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[reaction: see text] A new Pd(0) complex of a pendant cyclophosphazene-containing cross-linked polymer is found to be an effective heterogeneous catalyst for the Heck arylation reaction. The catalyst is robust and can be recycled without significant loss of activity.
We study the adsorption of homogeneous or heterogeneous polymers onto heterogeneous planar surfaces with exponentially decaying site-site correlations, using a variational reference system approach. As a main result, we derive simple equations for the adsorption-desorption transition line. We show that it is preferable to have a small amount of strongly adsorbing sites or monomers rather than a greater amount of weakly adsorbing ones. The results are discussed with respect to their implications for the physics of molecular recognition.
The rates of blunt-end and cohesive-end ligation of DNA by T4 DNA ligase are increased by orders of magnitude in the presence of high concentrations of a variety of nonspecific polymers such as polyethylene glycol, Ficoll, bovine plasma albumin, or glycogen. Blunt-end ligation of small self-complementary oligodeoxyribonucleotides is also stimulated. The use of polyethylene glycol 6000 in such systems is characterized in some detail. Conditions are suggested which either greatly increase the rate of formation of and size of linear ligation products or which allow smaller but significant improvements in the amounts of circular ligation products.
The effects of macromolecular crowding were tested on several reactions catalyzed by T4 RNA ligase. The rate of cyclization of oligoriboadenylates was stimulated up to 10-fold by relatively high concentrations of several polymers (polyethylene glycol (PEG) 8000 or 20,000; bovine plasma albumin; Ficoll 70). In addition, higher concentrations of PEG 8000 or PEG 20,000 allowed the novel formation of large linear products from the oligoriboadenylates. Also stimulated by high concentrations of PEG 8000 were the rate at which T4 RNA ligase joined p(dT)10 to oligoriboadenylates and the rate at which the enzyme activated p(dT)n by transfer of an adenylyl moiety from ATP to the oligonucleotides. These results with T4 RNA ligase are compared to earlier studies on the effects of crowding on DNA ligases.
The early-stage roughening of the interface between thin deuterated poly(methyl methacrylate) (d-PMMA) layers on thick polystyrene (PS) films was studied as a function of the temperature using real-time specular neutron reflectivity. By measuring the growth of the interface roughness as a precursor of the dewetting, the characteristic time constant of the early stages of the process was studied as a function of the temperature approaching the glass transition temperature (T(g)) of the two polymers from above and compared with the prediction of the growth of the interface by the spinodal process. Both solid and liquid regimes were probed, in which the viscosity of the thin film or the substrate dominates respectively. The characteristic growth time of the process also depends on the upper film thickness to a power of 5 or 6 in the solid or liquid regimes, respectively, as predicted by the theory of spinodal dewetting.
Compression of an adsorbed polymer layer distorts its relaxed structure. Surface force measurements from different laboratories show that the return to this relaxed structure after the compression is released can require tens of minutes and that the recovery time can grow rapidly with molecular weight. We argue that the arrested state of the free layer before relaxation can be described as a Guiselin brush structure (O. Guiselin, Europhys. Lett. 17, 225 (1992)), in which the monomer density falls off only weakly with distance from the surface. This brush structure predicts an exponential falloff of the force at large distance with a decay length that varies as the initial compression distance to the 6/5 power. This exponential falloff is consistent with surface force measurements. We propose a relaxation mechanism that accounts for the increase in relaxation time with chain length.
Reservoir-type microcapsules were prepared using a double emulsion solvent evaporation technique from a series of nine different poly-beta-hydroxybutyrate (PHB)-based polymers in which both molecular weight and hydroxyvalerate content were varied. Particles prepared from a low molecular weight (MW 43,000) homopolymer had a shrivelled appearance, but were not porous. When the molecular weight of the fabricating homopolymer was increased to 540,000, however, these features disappeared and non-wrinkled particles with microporous surfaces were observed. Microcapsules prepared from a high molecular weight copolymer of PHB with 10.8 per cent hydroxyvalerate (HV) had a similar appearance, but particles prepared from a high molecular weight 20.1 per cent HV copolymer had much smoother and less porous surfaces. Lowering the molecular weight of the copolymer had the effect of producing particles that were generally distorted in shape and had highly irregular, macroporous, surface morphologies. Increasing the double emulsion temperature to 40 degrees C at 2 min after phase combination typically generated smoother and/or less porous particles and improved the batch yield. The numerous factors potentially responsible for the development of morphological characteristics of the products are discussed in the light of these observations.
The capsular type-specific antigen of Haemophilus influenzae, type b, has been reported to have a unit structure composed of D-ribose and phosphate. Recently, the presence of ribitol was found in preparations of type b capsular antigen. Our analytical results show equimolar proportions of ribose, ribitol, and phosphate. Periodate oxidation studies, paper chromatography of acidic and alkaline hydrolysates, and NMR spectral data indicate the structure of the capular antigen of H. influenzae b, strain Eagan to be a polyribosylribitol phosphate polymer.
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