Inverse temperature transitions hydrophobic association

Due to the struggle to survive under circumstances of limited food supply, organisms evolve to use the most efficient mechanism available to their composition. The most efficient mechanism available to the proteins that sustain Life would seem to be the apolar-polar repulsive free energy of hydration as observed for the inverse temperature transitions for hydrophobic association. The efficiency of designed elastic-contractile protein-based machines and a number of additional properties make designed protein-based materials of substantial promise for the marketplace of the future. 

Figure 5.5. Transitions, plotted as independent variable versus dependent variable, showing a response limited to a partieular range of independent variable. (A) Representation of the thermally driven contraction for an elastic-contractile model protein, such as the cross-linked poly(GVGVP), plotted as the percent contraction (dependent variable) versus temperature (independent variable). The plot shows a poorly responsive range below the onset of the transition, the temperature interval of the inverse temperature transition for hydrophobic association, and another poorly responsive region above the tem-...

The protein-based polymer is soluble in water at temperatures below its coexistence line where the hydrophobic residues are surrounded by hydrophobic hydration. As the positive (-TAS) term due to hydrophobic hydration becomes larger than the negative AH term, simply due to increasing the value of T, solubility of a protein-based polymer is lost, and it hydrophobicaUy folds and assembles. The inverse temperature transition is a hydrophobic association transition. 

T, to a new value of T, caused by an energy input represented by % to provide a measure of the change in Gibbs free energy for hydrophobic association of the protein-based polymer. Therefore, Tt, the onset temperature for the inverse temperature transition, represents an intrinsic property of the hydrophobic consilient mechanism of energy conversion. 

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...

Table 5.3. Hydrophobicity Scale in terms of AGha, the change in Gibbs free energy for hydrophobic association, for amino acid residue (X) of chemically synthesized poly[fv(GVGVP), fx(GXGVP)], 40m ml, mw = 100 kDa in 0.15 N NaCl, 0.01 M phosphate, using the net heat of the inverse temperature transition, AGha = [AH,(GGGVP) - AH.(GXGVP)] for the fx = 0.2 data extrapolated to f = 1.

At any temperature within the temperature interval for the thermally driven transition, there exists an equilibrium between hydrophobic association and dissociation. Obviously, at the low temperature side of the temperature interval, limited hydrophobic association occurs. At the high temperature side of the temperature interval, hydrophobic association is limited only by the extending load. The relationship is such that hydrophobic association is inversely proportional to the load. 

Our focus now turns to the physical basis whereby the energy conversions of the hydrophobic consilient mechanism occur, and, of course, it becomes an issue of what controls the inverse temperature transition of hydrophobic association. 

Hydrophobic association on raising the temperature is the most fundamental aspect of the consilient mechanism, arising as it does from the inverse temperature transition. An equivalent statement would be that hydrophobic dissociation on lowering the temperature is fundamental to the consilient mechanism. Historically, this has been called cold denaturation of enzymes. In our view, those protein systems that associate on heating to physiological temperatures in order to achieve a functional state should be considered in terms of the consilient mechanism. 

D.W. Urry, The Change in Gibbs Free Energy for Hydrophobic Association Derivation and Evaluation by means of Inverse Temperature Transitions. Chem. Phys. Letters, 399,177-183, 2004. 

Figure 7.1. The movable cusp of insolubility represented as the calorimetry curve for the inverse temperature transition of (GVGVP)25, due to hydrophobic association. Hydrophobic association occurs either by raising the temperature from below to above the temperature interval of the transition or by introducing an energy that lowers the cusp of insolubility, that is, lowers the temperature at which the transition occurs. Hydrophobic dissociation occurs either by lowering the temperature from above to below physiological temperature or by moving the cusp to higher temperatures by the introduction of an energy that increases the temperature at which the transition occurs from below to above physiological temperature.

The hydrophobically associated state represents the insolubility side of the T,-(solubility/insolubility)divide. As discussed in Chapter 5, contraction of the model elastic-contractile model proteins capable of inverse temperature transitions arises due to hydrophobic association. Hydrophobic association occurs, most fundamentally, on raising the temperature, on adding acid (H" ) to protonate and neutralize carboxylates (-COO ), and on adding calcium ion to bind to and neutralize carboxylates. Most dramatically, hydrophobic association occurs on dephosphorylation of (i.e., phosphate release from) protein, and it commonly occurs with formation of ion pairs or salt bridges between associated hydrophobic domains. 

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... 

Scenario of Muscle Contraction by the Inverse Temperature Transition of Hydrophobic Association... 

Yet much more phenomenological data, showing the coherence of phenomena between hemoglobin function and the consilient mechanism of the inverse temperature transition for hydrophobic association, continues below, before direct examination of the molecular structures, as generally presented. Subsequently, in section 7.3, the molecular structures are examined to look for the specific interactions most significant to the consilient mechanism. 

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