Entropic destabilization

Thermodynamic studies performed on a number of other guanosine adducts show some examples where binding represents an enthalpic destabilized, but entropy stabilized, process and others that are enthalpically stabilized, but entropically destabilized. The net thermodynamic result, however, appears to be a relatively modest localized free-energy destabilization that could form the basis for some recognition mechanism. 

Another key feature is the availability of a nearly perfect acceptor dopant (i.e. a dopant which does not change the electronic structure of the oxygen). While in all other reported cases, the increase of the acceptor dopant concentration leads to a reduction of the proton mobility and an entropic destabilization of protonic defects [222], both the proton mobility and the thermodynamics of hydration are practically unchanged for dopant levels up to 20% Y in BaZr03 (Fig. 3.2.13). High proton mobility and entropically stabilized protonic defects even at high dopant concentrations and the high-solubility limit lead to the enormous proton... 

Another way to interpret the above observations would be in terms of the general principle that effective steric stabilization of polymer-coated droplets requires that the continuous phase be a good quality solvent for the polymeric stabilizer. Under poor quality solvent conditions (asi-casein at high ionic strength), the required entropic stabilizing repulsion of the adsorbed protein layer is converted into a destabilizing polymer-mediated attraction (Dickinson and Stainsby, 1982 Dickinson, 2006). 

As might be anticipated, the majority of solvent effects of the structures of pyridines relate to conformational changes and, where appropriate, this has been mentioned above. It may be further exemplified by the conformational changes observed on addition of alcohols and fluorinated alcohols to solutions of the pyridinium salts such as 51 in water <2000JA738>. Helical coils are favored by the fluorinated alcohols as the co-solvent destabilizes the exposed hydrophobic side chains in other conformations along with favoring the helical conformation entropically. The former effect is less marked in nonfluorinated alcohols. 

The disulfide bond differs from other types of interactions in folded proteins, such as hydrogen bonds and hydrophobic, electrostatic and van der Waals interactions. The disulfide bond is a covalent bond that is able to significantly stabilize folded conformations by 2-5 kcal/ mol for each disulfide.11 The effect is presumed to be due mainly to a decrease in the configurational chain entropy of the unfolded polypeptide.21 On the other hand, another view is that the disulfide bond destabilizes folded structures entropically, but stabilizes them enthalpically to a greater extent.31... 

The molecular mechanisms by which the extension of the N-terminus by the extra methionine residue destabilized recombinant a-lactalbumin remain unclear. Additional conformational entropy of the extra methionine residue in the unfolded state could account for the destabilization and unfolding-rate acceleration of the recombinant protein [22]. Ishikawa and coworkers reported the destabilization of recombinant bovine a-lactalbumin, similarly induced by the extra N-terminal methionine residue, and showed that the enthalpy change of thermal unfolding was the same for the authentic and recombinant proteins, indicating that the destabilization was caused by an entropic effect [42]. However, the destabilization by the extra methionine residue in the lysozyme homologous to a-lactalbumin was rather enthalpic and accompanied by a disruption of hydrogen-bond networks in the N-terminal region [43,44]. 

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