Aqueous monomeric, oligomerization

Fig. 8 Proposed model for gramicidin S in a membrane according to the orientational constraints obtained from and N-NMR. The upright backbone alignment (r 80°) and slant of the /3-sheets (p -45°) are compatible with the formation of an oligomeric /3-barrel that is stabilized by hydrogen bonds (dotted lines). A The oligomer is depicted sideways from within the lipid bilayer interior (showing only backbone atoms for clarity, but with hydrophobic side chains added to one of the monomers). Atomic coordinates of GS were taken from a monomeric structure [4], and the two DMPC lipid molecules are drawn to scale (from a molecular dynamics simulation coordinate file). The bilayer cross-section is coloured yellow in its hydrophobic core, red in the amphiphilic regions, and light blue near the aqueous surface. B Illustrates a top view of the putative pore, although the number of monomers remains speculative...

Figure 1 gives results obtained by Alexander et al. (I) and Baumann (2) by dissolving fine particles of commercially available vitreous silica powders in aqueous solutions. Similar data obtained in polymerization and depolymerization experiments by Scheel et al. (15) and Friedberg (10) indicate that the curve shown in Figure 1 represents an equilibrium concentration for oligomeric acid. It can be approached from the supersaturated state of monomeric silicic acid as well as from a solution of pure polymeric silicic acid. 

Regardless of the t)q)e of soluble metal silicates used, they are subject to the same molecular speciation in aqueous solution resulting in a mixture of monomeric tetrahedral ions, oligomeric linear or cyclic silicate ions, and polysilicate ions. Sodium metasilicate, an example of a soluble metal silicate, can be prepared in anhydrous form or in the presence of water of crystallization as the penta- or nona-hydrate. It is readily soluble in water [8]. 

The unsubstituted para-t-butyl calixarenes themselves complex metals via their aryloxide groups. Since aryloxide complexes are frequently oligomeric, the simple calixarenes do not give monomeric complexes. Aryloxides are hard ligands, therefore they readily form complexes with oxo-philic hard metal ions such as alkali metals, early transition metals, lanthanides, and actinides. Complexation is often inferred because the calixarene acts as a carrier for the metal ion from an aqueous to an organic phase. With the /wa-/-butylcalix[ ]arenes in alkaline solution, a value of n = 6 gives the best carrier for lithium(I), sodium(I), and potassium(I), with a value of n 8 giving the best carrier for rubidium(I) and caesium(I).15,16 Titanium(IV) complexes have been characterized,17-19 as well as those of niobium(V) and tantalum(V).20-22 These complexes are classified as... 

Long chain limiting propagation rate coefficient in the latex phase (L/mol/sec) Monomeric radical propagation rate coefficient in the latex phase (L/mol/sec) aqueous phase propagation rate coefficient for oligomeric radicals of degree / (L/mol/sec)... 

The hairy particles stabilized by non-ionic emulsifier (electrosteric or steric stabilization) enhance the barrier for entering radicals and differ from the polymer particles stabilized by ionic emulsifier [35]. For example, the polymer lattices with the hairy interface have much smaller values of both the radical entry (p) and exit (kdes) rate coefficients as compared to the thin particle surface layer of the same size [128,129]. The decrease of p in the electrosterically stabilized lattices is ascribed to a hairy layer which reduces the diffusion of oligomeric radicals, so that these radicals may be terminated prior to actual entry. For the electrostatically stabilized lattices with the thin interfacial layer, exit of radicals occurs by the chain transfer reaction [35]. This chain transfer reaction results in a monomeric radical which then exits out of the particle by diffusing through the aqueous phase and this event is competing with the propagation reaction in the particle [130]. The decrease of kdes in the electrosterically stabilized latex... 

This phenomenon was confirmed by the introduction of symmetric tetraalkylammonium hydroxides in the dissolution of silica gel. In TMAOH the observed rate of dissolution was slow compared to the rate observed for cesium hydroxide dispersions, and cesium hydroxide has the lowest rate for the different alkali metal hydroxides. Results in Figure 3 clearly reveal an inhibition time between mixing of the silica gel with the aqueous TMAOH and the onset of dissolution. This observation is attributed to the strong interaction of the rather apolar TMA cation with the negatively charged silica gel surface. Because in this case no hydration shell is present, dissolution only occurs very slowly. The observed inhibition period of the dissolution reaction can be related to specific interactions of TMA cations with relatively large oligomeric species of the monomeric... 

Because polymerization of the water-glass solution gives rise to a broad distribution of silicate anions and thus results in poorly resolved Si NMR fines, the application of Si NMR spectroscopy in this type of reaction provides minor information about the different oligomerization steps and the reaction mechanism of monomeric silicic acid. Therefore a different approach to the study of aqueous silicate solutions was apphed. Because of the... 

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