Drag of a Solid Particle Trapped in a Thin Film or at an Interface: Influence of Surface Viscosity and Elasticity

We propose a theoretical model for the motion of a spherical particle entrapped in a thin liquid film or in a monolayer of insoluble surfactant at the air/water interface. Both surface shear and dilational viscosity, surface diffusion, and elasticity of the film are taken into consideration. The drag force acting on the particle is analytically calculated and asymptotic expressions of the problem are provided. The relevance of the model is discussed by comparing the calculated "viscoelastic" drag, gamma(vel), to the one predicted by Saffman's theory, gamma(S), for cylindrical inclusions in membranes. Numerical analyses are performed to evaluate the contributions of the surface viscosity and the diffusion coefficient of the layer on the hydrodynamical resistance experienced by the particle. Copyright 2000 Academic Press.     

Photogeneration and transport of charge carriers in polysilylenes

The attachment of π-conjugated chromophores that absorb the radiation with long wavelengths to poly(methylphenylsilylene) ( 1 ) via reactions of its formylated derivative is described. Some of the polymers obtained show improved photostability and higher quantum photogeneration efficiency in comparison with the parent polymer. Photoconductive ultra-thin layers can be prepared from polar derivatives of ( 1 ) by the Langmuir–Blodgett technique.     

Note on the Coulson integral formula

Let P be a polynomial of degree n, whose zeros λ₁, λ₂, ..., λ _n are real-valued. The Coulson integral formula (first reported in 1940) is an expression for the sum of the positive–valued zeros of P, in terms of P. We show that the Coulson formula holds if and only if the condition λ₁+λ₂+...+λ _n =0 is obeyed. We also show how the formula has to be modified, so that it be applicable in the case when λ₁+λ₂+...+λ _n ≠ 0. Ams Classification (2000): primary: 92E10, secondary: 32A27, 81Q99     

Kinetic studies and analytical application of cholesterol oxidase and peroxidase immobilized to synthetic polymer

A method for individual and simultaneous covalent immobilization of cholesterol oxidase and peroxidase to copolymer of acrylonitrile with acrylamide is described. The effect of immobilization on the catalytic properties of the covalently bound enzymes was studied. The immobilized enzymes showed no change in pH optima and an increase in temperature optima, activation energy, and K _m , compared to data received from experiments with soluble enzymes. A small glass column packed with immobilized multienzyme complex was used to develop a method for manual determination of cholesterol in foodstuffs (e.g., in mayonnaise “Olinease”). The method was characterized by high analytical precision (coefficient of variation = 2.67%). The results show high correlation with those obtained by the Kageyama method (r=0.986). The method is economical (the enzyme-carrier conjugate may be used more than 300 times), precise, easy to perform, and less time-consuming than the manual methods utilizing soluble enzymes. The established manual method can be proposed for cholesterol determination in foodstuffs.     

Detection of hydrogen atom adduct of spin-trap DEPMPO. The relevance for studies of biological systems

We proposed EPR spectroscopy using spin-trap DEPMPO as a novel method for the detection of a hydrogen atom (*H) produced by chemical and biological systems. In complex EPR spectra of DEPMPO adducts in biological systems, spectral lines of unknown origin have been observed. We have assumed (Baci?, G.; Mojovi?, M. Ann. N. Y. Acad. Sci. 2005, 1048, 230-243) that those lines represent the spectrum of a hydrogen atom (*H) adduct i.e., DEPMPO/H. An electrochemical system known to produce only *H radicals was used here in order to obtain a separate spectrum of the DEPMPO/H adduct. An acquired spectrum as well as a computer spectral simulation of the DEPMPO/H adduct showed considerable resemblance with additional lines in the EPR spectra of DEPMPO adducts in biological systems-plant plasma membranes and cell walls. This shows that such a radical is produced by plants as well as that DEPMPO is suitable for detection in both electrochemical and biological systems.     

Laser annealing of GaAs dual implanted with Si and P ions

Laser annealing of SI(100) GaAs:Cr implanted either with Si⁺ ions (150 keV, 6×10¹³-1×10¹⁵cm^–2) or dual implanted with Si⁺ ions (150 keV, 6×10¹⁴–1×10¹⁵cm^–2) and P⁺ ions (160 keV, 1×10¹⁴–1×10¹⁵cm^–2) has been examined using backscatteringchannelling technique and via electrical measurement of Hall effect. It has been found that at laser energy densities 0·8 J cm^–2 a full recovery of the sample surface occurs. In dual implanted samples (1×10¹⁵ Si⁺ cm^–2+1×10¹⁵P⁺cm^–2) up to 46% of Si atoms become electrically active after the laser annealing. Resultant Hall mobility of carriers is, however,lower than that obtained after common thermal annealing.The authors are pleased to take the opportunity of thanking Professor M. Kubát for his encouragement and continuous support. Accelerator staff is gratefully acknowledged for its assistance in the course of experiments.     

The UHF-CNDO/2 study of tetra-atomic radicals containing silicon and phosphorus

The UHF-CNDO/2 calculations have been performed for several radicals containing C, Si or P. The equilibrium geometries found by the minimization of the total energies and calculated spin densities are compared with results from ESR experiments and INDO calculations. The electronic structure of the radicals is discussed for their equilibrium geometries.     

Structural basis of swinholide A binding to actin

Marine toxins targeting the actin cytoskeleton represent a new and promising class of anti-cancer compounds. Here we present a 2.0 A resolution structure of swinholide A, a marine macrolide, bound to two actin molecules. The structure demonstrates that the actin dimer in the complex does not represent a physiologically relevant entity, for the two actin molecules do not interact with each other. The swinholide A actin binding site is the same as that targeted by toxins of the trisoxazole family and numerous actin binding proteins, highlighting the importance of this site in actin polymerization. The observed structure reveals the mechanism of action of swinholide A and provides a structural framework about which to design new agents directed at the cytoskeleton.     

Motional heterogeneity and phase separation of functionalized polyester polyurethanes

The polyester polyurethanes, PU based on isophoronediisocyanate, polycaprolactone, and 1,4-butanediol with different amounts of functional groups introduced into the hard segments via second chain extender, 2,2′-bis-(hydroxymethyl) propionic acid, were investigated by electron spin resonance, ESR, spin label method, wide-angle X-ray diffraction, WAXD, optical microscopy and differential scanning calorimetry, DSC. The objective of this study is to clarify the effect of functional groups on the motional heterogeneity, microphase separation and crystallisation of the polyurethanes. The concentration of carboxylic groups varied from 0 to 0.45 mmol g⁻¹. The temperature-dependent ESR spectra of spin labelled PU hard segments chain ends with stable nitroxide radical 2,2,6,6-tetramethyl-4-aminopiperidin-1-yloxyl are sensitive to the amount of functional groups attached to the hard segments. Composite ESR spectra of functionalized PU, with fast and slow component, suggest that PU hard segments are partitioned in two motionally different environments. According to the ratio of fast and slow component motional heterogeneity increases with an increase of functional groups up to 0.35 mmol g⁻¹ and above this concentration slow component decreases indicating higher degree of phase mixing and stronger effect of soft segments. Polarized micrographs and the extent of ordering from WAXD measurements reveal the changes of phase morphology with the carboxylic groups content in a similar way as shown from the motional behaviour of spin label on the segmental level. The degree of crystallinity and the separation of spherulitic rings are decreasing above a certain concentration of functional groups. The effect of functional groups in PU on the hard and soft segment mixing is discussed in terms of additional noncovalent interactions and chain structure which at critical level of interactions lead to a formation of more open hard segment structure accessible to interaction with the soft segment.