Membrane surface dynamics of DNA-threaded nanopores revealed by simultaneous single-molecule optical and ensemble electrical recording

We describe a method for simultaneous single-molecule optical and electrical characterization of membrane-based sensors that contain ion-channel nanopores. The technique is used to study the specific and nonspecific interactions of streptavidin-capped DNA polymers with lipid bilayers composed of diphytanoyl phosphatidylcholine and diphytanoyl phosphatidylglycerol. Biotinylated DNA that is bound to fluorescently labeled streptavidin is electrophoretically driven into, or away from, the lumen of alpha hemolysin (alphaHL) ion channels by an external electric field. Confocal microscopy simultaneously captures single-molecule fluorescence dynamics from the membrane interface at different applied potentials. Fluorescence correlation analysis is used to determine the surface number density and diffusion constant of membrane-associated complexes. The dual optical and electrical approach can detect membrane-associated species at a surface coverage below 10(-5) monolayers of streptavidin, a sensitivity that surpasses most other in vitro surface analysis techniques. By comparing the change in transmembrane current to the number of fluorescent molecules leaving the bilayer when the electrical potential is reversed, we demonstrate the general utility of the approach within the context of nanopore-based sensing and discuss a mechanism by which DNA-streptavidin complexes can be nonspecifically retained at the membrane interface.     

A plasticity model for powder compaction processes incorporating particle deformation and rearrangement

This paper develops a mechanistic model of granular materials that can be used with a commercial finite element package (ABAQUS). The model draws on the ideas of critical state soil mechanics and combines them with the theory of envelopes to develop an elasto-plastic model with a non-associated flow rule. The model incorporates both local deformation at the granule contacts, and rearrangement of the granules so that jointly they account for any bulk deformation. The mechanics of the model closely reflect the physicality of the material behaviour and the model parameters are closely linked (although not simplistically identical) to the characteristics of the granules. This not only gives an insight into the material behaviour, but also enables the model to be used to facilitate design of the material, its processing properties and, hence, component development. The model is used to simulate drained triaxial tests, settlement of a powder in a bin, and some examples of die pressing. Simulations are compared with experimental data and with predictions obtained using other models.     

Effect of solids concentration in dense suspensions of silica-iron(III) oxide and water

The rheological properties of dense suspensions, of silica, iron (III) oxide and water, were studied over a range of solids concentrations using a viscometer, which was modified so as to prevent settling of the solid components. Over the conditions studied, the material behaved according to power—law flow relationships. As the concentrations of silica and iron(III) oxide were increased, an entropy term in the flow equation was identified which had a silica dependent and an iron (III) oxide dependent component. This was attributed to a tendency to order into some form of structural regularity. A, A, B, C pre-exponential functions (K Pa^–n s^–1) - C _ox volume fraction iron (III) oxide - Q activation energy (kJ mol^–1) - R gas constant (kJ mol^–1 K^–1) - R _v silica/water volume ratio - T temperature (K) - n power-law index - H enthalpy (kJ mol^–1) - S entropy change (kJ mol^–1 K^–1) - shear strain rate (s^–1) - shear stress (Pa)     

A Novel Application of Sodium Borohydride for the Regiospecific and Stereoselective Reduction of Conjugated Alkadiynes

Alkynes, when carrying an α-sulfonyl group, are shown to be stereoselectively reduced to z-alkenes using sodium borohydride under mildly basic conditions.     

Molecular explanation for why talc surfaces can be both hydrophilic and hydrophobic

While individual water molecules adsorb strongly on a talc surface (hydrophilic behavior), a droplet of water beads up on the same surface (hydrophobic behavior). To rationalize this dichotomy, we investigated the influence of the microscopic structure of the surface and the strength of adhesive (surface-water) interactions on surface hydrophobicity. We have shown that at low relative humidity, the competition between adhesion and the favorable entropy of being in the vapor phase determines the surface coverage. However, at saturation, it is the competition between adhesion and cohesion (water-water interactions) that determines the surface hydrophobicity. The adhesive interactions in talc are strong enough to overcome the unfavorable entropy, and water adsorbs strongly on talc surfaces. However, they are too weak to overcome the cohesive interactions, and water thus beads up on talc surfaces. Surprisingly, even talc-like surfaces that are highly adhesive do not fully wet at saturation. Instead, a water droplet forms on top of a strongly adsorbed monolayer of water. Our results imply that the interior of hydrophobic zeolites suspended in water may contain adsorbed water molecules at pressures much lower than the intrusion pressure.     

Reaction Mechanisms in a Multichannel Scattering Theory for Identical Particles

Exchange effects in rearrangement reactions are studied in the framework of a general algebraic theory of identical-particle scattering. An algebraic definition of reaction mechanisms proposed previously is used to distinguish the contributions to scattering amplitudes made by different exchange processes. Previous statements concerning the number of possible exchange mechanisms and their relative weight factors are rigorously proved. The results of this paper differ from those obtained in previous studies, as here identical clusters of particles are also treated as indistinguishable. Received March 31, 1994; revised Juli 22, 1994; accepted for publication September 29, 1994     

Synthesis and bioactivity of the gibberellin, 18-hydroxy-GA1 (GA132)

As part of a study to confirm putative structural assignments to new gibberellins and to furnish sufficient quantities for biological investigations, a twenty step synthesis of 18-hydroxy GA1 from gibberellic acid (GA3) is described, allowing the confirmation of structure for a new gibberellin, GA132, that occurs in developing grains of barley (Hordeum vulgare). The early part of the sequence involved cleavage of the C(3)-C(4) bond in the A-ring of a 3-oxo intermediate. The ring was then reformed as part of a "domino" process involving the conjugate addition of alkoxide to an alpha-methylene lactone moiety followed by an intramolecular aldol reaction. The bioactivities of the new GA, and its 18-hydroxy-GA4 relative, have been confirmed in dwarf barley growth and alpha-amylase induction assays.