Entropic stabilisation

These effects have been observed for both aqueous and non-aqueous media and good correlation between the point of incipient flocculation and the 0-temperature is well established112. The transition from stability to instability usually occurs over a very narrow temperature range (1 or 2 K). Enthalpic stabilisation tends to be the more common in aqueous media and entropic stabilisation the more common in non-aqueous media. Owing to the elastic effect, aggregation into a deep primary minimum does not take place (as is possible with lyophobic sols) and redispersion takes place readily on reverting to better than 0-solvent conditions. 

Figure 1. The calcite-aragonite phase boundary in P/T space is curved, due to the temperature-dependence of the entropy of calcite. The entropic stabilisation of calcite at high-T arises from the order-disorder phase transition from orientationally ordered R3c calcite at low temperatures to the orientationally disordered R3c stmcture, stable above 1260 K. Figure after Redfern etal. (1989a).

The crystal structure of the Escherichia coli galactose-binding protein with a bound molecule of p-o-glucose was obtained." " From the structure, it can be observed that recognition is achieved by sequestering the monosaccharide entirely beneath the protein surface and expelling bulk water from the active site. This not only fosters a favourable entropic stabilisation but also allows the surrounding amino acid residues unfettered access to the substrate. In the... 

In the SCF analysis of curved bilayers, it was found that all results could be fitted with the Helfrich equation, without the need to invoke a nonzero Jo. This means that the vesicles are typically stabilised by translational and undulational entropic contributions. This result is consistent with results by Leermakers [114] for uncharged lipid bilayers, and can be rationalised by symmetry arguments as discussed above. 

Thermodynamic measurements on the analogous (unmethylated) Zn2+ complexes reveal that the stabilisation by macrocyclic preorganisation has both enthalpic and entropic contributions (Table 1.4). 

One must conclude that hydrophobic interactions may stabilise the multilayer TRP-ARG sandwich of gpl30, in spite of the different character of these two amino acids, and in spite of the entropic penalty mentioned above. However the gpl30 crystals themselves came from a solution which contained glycerol molecules and sulfate ions, and both of these components were trapped in the crystals where they may have helped to stabilise the observed protein structure. It would not be altogether surprising if some alternative conformation or conformations of gpl30 may also occur in vivo, if those somewhat unphysiological substances are not present in the human body in sufficient concentrations to stabilise the structure as observed. 

Rgure 7.8 Entropic (steric) stabilisation the potential energy-distance plots for (a) particles with no electrostatic repulsion, = Vs + a i (b) with electrostatic repulsion, V ,oi = Vs + + Vr. 

Entropic, volume restriction or elastic interaction, G p This results from the loss in configurational entropy of the chains on significant overlap. Entropy loss is unfavourable and, therefore, G j is always positive. A combination of G,. with G gives the total energy of interaction Gj (theory of steric stabilisation). 

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