Alloy model for high-temperature superconductors

An alloy model is proposed for the electronic structure of high-temperature superconductors. It is based on the assumption that holes and extra electrons are localized in small copper-oxygen clusters, that would be the components of such an alloy. This model, when used together with quantum chemical calculations on small clusters, can explain the structure observed in the experimental densities of states of both hole and electron superconductors close to the Fermi energy. The main point is the strong dependence of the energy level distribution and composition on the number of electrons in a cluster. The alloy model also suggests a way to correlate T_c with the number of holes, or extra electrons, and the number of adequate clusters to locate them.     

A multivalued version of Sharkovskiĭ’s theorem holds with at most two exceptions

A multivalued version of Sharkovskiĭ’s theorem is formulated for M-maps on linear continua and, more generally, for triangular M-maps on a Cartesian product of linear continua. This improves the main result of [AP1] in the sense that our multivalued analogue holds with at most two exceptions. A further specification requires some additional restrictions. For instance, 3- orbits of m-maps imply the existence of k-orbits for all k ? \mathbbNk \in {\mathbb{N}} , except possibly for k ?k \in {4, 6}. It is also shown that, on every connected linearly ordered topological space, an M-map with orbits of all periods can always be constructed. This demonstrates that Baldwin’s classification of linear continua in terms of admissible periods [Ba] is useless for multivalued maps.     

Parallel iterative reaction path optimization in ab initio quantum mechanical/molecular mechanical modeling of enzyme reactions

The determination of reaction paths for enzyme systems remains a great challenge for current computational methods. In this paper we present an efficient method for the determination of minimum energy reaction paths with the ab initio quantum mechanical/molecular mechanical approach. Our method is based on an adaptation of the path optimization procedure by Ayala and Schlegel for small molecules in gas phase, the iterative quantum mechanical/molecular mechanical (QM/MM) optimization method developed earlier in our laboratory and the introduction of a new metric defining the distance between different structures in the configuration space. In this method we represent the reaction path by a discrete set of structures. For each structure we partition the atoms into a core set that usually includes the QM subsystem and an environment set that usually includes the MM subsystem. These two sets are optimized iteratively: the core set is optimized to approximate the reaction path while the environment set is optimized to the corresponding energy minimum. In the optimization of the core set of atoms for the reaction path, we introduce a new metric to define the distances between the points on the reaction path, which excludes the soft degrees of freedom from the environment set and includes extra weights on coordinates describing chemical changes. Because the reaction path is represented by discrete structures and the optimization for each can be performed individually with very limited coupling, our method can be executed in a natural and efficient parallelization, with each processor handling one of the structures. We demonstrate the applicability and efficiency of our method by testing it on two systems previously studied by our group, triosephosphate isomerase and 4-oxalocrotonate tautomerase. In both cases the minimum energy paths for both enzymes agree with the previously reported paths.     

Design and synthesis of a transferable farnesyl pyrophosphate analogue to Ras by protein farnesyltransferase

The posttranslational addition of a farnesyl moiety to the Ras oncoprotein is essential for its membrane localization and is required for both its biological activity and ability to induce malignant transformation. We describe the design and synthesis of a farnesyl pyrophosphate (FPP) analogue, 8-anilinogeranyl pyrophosphate 3 (AGPP), in which the omega-terminal isoprene unit of the farnesyl group has been replaced with an aniline functionality. The key steps in the synthesis are the reductive amination of the alpha,beta-unsaturated aldehyde 5 to form the lipid analogue 6, and the subsequent conversion of the allylic alcohol 7 to the chloride 8 via Ph(3)PCl(2) followed by displacement with [(n-Bu)(4)N](3)HP(2)O(7) to give AGPP (3). AGPP is a substrate for protein farnesyltransferase (FTase) and is transferred to Ras by FTase with the same kinetics as the natural substrate, FPP. AGPP is highly selective, showing little inhibitory activity against either geranylgeranyl-protein transferase type I (GGTase I) (K(i) = 0.06 microM, IC(50) = 20 microM) or squalene synthase (IC(50) = 1000 microM). AGPP is the first efficiently transferable analogue of FPP to be modified at the omega-terminus that provides a platform from which additional analogues can be made to probe the biological function of protein farnesylation. AGPP is the first example of a class of compounds that are alternate substrates for protein isoprenylation that are not inhibitors of squalene synthase.     

New adsorbents from oil shales: Preparation, characterization and U, Th isotope adsorption tests

New activated adsorbents for radionuclides have been produced from Moroccan oil shales by pyrolysis of the natural material at 550 °C flowed by a KMnO₄ activation. The texture and composition of the native rock and the adsorbents were studied before their use in tests for adsorption of radionuclides from standard solutions prepared from uranylnitrate and thorium nitrate in equilibrium with their daughters. The distribution coefficients between solutions containing U, Th and Ra and the adsorbents were evaluated by means of specific activities, measured by g-ray spectrometry. The adsorbents were observed to eliminate U, Th, Ra, Ac and Tl from aqueous solutions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Determination of water-soluble vitamins in pharmaceutical preparations by reversed-phase high-performance liquid chromatography with a mobile phase containing sodium dodecylsulphate andn-propanol

Summary A reversed-phase liquid chromatographic method with sodium dodecylsulphate-n-propanolwater as mobile phase has been used to separate and determine six water-soluble vitamins in twelve minutes. The analytical characteristics linear range, sensitivity, detection limits, and precision were evaluated. The lowest detection limits were those of nicotinic acid (not usually present in pharmaceutical products), 0.7 mgL⁻¹, nicotinamide, 1.3 mg L⁻¹, and pyridoxine, 1.4 mg L⁻¹. When the method was applied to the determination of the vitamins in pharmaceutical samples the values found agreed with those on the labels.     

Cooperative hydrogen-bonding effects in a water square: a single-crystal neutron and partial atomic charges and hardness analysis study

Four isomorphous complexes of formula [M(L)(4)(H(2)O)(2)]SO(4).2H(2)O (M = Co, 1a; Ni, 1b; Cu, 1c; Zn, 1d) have been isolated and characterized by single-crystal X-ray diffraction and neutron diffraction using the quasi-Laue diffractometer VIVALDI at the Institut Laue-Langevin as well as by thermogravimetric analysis. The structures contain a discrete, strongly hydrogen-bonded water tetramer which causes a significant distortion of the metal coordination sphere in each case. Partial atomic charges and hardness analysis (PACHA) calculations reveal that the shortest hydrogen bonds are not the strongest in this constrained, cyclic solid-state structure and show that the distortion at the metal center is caused by the drive to maintain the integrity of the water tetramer. The system undergoes a disorder-order transition on slow cooling that provides insight into the nature of communication between water squares.     

Antifungal activity of crown ethers

Some crown ethers were found to show significant antifungal activity against some wood-decay fungi, phytopathogenic fungi and eumycetes,Trichophytons for dermatomycosis. Their toxicity was evaluated by the paper disc method as well as by determining the values of ED₅₀, i.e., the concentration which inhibits the mycelium growth by 50%. The fungi examined areTyromyces palustris, Picnoporous coccineus, Coriolous versicolor, Pyricularia filamentosa, Fusarium sp., Trichophyton rubrum, Trichophyton sp., etc. Among the 26 crown ethers tested, 3,5-di-t-butyl-benzo-15-crown-5 showed relatively high activity, the highest ED₅₀ value of which being 8 M or 3 ppm. Other alkylbenzocrown ethers, dicyclohexyl crown ethers and Kryptofix 22DD also showed considerable activity. On the other hand, unsubstituted crown ethers, benzocrown ethers with a polar substituent, Kryptofix 222B and Kryptofix 221 were inactive.     

Autoxidation kinetics of atactic polypropylene in bulk

The kinetics of the reaction between atactic polypropylene (APP) and oxygen in bulk at temperatures ranging from 170 to 210°C and oxygen partial pressures from 160 to 760 torr have been studied by thermal differential analysis. The reaction takes place in two successive steps, both giving hydroperoxide groups as product. Partial reaction orders with respect to APP and oxygen for the first step, which corresponds to the uncatalyzed attack of a C? H tertiary bond to give a hydroperoxide, are one and two, respectively. In the second step, interpreted as another attack on a tertiary C? H by oxygen, catalyzed by a neighboring hydroperoxide group, reaction orders are one and one-half for APP and oxygen, respectively. Activation parameters have been determined and a reaction sequence is proposed. Hydroperoxidated APP subsequently decomposes via a zero-order process giving methylketone groups as its main product. An interpretation of this process is also given.