Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
Fibrillar Proteins
X-ray diffraction analysis shows that keratins contain repeating structural units

To determine the distances between atoms in a crystal, a beam of X-rays of a specific wavelength is directed at it, and the angles of deflection of the X-rays by the electron shells of the crystal lattice atoms (diffraction angles) are measured, along with the intensities of the deflected (diffracted) rays. X-Ray Diffraction Analysis of sodium chloride crystals, for example, demonstrates that Na ⁺ and Cl^- ions form a simple cubic lattice. Methods based on X-ray diffraction can also establish interatomic distances in complex organic molecules, even in macromolecules as large as Proteins. However, this is a more challenging task than investigating simple salt crystals, as the enormous number of atoms in a protein molecule yields thousands of diffraction spots, the intensities of which can only be analyzed using computers.

In the early 1930s in England, William Astbury conducted the pioneering X-ray structural studies of proteins, which laid the foundation for a new scientific field. He demonstrated that an X-ray beam directed at a Hair or wool fiber—composed primarily of α-keratin—produces a characteristic diffraction pattern. Based on the analysis of this pattern, Astbury concluded that hair and wool exhibit periodicity, meaning they contain repeating structural units approximately 0.54 nm in length arranged along the fiber axis. His observations subsequently led to the hypothesis that The polypeptide chains in this family of proteins are not fully extended, but rather twisted or folded in some regular manner.

In addition, Astbury discovered that Fibroin (the protein comprising silk fibers), which represents β-keratin, yields a slightly different diffraction pattern corresponding to a Structure with a repeat period of 0.70 nm. Interestingly, the diffraction pattern obtained after stretching hair or wool under hot steam exhibits a periodicity close to 0.70 nm and closely resembles that of silk. For A number of years, no further Conclusions could be drawn from these observations due to the lack of data on the three-dimensional structure of Polypeptides.