On the source of entropic elastomeric force in polypeptides and proteins: Backbone configurational vs. side‐chain solvational entropy

At physiological temperature in water, the relaxed state of the protein, elastin, and model elastomeric sequential polypeptides derived from this biological elastomer is the result of an inverse temperature transition. At lower temperatures, say at 20°C, the hydrophobic side chains of the elastomers are hydrated with a low‐entropy net of water. On raising the temperature, the more ordered waters of hydrophobic hydration are converted to less ordered bulk water as the polypeptide chains fold increasing intra‐ and intermolecular hydrophobic contacts. Following Flory and colleagues, thermoelasticity studies suggests that these polypeptide elastomers are dominantly entropic elastomers above the temperature of the inverse temperature transition. A central question becomes the source of the entropic elastomeric force. On stretching, hydrophobic side chains become exposed to water, resulting in an exothermic reaction of hydrophobic hydration. The issue addressed by the present report is whether this decrease in solvent entropy on stretching might make a major contribution to the entropic elastomeric restoring force. It has previously been argued that the free energy of solvation can be made very small by a 30% ethylene glycol (EG):70% water solvent mixture. This is demonstrated here using the repeating pentapeptide sequence of elastin (Val¹‐Pro²‐Gly³‐Val⁴‐Gly⁵)_n or poly(VPGVG) and its γ‐irradiation cross‐linked elastomeric matrix. Differential scanning calorimetry of the inverse temperature transition of poly(VPGVG) shows the endothermic heat of the transition to become very small in EG/H₂O when compared with H₂O alone, which also indicates a very small entropy change for the transition on exposure of the hydrophobic side chains to the EG/H₂O solvent mixture. A similar result is found for the cross‐linked elastomeric matrix. Significantly, however, in spite of the lower heats for hydrophobic solvation, the elastic modulus and the entropic elastomeric forces generated are greater in EG/H₂O. Thus, even though the heat of the transition and the entropy change on solvation are markedly reduced, the entropic elastomeric force is increased. Accordingly, it is argued that the entropy change due to hydrophobic side‐chain solvation on extension is considered not to be a primary source of the entropic elastomeric force.