Ruled surfaces generated by some special curves in Euclidean 3-Space

In this paper, a family of ruled surfaces generated by some special curves using a Frenet frame of that curves in Euclidean 3-space is investigated. Some important results are obtained in the case of general helices as well as slant helices. Moreover, as an application, circular general helices, spherical general helices, Salkowski curves and circular slant helices, which illustrate the results, are provided and graphed.     

Lie-group method of solution for steady two-dimensional boundary-layer stagnation-point flow towards a heated stretching sheet placed in a porous medium

The boundary-layer equations for two-dimensional steady flow of an incompressible, viscous fluid near a stagnation point at a heated stretching sheet placed in a porous medium are considered. We apply Lie-group method for determining symmetry reductions of partial differential equations. Lie-group method starts out with a general infinitesimal group of transformations under which the given partial differential equations are invariant. The determining equations are a set of linear differential equations, the solution of which gives the transformation function or the infinitesimals of the dependent and independent variables. After the group has been determined, a solution to the given partial differential equations may be found from the invariant surface condition such that its solution leads to similarity variables that reduce the number of independent variables of the system. The effect of the velocity parameter λ, which is the ratio of the external free stream velocity to the stretching surface velocity, permeability parameter of the porous medium k ₁, and Prandtl number Pr on the horizontal and transverse velocities, temperature profiles, surface heat flux and the wall shear stress, has been studied.     

Binding of Biotin to Streptavidin: A combined fluorescence correlation spectroscopy and time-resolved fluorescence study

The Biotin-Streptavidin complex is a widely studied system in biology and biophysics, because of its extremely strong non-covalent binding affinity. The latter is often exploited to link molecules to substrates or to one another. However, the details of the Biotin-Streptavidin binding have not been fully elucidated so far. Particularly, the role of cooperative effects in enhancing the binding affinity has not been clarified. Our long-term aim is to investigate this point by implementing two complementary approaches, fluorescence correlation spectroscopy and time-correlated single-photon counting. As both methods rely on the analysis of fluorescence signals, biotin labeled with Atto-550-dye was used. In this work, in order to get a first overview of the system, we analyzed solutions in three paradigmatic ranges of Biotin-to-Streptavidin concentration ratio. Fluorescence correlation spectroscopy measurements allowed us to extract diffusion times of free biotin and of biotin-Streptavidin complexes, and also to gain information about the dynamics of the intersystem crossing between the first excited triplet and the first excited singlet states. Time-correlated single-photon counting made it possible to derive the lifetimes of the different species in solution, as well as to deduce relevant information about the relative abundance of Streptavidin-complexed and free Biotin.     

A new synthesis of 1,5-dioxepan-2-one

A new method for the synthesis of 1,5-dioxepan-2-one (DXO)¹ was developed and advantages over an earlier synthetic scheme² were documented.     

Lipophilicity measurement of drugs by reversed phase HPLC over Wide pH range using an alkaline-resistant silica-based stationary phase, XBridge Shield RP(18)

We propose a reversed phase HPLC (RP-HPLC) with an alkaline-resistant silica-based stationary phase, XBridge Shield RP(18), for the determination of the lipophilicity of drugs with diverse chemical nature ranging from acidic to basic. A set of 40 model compounds with well-defined solvatochromic parameters was selected to allow a broad distribution of structural properties. The chromatographic results showed that the lipophilicity index log k(w) obtained with XBridge Shield RP(18) was well correlated with experimental log P(oct) values (r(2)=0.96). Linear solvation free-energy relationship (LSER) analyses revealed that the retention mechanism of the stationary phase and 1-octanol/water partitioning were controlled by almost the same balance of intermolecular forces (hydrophobicity as expressed by the van der Waals volume V(w), H-bond acceptor basicity beta, and dipolarity/polarizability pi*). The results showed that XBridge Shield RP(18) phase overcomes the shortcomings of the silica-based stationary phases, the application of which to lipophilicity measurements had been limited to neutral and acidic compounds.     

Constructing the Spectrum of Packings and Coverings for the Complete Graph with Stars with up to Five Edges

The packing and covering problems have been considered for several classes of graphs. For instance, Bryant et. al. have investigated the packing problem for paths and cycles, and the packing and covering problems for 3-cubes. The packing and covering problems were settled for stars with up to six edges by Roditty. In this paper, for every possible leave graph (excess graph), we find a corresponding maximum packing (minimum covering) of the complete graph with stars with up to five edges.     

Simultaneous detection of ascorbate and uric acid using a selectively catalytic surface

The direct and selective detection of ascorbate at conventional carbon or metal electrodes is difficult due to its large overpotential and fouling by oxidation products. Electrode modification by electrochemical reduction of diazonium salts of different aryl derivatives is useful for catalytic, analytical and biotechnological applications. A monolayer of o-aminophenol (o-AP) was grafted on a glassy carbon electrode (GCE) via the electrochemical reduction of its in situ prepared diazonium salts in aqueous solution. The o-aminophenol confined surface was characterized by cyclic voltammetry. The grafted film demonstrated an excellent electrocatalytic activity towards the oxidation of ascorbate in phosphate buffer of pH 7.0 shifting the overpotential from +462 to +263 mV versus Ag/AgCl. Cyclic voltammetry and d.c. amperometric measurements were carried out for the quantitative determination of ascorbate and uric acid. The catalytic oxidation peak current was linearly dependent on the ascorbate concentration and a linear calibration curve was obtained using d.c. amperometry in the range of 2-20 μM of ascorbate with a correlation coefficient 0.9998, and limit of detection 0.3 μM. The effect of H₂O₂ on the electrocatalytic oxidation of ascorbate at o-aminophenol modified GC electrode has been studied, the half-life time and rate constant was estimated as 270 s, and 2.57 × 10⁻³ s⁻¹, respectively. The catalytically selective electrode was applied to the simultaneous detection of ascorbate and uric acid, and used for their determination in real urine samples. This o-AP/GCE showed high stability with time, and was used as a simple and precise amperometric sensor for the selective determination of ascorbate.     

Surface/interface phenomena in nano‐multilayer coating under severing tribological conditions

An extensive study of surface/interface phenomena during wear of an adaptive TiAlCrSiYN/TiAlCrN nano‐multilayer coating deposited using physical vapor deposition was undertaken under increasingly severe tribological conditions associated with dry end milling of H13 hardened tool steel. The results of FEM modeling on the temperature/stress distribution at different cutting speeds outline actual cutting conditions on the both rake and flank frictional surfaces of the coated tool. Studies of the surface/interface phenomena were made by means of SEM/high‐resolution transmission electron microscopy/XPS analyses. Results demonstrate that intensifying tribological conditions facilitates improved wear performance of the adaptive coating layer. In extreme tribological conditions of ultra‐performance machining (cutting speed of 500 m/min), the self‐organization process establishes entirely through the formation of a nano‐scale layer of dynamically re‐generating tribo‐ceramic films. The formation of these surface nano‐films results in exceptionally efficient protection of the underlying coating layers. In response to the extreme external environment, the coating layer remained almost undamaged during a long run, demonstrating the capacity to efficiently replenish necessary tribo‐ceramic films. In this way, interconnection of various surface and undersurface processes is established in the hierarchically structured tribo‐films/coating layer. This integral performance is responsible for exceptional wear resistance under intensifying and extreme tribological conditions. Copyright © 2016 John Wiley & Sons, Ltd.