Hydration competition hydrophobic-polar

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... 

Does There Exist a Competition for Hydration Between Hydrophobic and Polar (e.g.. Charged) Groups as a Key Part of the Functioning of Protein-based Machines of Biology ... 

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. 

Our View Competition for Hydration with Hydrophobic Moieties Energizes Polar Species... 

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. 

---