Hair versus feathers provides an informative and interesting example of multi-scale structure controlling biological function. At the atomic level, these structures are both built of proteins called keratins that typify the two major types of secondary structure in proteins. Linus Pauling with Robert Corey and Herman Branson defined the two major forms of protein secondary structures based upon the x-ray diffraction results of Bragg, Kendrew, and Perutz on α-helical proteins and upon the x-ray diffraction patterns of β-keratin (see David Eisenberg PNAS, 2003 100, 11207-11210).

β-keratins in feathers are built of polypeptide strands hydrogen-bonded into β-pleated sheets. The component extended β-stands within the sheet are twisted and cross-linked by covalent disulfide bonds into extremely tough structures that are stronger than the α-keratins of human hair. α-keratins in the hair of mammals are built of polypeptide strands that form an extended spiral chain or helix with 3.5 amino acid residues/turn (versus the classical protein alpha helix with 3.6 residues/turn). The α-helix, predicted by Linas Pauling, is the most common form of secondary structure in proteins. The stability of α-keratin helices comes in part from a heptad repeat of leucine residues, which favor helix conformation via hydrophobic stacking interactions.

α-keratin x-ray fiber diffraction results actually slowed Pauling in his publication of the discovery of the α-helix because α-keratin shows a strong reflection at 5.15-Å resolution, whereas the α-helix repeat was calculated as 5.4 Å, so α-keratin seemed to rule out the α-helix to Pauling for several years. Francis Crick, a graduate student with Perutz, realized that keratin is a coiled-coil of alpha helices that provides further stability and results from α-helices winding around each other to cause a wider winding in the coiled-coil that in single helices and so reduces the repeat distance to 5.1 Å.

Thus hair as one of the defining characteristics of mammals and feathers as complex structures that distinguish birds from other living groups derive their different physical features from distinctly different fundamental secondary structures in the keratin proteins. The study of these gross physical structures by x-ray diffraction provided key insights into the fundamental structures of proteins.