摘要：

Numerous physical characterizations clearly demonstrate that the polypentapeptide of elastin (Val 1 -Pro 2 -Gly 3 -Val 4 -Gly 5 ) u in water undergoes an inverse temperature transition. Increase in order occurs both intermolecularly and intramolecularly on raising the temperature from 20 to 40°C. The physical characterizations used to demonstrate the inverse temperature transition include microscopy, light scattering, circular dichroism, the nuclear Overhauser effect, temperature dependence of composition, nuclear magnetic resonance (NMR) relaxation, dielectric relaxation, and temperature dependence of elastomer length. At fixed extension of the cross-linked polypentapeptide elastomer, the development of elastomeric force is seen to correlate with increase in intramolecular order, that is, with the inverse temperature transition. Reversible thermal denaturation of the ordered polypentapeptide is observed with composition and circular dichroism studies, and thermal denaturation of the crosslinked elastomer is also observed with loss of elastomeric force and elastic modulus. Thus, elastomeric force is lost when the polypeptide chains are randomized due to heating at high temperature. Clearly, elastomeric force is due to nonrandom polypeptide structure. In spite of this, elastomeric force is demonstrated to be dominantly entropic in origin. The source of the entropic elastomeric force is demonstrated to be the result of internal chain dynamics, and the mechanism is called the librational entropy mechanism of elasticity. There is significant application to the finding that elastomeric force develops due to an inverse temperature transition. By changing the hydrophobicity of the polypeptide, the temperature range for the inverse temperature transition can be changed in a predictable way, and the temperature range for the development of elastomeric force follows. Thus, elastomers have been prepared where the development of elastomeric force is shifted over a 40°C temperature range from a midpoint temperature of 30°C for the polypentapeptide to 10°C by increasing hydrophobicity with addition of a single CH 2 moiety per pentamer and to 50°C by decreasing hydrophobicity. The implications of these findings to elastic processes in protein mechanisms are (1) When elastic processes are observed in proteins, it is unnecessary, and possibly incorrect, to attempt description in terms of random chain networks and random coils; (2) rather than requiring a random chain network characterized by a random distribution of end-to-end chain lengths, entropic elastomeric force can be exhibited by a single, short peptide segment; (3) perhaps of greatest significance, whether occurring in a short peptide segment or in a fibrillar protein, it should be possible reversibly to turn elastomeric force on and off by reversibly changing the hydrophobicity of the polypeptide. Phosphorylation and dephosphorylation would be the most obvious means of changing the hydrophobicity of a polypeptide. These considerations are treated in Part 2: Simple (Passive) and Coupled (Active) Development of Elastic Forces (see Urry, 1988).

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