Hydration competition apolar-polar

Figure 2.18. Energies are shown that can be inter-converted by means of elastic-contractile model proteins capable of exhibiting inverse temperature transitions functioning by means of the competition for hydration between oil-like and charged groups called an apolar-polar repulsive free energy of hydration. See Chapter 5 for a more complete development of the phenomenology and physical basis and Chapter 8 for details of the molecular process.

Competition for Hydration Between Apolar (Hydrophobic) and Polar (e.g., Charged) Groups Controls Amount of Hydrophobic Hydration... 

Competition for hydration between polar (e.g., charged) and hydrophobic (apolar) groups, called the apolar-polar repulsive free energy of... 

With the protein-based polymer poly [0.8GVGVP),0.2GEGVP)], at low pH when aU 4 of the Glu (E) residues/lOO residues are as COOH, the transition temperature is near 25° C and the heat of the transition, AH, = 0.97kcal/mole-pentamer (see Rgure 5.28). On raising the pH to the point of less than two C00"/100 residues, the heat of the transition has been reduced, and AH, = 0.27kcal/mole pentamers. The preferred interpretation over a decade ago was (1) that the formation of 2 C00 /100 residues structured almost three-fourths of the thermodynamically measured waters of hydrophobic hydration, and (2) that there exists a competition for hydration between apolar and polar residues, referred to as an apolar-polar repulsive free energy of hydration. 

Figure 5.34. Acid-base titration curves of the series of elastic Model Proteins I and i through v of Table 5.5 that exhibit systematic increases in hydrophobic-induced pK shifts and positive cooperativity resulting from competition for hydration between apolar and polar groups. (Inset) Slope of the Henderson-Hasselbalch equation with n = 1, and the slopes for...

From the analysis of the acid-base titration data in Figures 5.30 through 5.34, positive cooperativity results from the apolar-polar repulsive free energy of hydration, that is, from the competition for hydration between apolar (hydrophobic) and polar (e.g., charged) species. The general statement can be that the appearance on the scene of the first polar, for example, charged, species must do the work of destructuring hydrophobic hydration in order to achieve adequate hydration for itself. 

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. 

Stretch activation of muscle is a well-described phenomenon it was the subject of The Croonian Lecture (1977) given by Pringle,and it has been extensively researched and reported in the literature over the ensuing decades. For example, the basic description becomes When active insect flight muscle is stretched, its ATPase rate increases.. . This we take as yet another demonstration of a fundamental process whereby a phosphate present in a protein can be activated, energized, as the result of an increase in hydrophobicity. It is an example of the competition for hydration between apolar and polar species, that is, an example of the apolar-polar repulsive free energy of hydration active in muscle contraction. 

The apolar-polar repulsive free energy of hydration, AG,p, as reviewed above, results from a competition for hydration between charged species and hydrophobic groups. In the most extreme case reported thus far, this competition raised the pKa of a carboxyl from about 4 to about 11. This constitutes an amount sufficient to raise the free energy of carboxylates by 8 to lOkcal/mole. The basic issue to be addressed here is whether there is reason to believe that the free energy of the phosphate group would also be subject to competition for hydration with hydrophobic groups. 

The reduction of ubiquinone by receipt of two electrons to produce within the lipid bilayer membrane, but yet on the matrix side, would allow the pick up of two protons from the matrix. The process would result from formation of the negatively charged whereby the competition for hydration due to the apolar-polar repulsive free energy of hydration, AGap, would open an aqueous channel to the matrix side of the membrane for entry of two protons, 2H. With the above-preferred location for reaction (8.9b) combined with the just noted location of reaction (8.9c), there would be a net transport of two protons from the matrix side to the cytoplasmic side of the membrane. This would be in keeping with the direction of proton transport for Complexes I, III, and IV. As shown below in Figures 8.8 and 8.9, however, transport could not occur by this... 

The operative component of the comprehensive hydrophobic effect arises from the competition between charged and oil-like groups. This was shown to result in a previously unknown repulsive force embodied within an interaction energy called an apolar-polar repulsive free energy of hydration, AG,p. During function, AG,p works in conjunction with elastic force development by the restriction of internal chain dynamics. These have been called the hydrophobic and elastic consilient mechanisms. In Chapters 6,7, and 8, these consilient mechanisms were demonstrated to be fundamental to understanding the functions of biology s proteins. 

Both of these contractions result from hydrophobic association due to protonation of the carboxylate group. The underlying physical process is the competition for hydration between the charged carboxylate and the hydrophobic side chains of the valine (Val, V) residues (Urry, 1997). This competition, described as an apolar-polar repulsive free energy of hydration, results in increasing positive cooperativity of the add-base titration... 

As shown in Figure 7.34, the association of parallel and antiparallel P-chains in the formation of P-sheets is exactly the inverse all peptide groups form hydrogen bonds between chains. The consequences of this are significant when the protein chains have a composition within which exists substantial competition for hydration between hydrophobic (apolar) and polar (e.g., charged or peptide) groups. Of the... 

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