Insolubility hydrophobic hydration

The thermodynamic and structural processes that occur when water molecules are in the vicinity of hydrophobic entities (water fearing, insoluble in water) are referred to collectively as hydrophobic hydration (Tanford, 1973 Privalov and Gill, 1988 Blokzijl andEngberts, 1993 Chau and Mancera, 1999). Hydrophobic hydration is important in gas hydrate formation, which usually starts with hydrophobic gas molecules (e.g., methane) being introduced into liquid water. 

In Summary, Hydrophobic Hydration Disappears for Two Different Reasons with Opposite Consequences of Insolubility and Solubility... 

Whether a particular hydrophobic region or domain of a model protein or of a natural protein associates with a second hydrophobic domain in the same molecule or a separate molecule, the same process of loss of solubility occurs. If the two hydrophobic domains can associate and if together they have so much hydrophobic hydration that their T, is below the temperature of the environment, they associate they are insoluble AG(solubility) is positive. 

Hydrophobic Hydration Disappears When a Pair of Hydrophobic Domains Associate, That Is, When the Pair of Domains Becomes Insoluble... 

Increasing the temperature to result in insolubility is the hallmark of hydrophobic association by inverse temperature transitions. As temperature is a factor in the positive (-TAS) term for formation of hydrophobic hydration, an increase in temperature obviously increases the magnitude of the positive (-TAS). An increase in the magnitude of the positive (-TAS) term means that AG(solubility) = AH -TAS becomes positive, and solubility is lost. The cusp of insolubility, the Tt-(solubility/insolubil-ity) divide, is surmounted by heating. On... 

Death is the ultimate manifestation of excursion excess into the realm of insolubility. ATP Adenosine Triphosphate (ATP), the universal biological energy currency, is the ultimate solubilizer of protein. In our view, the negative hypercharged phosphate functions as a super-carboxylate to destroy hydrophobic hydration in the process of satisfying its own thirst for hydration. Thus, as paired associated hydrophobic surfaces undergo an opening fluctuation, hydrophobic hydration that would form is immediately recruited for hydration of added phosphate. As substantial hydrophobic hydration is required for the positive (-TAS) to dominate and to result in insolubility, the result of phosphorylation is solubility. In this way bound phosphate provides for protein solubilization by separation of hydrophobicaUy associated domains. 

The phenomenon of hydrophobic association on raising the temperature, as noted above and treated in detail in Chapter 5, derives from the thermodynamics of structured water surrounding hydrophobic moieties. Hydrophobic hydration disappears, due to an unfavorable Gibbs free energy for solubility, as the temperature is raised from below to above the transition temperature that reaehes the cusp of insolubility represented in Figure 7.1. This causes the hydrophobic domains to separate from water by means of intra- and intermolecular hydrophobic association. 

Phosphate attached to a model protein is three to four times more effective on a mole fraction basis than carboxylate in raising the T,-divide for hydrophobic association, which we have shown is due to a decrease in hydrophobic hydration (see Figures 5.25 and 5.27). Dephosphorylation, therefore, would re-establish hydrophobic hydration and dramatically lower the Trdivide, which is to lower the temperature range of the cusp of insolubility to below physiological temperature. The result would be an insolubilization of hydrophobic domains (a hydrophobic association) that we consider to be the power stroke of muscle contraction. 

Recall from Figure 5.27 and associated discussion that, as the amount of hydrophobic hydration increases, the value of T, decreases. The cusp of insolubility moves to lower temperatures, from above to below physiological temperature. Accordingly, hydrophobic association between two surfaces occurs because the surfaces are sufficiently hydrophobic that during a transient opening so much hydrophobic hydration forms that the T,-divide is below the operating temperature. This causes the transiently... 

Recall from Chapter 5, section 5.1.3.3, that insolubility (association) of hydrophobic groups occurs when there has developed too much hydrophobic hydration. In particular, AG(solubility) = AH - TAS, where AH is nega-... 

Release of the y-phosphate from protein-bound ATP, leaving bound ADP, restores much hydrophobic association that existed in the protein before ATP binding. Release of the y-phosphate permits reconstitution of sufficient hydrophobic hydration to drive hydrophobic association. In the broad view of the consilient mechanism, as regards ATPases, ATP binding causes hydrophobic dissociation, and P and ADP release re-establishes maximal hydro-phobic association. In terms of the movable cusp of insolubility, binding of the polar ATP molecule raises the temperature of the movable cusp of insolubility to give solubility, and the decrease in polaritys on phosphate release lowers the movable cusp of insolubility to re-establish the insolubility of hydrophobic association... 

Before electron transfer, as fluctuations toward dissociation occur with occupancy by the hydrophobic QH2 molecule, sufficient hydrophobic hydration would form to result in water insolubility such that reassociation results. If, on the other hand, fluctuations toward dissociation occur for QH occupancy, as hydrophobic hydration formed it would be recruited to align for hydration of the positively charged QHi. Thus, increased hydrophobic hydration that ensured hydrophobic reassociation would not occur and hydrophobic dissociation would be the result. Without the favorable decrease in Gibbs free energy for association, the deforming force that caused extension of the tether would be gone, and elastic retraction would result. Additional mechanical details of the resulting domain movement follow below. 

The right hand side of Equation (16) is negative because ((GVGIP) < ((GVGVP) and A5,(GVGVP) is positive for a transition whereby hydrophobic hydration becomes less ordered bulk water during hydrophobic association, that is, as insolubility ensues. 

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