Transitions hydrophobic association

Lavialle et al. [74] have also obtained information on the lipid-protein boundary. These authors studied the interaction of melittin, a polypeptide consisting of 26 amino acid residues, with dimyristoyl phosphatidylcholine. The results illustrated in Fig. 20 show that for a lipid-melittin molar ratio of 14 1 two order-disorder transitions are observed, one above (at 29 °C) an one below (at 17 °C) the transition for the pure lipid (at 22.5 °C). The low temperature transition is associated with a depression of the main lipid phase transition while the 29°C transition is associated with the melting behavior of approximately seven inunobilized boundary lipids which surround the hydrophobic portion of the melittin. 

Table 2 Dependence of transition temperature and standard Gibbs free energy of hydrophobic association for elastin-mimetic protein polymers based on the pentapeptide sequences (Val-Pro-Gly-Xaa-Gly)...

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. 

The position of the T,-divide that separates soluble from insoluble (hydrophobically associated) states in the phase diagram depends on seven variables on the six intensive variables of temperature, chemical potential, electrochemical potential, mechanical force, pressure, and electromagnetic radiation, and on polymer volume fraction or concentration. Therefore, diverse protein-catalyzed energy conversions by the consilient mechanism result from designs that control the location of the Tfdivide in this seven-dimensional phase transitional space. Complete mathematical description has yet to be written for representation of the T,-divide in seven-dimensional phase transitional space, but it may prove to be more relevant to... 

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. 

Different Estimates of Transition Temperature Used in Calculating the Gibbs Free Energy for Hydrophobic Association, AGha, by Equation (5.10a)... 

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. 

Figure 5.15. Plot of T, as a function of log(calcium ion concentration), log[Ca ], which is proportional to chemical energy. Considering the biological transition zone of lowering T, from 37°C to 25°C, it is possible visually to gain a sense of how the amount of chemical energy differs as the composition of the model protein changes. Clearly, as the hydrophobicity increases per carboxylate, the chemical energy required to drive hydrophobic association decreases, and it further decreases by having the carboxylates of the Glu (E) residues proximal for bidentate interaction with the doubly charged calcium ion. (D. Urry and C.-H. Luan, unpublished results.)...

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