Hydrophobic effect inverse temperature

With rising temperature the volume of the aqueous phase grows, the micelles swell until suddenly, at the so-called phase inversion temperature, the oil phase has the larger volume. This effect is explained by the polyoxyethylene chains dehydrating as the temperature rises. The hydrophilic/hydrophobic balance of the molecule is thereby altered and the solubility in the oil phase grows. When concentration is great enough, micelles are formed in the oil phase and water is solubilised. If the two phases do not exist as stratified layers but as emulsion a... 

Figure 5.10. An embodiment of the comprehensive hydrophobic effect in terms of a plot of the temperature for the onset of phase separation for hydrophobic association, Tb, versus AGha. the Gibbs free energy of hydrophobic association for the amino acid residues, calculated by means of Equation (5.10b) using the heats of the phase (inverse temperature) transition (AH,). Values were taken from Table 5.3. Tb and T, were determined from the onset of the phase separation as defined in Figure 5.1C,B, respectively. The estimates of AGha utilized the AH, data listed in Table 5.1 for fx = 0.2 but extrapolated to fx = 1, and the Gly (G) residue was taken as the...

Early in our studies it was expected that the post-translational modification of proline hydroxylation, so important to proper collagen structure and function, would raise the value of the temperature, T, for the onset of the inverse temperature transition for models of elastin. Accordingly, hydroxyproline (Hyp) was incorporated by chemical synthesis into the basic repeating sequence to give the protein-based polymers poly[fvs,i(Val-Pro-Gly-Val-Gly), fHyp( al-Hyp-Gly-Val-Gly)], where f sl -i- fnyp = 1 and values of fnyp were 0, 0.01, and 0.1. The effect of prolyl hydroxylation is shown in Figure 7.49. Replacement of proline by hydroxyproline markedly raises the temperature for hydrophobic association. Prolyl hydroxylation moves the movable cusp of... 

Figuke 7.49. Effect of prolyl hydroxylation on hydrophobic association (insolubility). Temperature profiles show turbidity formation due to the aggregation attending the onset of the inverse temperature transition. The value for the onset temperature is taken at 50% of the maximal turbidity and is called T,. Hydroxylation markedly shifts the polymer to solubility. (Reproduced with permission from Urry et al. )... 

To date the sigmoid curve in Figure 5.10 offers perhaps the most effective introduction to the comprehensive hydrophobic effect. It is a plot of the reference temperature, either Tb or Tt, for the onset of the inverse temperature transi-... 

Net Heat Changes for the Inverse Temperature Transitions of the Comprehensive Hydrophobic Effect Are Endothermic Due to the Conversion of Hydrophobic Hydration to Bulk Water ... 

As reviewed in Chapter 7 with a focus on the issue of insolubility, extensive phenomenological correlations exist between muscle contraction and contraction by model proteins capable of inverse temperature transitions of hydrophobic association. As we proceed to examination of muscle contraction at the molecular level, a brief restatement of those correlations follows with observations of rigor at the gross anatomical level and with related physiological phenomena at the myofibril level. Each of the phenomena, seen in the elastic-contractile model proteins as an integral part of the comprehensive hydrophobic effect, reappear in the properties and behavior of muscle. More complete descriptions with references are given in Chapter 7, sections 7.2.2, and 7.2.3. 

In Chapter 5, based on an inverse temperature transition due to hydrophobic association in water, a set of Axioms were derived from the phenomenological demonstration that de novo designed model proteins could efficiently interconvert the set of energies interconverted by living organisms. Then there followed a series of experimental results and analyses that defined the comprehensive hydrophobic effect. 

The effect of temperature is shown in Figure 4, where binding isotherms at 4 temperatures are plotted. At a glance, it is clear that there is very little temperature dependence even with an inversion temperature. Insensitivity to temperature is thermodynamically related to a small value of binding enthalpy. This might have to do with a delicate balance or cancellation of enthalpy changes related to electrostatic, (de)hydration, conformation, and hydrophobic interactions. 

However, surfactants incorporated into the electrolyte solution at concentrations below their critical micelle concentration (CMC) may act as hydrophobic selectors to modulate the electrophoretic selectivity of hydrophobic peptides and proteins. The binding of ionic or zwitterionic surfactant molecules to peptides and proteins alters both the hydrodynamic (Stokes) radius and the effective charges of these analytes. This causes a variation in the electrophoretic mobility, which is directly proportional to the effective charge and inversely proportional to the Stokes radius. Variations of the charge-to-hydrodynamic radius ratios are also induced by the binding of nonionic surfactants to peptide or protein molecules. The binding of the surfactant molecules to peptides and proteins may vary with the surfactant species and its concentration, and it is influenced by the experimental conditions such as pH, ionic strength, and temperature of the electrolyte solution. Surfactants may bind to samples, either to the... 

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