Optical Activity of Structural Proteins

Abstract

The rotatory dispersion and conformation of various globular proteins was reviewed in the preceding two chapters. Spectropolarimetry has been applied successfully in the study of these proteins which, as a rule, are soluble. The fibrous structural proteins are more difficult to study by this method, because only a few of them can be dissolved without degradation and possible change of conformation. The main reason for their resistance to mild solvents is their gross conformation, the highly asymmetric fibrous shape of the macromolecules. This can be illustrated by simple geometric consideration. It has been calculated that 8000 carbon atoms in a compact diamond packing form a cube with an edge length of 36.5 angstroms (Å) and a surface area of 7993.5 Ų, whereas in a fully extended chain the same 8000 atoms form a 10,200 Å long chain which has a surface area of 177,0 Ų. The larger the surface area the greater the number of contact points and the possibility of secondary bonding. Thus in highly extended polypetide chains, such as in silk or keratin, the secondary hydrogen bonds and van der Waals interactions link the chains at very many segments. Mechanical enmeshing may form structures from which the separation of any of the constituting macromolecules becomes quite difficult. Furthermore, in many protein fibers, such as the keratins, resilin, or elastin, the extended polypeptide chains are linked by covalent bonds. In such cases dissolution without chemical degradation is impossible.