Optical properties of single elastin fibres indicate random protein conformation.

In describing the properties of fibrous proteins it has been common practice to attribute the macroscopic mechanical properties to the organization at the molecular level. Hence, the high tensile stiffness of collagen and silk has been viewed as an inevitable consequence of their crystalline structure. For elastin, however, there has been considerable controversy and confusion in assigning a conformation to this rubber-like protein. The mechanical and thermodynamic properties of elastin are consistent with the kinetic theory of rubber elasticity, and this theory is based on an isotropic network of kinetically free, random-coil molecules. In contrast, numerous electron microscope studies of negatively stained elastins obtained by autoclave and alkali purification, as well as coacervates of the various soluble elastins, reveal a highly ordered (anisotropic) structure, consisting of 3- to 5-nm filaments that apparently run parallel to the long axis of elastin fibres. These filaments have been accepted as evidence for an anisotropic molecular structure in elastin that is dramatically different from the random network of the kinetic theory. We have now used polarized light microscopy to distinguish these two types of structure. We find that both purified and unpurified, water-swollen, single elastin fibres are optically isotropic, in agreement with the predictions of the kinetic theory of rubber elasticity.