Degradation of polymers in high speed rotary homogenizers: A hydrodynamic interpretation

Dilute solutions of DNA in neutral phosphate buffer and of polystyrene in toluene are degraded to limiting molecular weights by high speed stirring in a Virtis homogenizer equipped with special thin, highly sharpened blades. Results are interpreted and justified in terms of the hydrodynamic theory of laminar boundary layers. Estimates of critical stress values for chain scission are obtained at various limiting molecular weights for the two polymer systems, and the treatment is used to rationalize the dependence of degradation upon homogenizer speed. The agreement between the hydrodynamic theory and experimental data implies that limiting molecular degradation occurs entirely in the boundary layer region extending a distance equal to approximately the molecular contour length from the leading edge of the blade tip and that laminar flow conditions prevail in this region. It therefore seems likely that polymer molecules subjected to a critical shearing stress are essentially completely extended in the streamlines of flow. The hydrodynamic theory is finally used to explain the peculiar kinetics of molecular degradation in high speed rotary homogenizers.     

The thermal behaviour of some clathrates based upon Dianin's Compound

Clathrates based upon Dianin's Compound (4-p-hydroxyphenyl-2,2,4-trimethylchroman) exhibit shape and size selectivity towards included guest molecules in a manner similar to that shown by some commercially important zeolites. Clathrates derived from Dianin's Compound and a wide range of guest solvents were heated to their melting points and beyond, and their behaviour observed.

---

Zusammenfassung Auf Dianin's Verbindung (4-p-Hydroxyphenyl-2,2,4-trimethylchroman) basierende Klathrate weisen gegenüber eingeschlossenen Gastmolekülen eine ähnliche Form- und Grössenselektivität auf wie einige wichtige kommerzielle Zeolithe. Von Dianin's Verbindung und einer Reihe von Lösungsmitteln erhaltene Klathrate wurden bis zum Schmelzpunkt und darüber hinaus erhitzt und dabei ihr Verhalten beobachtet.

---

—4-- -2,2,4- — , , . , , , .

---

---

The assistance of Mr Carl Garland and Miss Marianne McCusker in the early stages and Miss Cathy Kennedy in the later stages of this work is acknowledged with thanks. Instruction in the use of the Thermal Analyser and interpretation of several of the calorimetric analyses were generously provided by Dr Geoff Irvine.     

Total synthesis of (+)-amphidinolide A. Assembly of the fragments

The structure elucidation of (+)-amphidinolide A, a cytotoxic macrolide, has been accomplished by employing a combination of total synthesis and NMR spectroscopic analysis. Amphidinolide A possesses two skipped 1,4-diene subunits which are accessible by ruthenium-catalyzed alkene-alkyne couplings. Previous total syntheses had revealed that the reported structure was incorrect; therefore, to incorporate maximum flexibility into the synthesis, with the ultimate goal of determining the correct structure, a highly convergent approach was chosen. Furthermore, liberal use was made of catalytic asymmetric transformations to set individual stereocenters. Three different strategies were envisioned for the end game, and due to the highly convergent nature of the synthesis, all three routes disconnect to the same three key intermediates, 5, 6, and 7. Diastereomers of 6 and 7 were easily prepared by modification of the synthetic routes to allow access to multiple diastereomers of 1 for structural determination.     

Simulation of anodic Pt oxide growth

A microscopic model of the growth of the thin anodic oxide film on Pt is presented. The slow step is migration of a Pt atom from the metal lattice to become a Pt(II) species. Pt(II) species can then rapidly diffuse across the surface. Sites of different types, e.g. terrace atoms or step atoms, have different reactivity, dependent on their number and type of bonds. Monte Carlo simulations are used to predict the kinetics and the surface reconstruction. The treatment of the structures of the oxide and metal is oversimplified by the assumption that they both have simple cubic unit cells with identical lattice parameters. The model predicts correctly the observed direct logarithmic growth law for potential step transients, without any change in behavior at full monolayer coverage. For cyclic voltammetry, it predicts an anodic peak on the first cycle, and plateau behavior for subsequent cycles. The reversible component is found, and interpreted in terms of a reversible reconstruction aided by the rapid surface diffusion. The height distributions predicted by this model agree with recent structural measurements, but the exact topography of the surface after multiple cycles is not reproduced.     

Polyethyleneglycol methacrylate 200 as an electrophoresis matrix in hydroorganic solvents.

The properties of gels composed of poly(polyethyleneglycol methacrylate) and copolymers of polyethyleneglycol methacrylate and acrylamide were studied. These novel electrophoresis matrices are amphigels (swellable in water and organic solvents) that have unprecedented organic solvent compatibility. Hydrophobic proteins which are poorly solubilized in aqueous detergent systems (e.g., zein) are well-resolved in these gels with hydro-organic solvents. This is especially relevant for isoelectric focusing, to avoid using either ionic detergents that may interfere with the focusing, or urea that may cause carbamylation of proteins. Variations of crosslinker, buffer, and solvent systems in these gels were explored.     

Fluoride sorption and associated aluminum release in variable charge soils

Fluoride sorption and related aluminum (Al) release are evaluated in two iron-oxide-rich soils as a function of soil depth, composition, and physical-chemical properties and potential mechanisms of fluoride-surface interaction are suggested. Measured Al concentrations at equilibrium fluoride sorption, reflective of the net balance between Al dissolution and sequestration of the released Al by the solid phase, suggest net fluoride-assisted dissolution of Al-bearing amorphous and crystalline soil minerals. Strikingly, soils of similar depth and horizonation from the same soil order but of distinct soil series exhibited markedly different susceptibility to Al loss in the presence of fluoride, possibly a combined result of differences in the mechanism of fluoride sorption, soil mineralogy, reactivity of the surficial Al and Fe, and soil solution chemistry. Fluoride sorption is strongly correlated with soil Al and Fe present as high-surface-area amorphous and crystalline oxide phases. Fluoride complexation to surficial Al and Fe ions via ligand exchange with surficial OH groups and water molecules appears to be the dominant sorption mechanism. At high dissolved fluoride concentrations (>7 mM), other mechanisms of fluoride retention including adsorption of AlF solution complexes, entrapment in the interparticle pore fluid, and precipitation into solution and/or onto the soil surface are also likely.     

Determination of ruthenium and tin on electrochemical anodes by energy-dispersive X-ray fluorescence

"Dimensionally Stable Anodes" (DSA)(R) have gained wide acceptance in electrochemical production of chlorine and caustic soda. The DSAs are usually composed of electrocatalytic layers of precious and non-precious metal oxides produced by thermal decomposition of salts on a valve-metal substrate (e.g., titanium). They have long lifetimes (some years) in commercial service, and accelerated aging is used in testing them. In these tests the cell potential is stable for most of the anode life. Failure of an anode is characterized by a rapid increase in potential to beyond the point of practical operation of the cell. Non-destructive X-ray techniques have been utilized to investigate the mechanism involved. It has been established that the precious metal content has been reduced by 50-60% when the anodes fail. Although present DSA coatings are more than adequate for commercial applications, there is continuing interest in improving them. The materials for DSA formulation include the precious metals iridium, ruthenium and rhodium, the non-precious metals tin, antimony and manganese, and the valve metals titanium and tantalum.     

Human cerebrospinal fluid protein database: edition 1992.

Two-dimensional electrophoresis maps of human cerebrospinal fluid proteins are presented in the form of labeled images. 931 protein spots are identified in spinal fluid from a normal volunteer. Distinct spots that represent variants of the same protein, especially posttranslational modifications, are estimated to reduce the 931 different spots to < 200 different proteins. 248 spots of 29 protein groups have been identified and are indicated on enlargements of specific gel regions. The distribution of protein abundance, mass, charge and shape characteristics of these normal 931 spinal fluid spots are graphically profiled. Analysis of the shape parameter "vertical height: width ratio" reveals that a ratio > 3.5 correlates with glycoproteins, enabling their identification simply by image analysis. Proteins that are not present on the normal map, but appear in spinal fluid in patients with schizophrenia and Creutzfeldt-Jakob disease are illustrated on additional maps.