Repulsion hydrophobic-induced

The ernes of ionic surfactants are usually depressed by tire addition of inert salts. Electrostatic repulsion between headgroups is screened by tire added electrolyte. This screening effectively makes tire surfactants more hydrophobic and tliis increased hydrophobicity induces micellization at lower concentrations. A linear free energy relationship expressing such a salt effect is given by ... 

The Hill plots in Figure 5.31 provide remarkable insight into the hydrophobic-induced pKa shifts. Starting in the hydrophobically associated state for Model Protein v, on decreasing acid concentration, that is, raising the pH, the pKa of the first carboxyl to form carboxylate is 7.0. In Figure 5.31 A, this is the pKa for the most tightly bound proton. As more carboxylates form, further ionization becomes easier, and finally the last carboxyl to ionize does so with a pKa of 5.7. Remarkably, the last carboxyl to ionize does so with a pKa shifted 1.7 pH units from that of the unperturbed Glu pKa of about 4. Even in the completely unfolded state there remains an apolar-polar repulsion of 2.4 (= 1.7 X 2.3RT) kcal/mole-Glu. ... 

Derivation of Equation (5.21) considers hydrophobic-induced pK shifts and hydrophobic-induced increases in positive cooperativity. In Figure 5.30, however, there is experimental delineation between the hydrophobic domain where the above considerations dominate and the electrostatic domain where charge-charge repulsion dominates with a negative cooperativity. Thus we require an expression that would properly include both effects. 

Other examples have emerged more recently. Prion proteins induce insolubility and cause the ravages of Alzheimer s and mad cow diseases. Then there are chaperones that reverse inappropriate insolubilities. In these latter cases considered mechanisms are not so deeply ingrained. In none of these, however, has the sense of an apolar-polar repulsive free energy of hydration, AG.p, emerged. In none of these has there been a suggestion of the competition for hydration between hydro-phobic and polar species that is the basis for repeated experimental demonstrations of large hydrophobic-induced pKa shifts. 

Hydrophobicity is repulsion between a nonpolar compoimd and a polar environment, such as water. The distribution and hydrophobicity of hydrophobic amino acids are characteristic of each protein, so a specific separation is possible with use of hydrophobic supports. Different models are used for the description of protein hydrophobicity and hydrophobic interactions, and different theories are proposed for the retention mechanism of proteins in HIC [31]. In addition to the important role of water in the strengthening—weakening of the hydrophobicity, these interactions can be induced by changing the ionic strength, presence of organic solvents, temperature, and the pH value of the chromatographic media. Based on these properties, the HIC separation occurs by the modulation of the frequency and distribution of surface-exposed hydrophobic amino-acid residues, the hydrophobicity of the medium, the nature and composition of the sample, as well as type and concentration of salt in the mobile phase (see Figure 7.6). 

The temperature-induced behavior of copolymers at two pHs (4.0 and 7.4) is observed as shown in Figure 8. At pH 4, no LCST was observed with poly-DMAEMA, and the LCST of all copolymers was increased compared to that at pH 7.4. At pH 4.0, (A,A-dimethylamino)ethyl groups of DMAEMA are fully ionized. An increasing electrostatic repulsion between charged sites on DMAEMA disrupts the hydrogen bonds between EAAm and DMAEMA. These interfere with the hydrophobic interactions between (A,A-dimethyl-... 

Figure 18.1 Models for different modes of peptide-lipid interaction of membrane-active peptides. The peptide remains unstructured in solution and acquires an amphipathic structure in the presence of a membrane. The hydrophobic face of the amphipathic peptide binds to the membrane, as represented by the grayscale. At low concentration, the peptide lies on the surface. At higher peptide concentrations the membrane becomes disrupted, either by the formation of transmembrane pores or by destabilization via the "carpet mechanism." In the "barrel-stave pore" the pore consists of peptides alone, whereas in the "toroidal wormhole pore" negatively charged lipids also line the pore, counteracting the electrostatic repulsion between the positively charged peptides. The peptide may also act as a detergent and break up the membrane to form small aggregates. Peptides can also induce inverted micelle structures in the membrane.

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