Interaction between proteins and salt solutions. II. Elasticity of collagen tendons.

Abstract

AbstractA study of the elastic properties of collagen tendons while in the crystalline state while undergoing shrinkage, and while in the amorphous state was carried out. An additional analysis of x‐ray diffraction patterns and birefringence was also performed. Our main observations are the following. The lattice spacings of a quinone‐crosslinked collagen tendon are not altered when the tendon is in equilibrium with KCl or KSCN solutions of different salt concentrations or with acid solutions having different pH. This occurs in spite of large alterations of the modulus of the crystalline tendon in salt and acid solutions. These results indicate that, within the highly ordered framework of the tendon, the salt or acid solution is able to perform a selective disordering action which may involve a considerable fraction of the material before shrinkage takes place. At temperatures and salt concentrations considerably larger than those corresponding to the shrinkage, the tendon exists in a mesomorphic state where residual ordered regions are constrained by the crosslinks in a tactoidal arrangement. The reversibility of length, volume, and birefringence observed within the mesomorphic state attest that it is indeed an equilibrium property of the tendon. Two types of reversible contractile systems can be obtained using the collagen tendon. If full shrinkage is attained, only a fraction of the original length can be recovered on reducing the temperature or salt concentration below the one causing shrinkage. A contraction‐extension cycle will then involve the fully denatured and the partly renatured tendon. In order to obtain a reversible contractile system involving recovery of the original length, it is necessary to maintain a more complex “memory” of the original organization than that assured by merely crosslinking in the oriented state and by the occurrence of the mesomorphic phase. This can be done by a selective melting as operated, for instance, by acid solutions. The occurrence of a glasslike state characterized by a high value of the modulus was revealed for the fully amorphous tendon in solutions of salts causing large deswelling. The unperturbed dimensions of collagen molecules are not greatly affected by alteration of salt or salt concentration in the case of KCl and KSCN. The mean‐square end‐to‐end distance \documentclass{article}\pagestyle{empty}\begin{document}$ \overline {r_0^2 } $\end{document} decreases with increasing temperature, i.e., d ln \documentclass{article}\pagestyle{empty}\begin{document}$ {{\overline {r_0^2 } } \mathord{\left/ {\vphantom {{\overline {r_0^2 } } {dT}}} \right. \kern-\nulldelimiterspace} {dT}}{\rm } = {\rm } - 0.6{\rm } \times {\rm 10}^{ - 3} {\rm deg}{\rm .}^{ - 1} $\end{document}. Elastic properties of quinone‐crosslinked tendons in the amorphous state are similar to those exhibited by other polymer systems crosslinked in the oriented state. Networks formed by chromium salts are, however, peculiar, since they exhibit negative stress‐temperature coefficients. The relevance of some of the above results on the aging of connective tissue and on the theories of muscle contraction is pointed out.