Stabilisation, electrostatic

The determination of the tertiary structure of the hens egg white lysozyme, and the subsequent proposal that the side-chain carboxyhc acid group of Glu 35 act as a general acid, and the ionised side chain of Asp 52 acted to electrostatically stabilise an oxocarbonium ion intermediate, led to a search for a manifestation of these phenomena in model systems [6]. In the scramble electrostatic stabilisation was invoked where there was no compelling reason to distinguish it from ordinary nucleophilic participation. Thus, 3,5-dichlorophenolate departs from compound XXXII 100 times faster than from its p-isomer [94] a similar 22-fold acceleration of the ortho compared to the para isomer was observed with compound XXXIII. The rate acceleration in both cases was attributed to electrostatic stabihsation, but since the 3-methoxyphthalide was in both cases the product, it is not clear why humdrum neighbouring group participation was not invoked. 

A very carefully thought out test of the reahty of electrostatic stabilisation of a developing oxocarbonium ion was applied by Loudon and Ryono [95] using compounds XXXIV and XXXV. (The oxocarbonium ion is generated by rate-determining proton transfer to the vinyl ether function, rather than by departure of a leaving group.) Neither carboxylate can participate nucleophihcally any lactone from XXXIV would be unconscionably strained. Furthermore, water molecules in XXXIV cannot be accommodated between the carboxylate and the oxocarbonium ion centre the endo face of the system is hydrophobic. 

Only insignificant differences on the rate of hydrolysis were observed on ionisation of the carboxylic acid group of XXXIV - about the same as those observed with XXXV. 

The idea of intramolecular electrostatic, as distinct from covalent, stabilisation of an oxocarbonium ion must therefore be considered as yet unproven. 

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At 45 °C (Fig. 20b), the situation is different. The electrostatically stabilised particles (E2) start to coagulate and form large aggregates in the presence... 

W. Nerinckx, T. Desmet, K. Piens, and M. Claeyssens, An elaboration on the syn-anti proton donor concept of glycoside hydrolases. Electrostatic stabilisation of the transition state as a general strategy,... 

Lyophilic stabilisation is particularly important in non-aqueous systems115, e.g. oil-based paints, and in systems of very high particle concentration where electrostatic stabilisation is of limited effectiveness. It is also essential in biological systems, e.g. blood, where the... 

Electrostatic stabilisation. Repulsion between the double layers formed by non-adsorbing counter-ions causes stabilisation. Simple salts can be used which do not have a steric effect. 

Stable suspensions are necessary to obtain homogeneous casts by colloidal filtration. Different kinds of stabilisation mechanisms for alumina suspensions are used in literature. Pure electrostatic stabilisation can be obtained using HNO3, while electrosteric stabilisation is obtained using polyelectrolytes like PolyMethylAcrylicAcid (PMAA) and PolyAcrylicAcid (PAA). In... 

Hidber et al. [9] described the colloidal processing of wet-milled a-alumina suspensions by centrifugal casting from a 80 wt-% suspension at pH 4.3. The mean particle diameter of the a-alumina powder was 0.3 pm and the BET surface area 8.59 m /g. Nitric acid or ammonia was used for electrostatic stabilisation. Very dense ceramics could be obtained with relative densities up to 99.9%. 

The major driving force for the catalytic effect of serine proteases appeared to be associated with the electrostatic stabilisation of the (- + -) charge distribution, representing the Ser...His...Asp triad at the active site. This hypothesis is further supported by the experiments of Corey et al. (1992) who displaced the ionised aspartate in trypsin from the site 102 to the more distant site 214 by designing the D102S/S214E(D) double mutant. The rate reduction for two different substrates, as compared to the wild-type enzyme, was found to be 3 orders of magnitude less than for... 

One of the early works on enzyme electrostatics involved a study of CPA (Hayes and Kollman, 1976). These authors stressed the importance of the MEP in understanding enzyme action. They suggested that the electrostatic environmental effects do contribute to the lowering the transition-state energy of the reaction. However, as stressed above, the actual calculations were basically unable to address this issue since they did not consider the protein dielectrics nor the fact that the reaction in solution involves major electrostatic stabilisation (what counts is the difference between the stabilisation in the enzyme and in water). Nevertheless, site-directed mutagenesis experiments have provided strong evidence that electrostatic effects play a major role in the reactions catalysed by carboxipeptidase A. 

The elucidation of the X-ray structure of lysozyme provided the first direct information about the structure of an enzyme-substrate complex (Blake et al., 1967). This and other biochemical studies led to the proposal of three major catalytic factors general acid catalysis, steric strain and electrostatic stabilisation. It was found that the effect of steric strain is unlikely to be important because it can be relaxed by small shifts in the substrate and enzyme geometries (Warshel and Levitt, 1976 Pincus et al., 1977). This point was further established by quantitative FEP calculations (Warshel, 1991). On the other hand, it was found that electrostatic interactions play a very important role in the enzymatic reaction. The total electrostatic stabilisation of the carbonium transition state, relative to the corresponding stabilisation in solution, was found to be 29 kJ/mol (cf. Figure 12., Warshel, 1978). This value appears to account... 

Careful analysis of the energetics of enzyme catalysis leads one to conclude that it must involve electrostatic stabilisation of transition states. It is also becoming clear that this is done by active sites with preoriented polar environment that can be viewed as a "supersolvent" for the transition state (Warshel, 1981). 

Fig. 6.24. Two pinholes. SEM picture of coating surface layer 2 coating obtained by film-coating of an electrostatically stabilised alumina suspension.

Fig. 6.47. Layer 2 film-coating after dr5fing cmd sintering, (a) Electrostatically stabilised suspension of a-Al203 powder at pH 3. (b) Strongly agglomerated suspension of the same powder at pH 8 (i.e.p.).

In Fig. 6.50 flow curves of Alox SF a-alumina in water at pH 4 and pH 8 are shown. The low pH suspension is electrostatically stabilised using the positive... 

Polymers are also essential for the stabilisation of nonaqueous dispersions, since in this case electrostatic stabilisation is not possible (due to the low dielectric constant of the medium). In order to understand the role of nonionic surfactants and polymers in dispersion stability, it is essential to consider the adsorption and conformation of the surfactant and macromolecule at the solid/liquid interface (this point was discussed in detail in Chapters 5 and 6). With nonionic surfactants of the alcohol ethoxylate-type (which may be represented as A-B stmctures), the hydrophobic chain B (the alkyl group) becomes adsorbed onto the hydrophobic particle or droplet surface so as to leave the strongly hydrated poly(ethylene oxide) (PEO) chain A dangling in solution The latter provides not only the steric repulsion but also a hydrodynamic thickness 5 that is determined by the number of ethylene oxide (EO) units present. The polymeric surfactants used for steric stabilisation are mostly of the A-B-A type, with the hydrophobic B chain [e.g., poly (propylene oxide)] forming the anchor as a result of its being strongly adsorbed onto the hydrophobic particle or oil droplet The A chains consist of hydrophilic components (e.g., EO groups), and these provide the effective steric repulsion. 

As discussed above, the total energy-distance of separation curve for electrostatically stabilised shows a shallow minimum (secondary minimum) at a relatively long distance of separation between the droplets. However, by adding small amounts of electrolyte, such minima can be made sufficiently deep for weak flocculation to occur. The same applies to stericaUy stabihsed emulsions, which show only one minimum, but whose depth can be controlled by reducing the thickness of the adsorbed layer. This can be achieved by reducing the molecular weight of the stabiliser and/or the addition of a nonsolvent for the chains (e.g., an electrolyte). 

This can occur if the energy barrier is small or absent (for electrostatically stabilised emulsions) or when the stabilising chains reach poor solvency (for sterically stabilised emulsions, that is if />0.5). For convenience, the flocculation of electrostatically and sterically stabilised emulsions will be discussed separately. 

Possible instabilities which might arise in the final formulations are (i) heteroflocculation of particles with differing charges (ii) electrolyte intolerance of electrostatically stabilised pigments (iii) competitive adsorption/desorption of a... 

Clearly, depends on the volume fraction of the dispersion, as well as the particle size distribution (which determines the number of contact points in a floe). Therefore, for quantitative comparison between various systems, it must be ensured that the volume fraction of the disperse particles is the same, and that the dispersions have very similar particle size distributions. also depends on the strength of the flocculated structure - that is, the energy of attraction between the droplets - and this in turn depends on whether the flocculation is in the primary or secondary minimum. Flocculation in the primary minimum is associated with a large attractive energy, and this leads to higher values of when compared to values obtained for secondary minimum flocculation (weak flocculation). For a weakly flocculated dispersion, as is the case for the secondary minimum flocculation of an electrostatically stabilised system, the deeper the secondary minimum the higher the value of (at any given volume fraction and particle size distribution of the dispersion). 

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