafo è domistabile, tale essendo ogni grafo in cui qualsiasi insieme dominante minimale di nodi è indipendente. L’algoritmo, che implica un numero di confronti polinomiale, rispetto al numero dei nodi, si basa sulla ricerca di insiemi di nodi dotati di particolari proprietà.

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Summary In the present paper an algorithm is proposed which verifies ?domistability? in graphs. A graph is called domistable when each minimal dominating set of nodes is also an independent one. The algorithm, polynomial in the number of nodes, is based on the systematic search for node subsets with particular properties.

     

Model compounds of nylons—I. The crystal structure of N,N′-Diethyladipamide

The conformation of the molecule of N,N′-diethyladipamide (NNDEA) has been investigated in the crystalline state by X-ray analysis. NNDEA crystallizes in the monoclinic system, space group $\text{P2}{}_1\text{/c}\text{with}\text{a = 13 · 76(2), b = 4 · 90(1), c = 8 · 99(2)}\text{A}\text{?}\text{and}\text{β = 92 · 0°(1)}$. 661 independent non-zero reflections collected by counter techniques with CuK_a were used in the refinement to the final value of R = 7 · 6 per cent. The conformation of the adipic acid moiety shows a succession of carbon-carbon bonds (from one carbonyl group to the other) of the type skew (+), gauche (?), trans, gauche (+), skew (?).Analogous conformations (instead of the all-trans planar) may be present as point or linear defects in the crystals of nylon 6-6.     

Study of ancient mortars from the Roman Villa of Pollio Felice in Sorrento (Naples)

The study of ancient mortars is an important aspect of building conservation: the choice of the materials has varied according to historical period, regional habits, and their specific function in the structure. Ancient mortars are composites, comprising hydraulic or aerial binding materials, and aggregates, passive or active, which may react with binding material. Moreover, they were modified during setting, hardening, and aging, according to processes not yet well known. In this paper, we present a study of ancient mortars from the Villa of Pollio Felice of Sorrento (Naples). The analysis has been performed by conventional techniques (grain-size distribution, lime-percentage analysis, optical and electron microscopy, and X-ray diffraction) and by means of a laboratory X-ray microdiffractometer equipped with an image plate detector. This system, applied for the first time to archaeological studies, can reach a spatial resolution of a few tenths of microns and it allows us to obtain separate phase identification of binder and filler particles. PACS 07.85.Nc; 61.10.Nz     

Conformational diseases and structure-toxicity relationships: lessons from prion-derived peptides

The physiological form of the prion protein is normally expressed in mammalian cell and is highly conserved among species, although its role in cellular function remains elusive. Available evidence suggests that this protein is essential for neuronal integrity in the brain, possibly with a role in copper metabolism and cellular response to oxidative stress. In prion diseases, the benign cellular form of the protein is converted into an insoluble, protease-resistant abnormal scrapie form. This conversion parallels a conformational change of the polypeptide from a predominantly alpha-helical to a highly beta-sheet secondary structure. The scrapie form accumulates in the central nervous system of affected individuals, and its protease-resistant core aggregates into amyloid fibrils outside the cell. The pathogenesis and molecular basis of the nerve cell loss that accompanies this process are not understood. Limited structural information is available on aggregate formation by this protein as the possible cause of these diseases and on its toxicity. A large amount of structure-activity studies is based on the prion fragment approach, but the resulting information is often difficult to untangle. This overview focuses on the most relevant structural and functional aspects of the prion-induced conformational disease linked to peptides derived from the unstructured N-terminal and globular C-terminal domains.