The Structure and Chemistry of Human Hair 22 1930s Astbury in Leeds, England, found a highly characteristic X-ray diffraction pattern was obtained from all mammalian keratin fibers (the so-called α-keratin pattern) and particularly prominent were strong 5.1 Å meridional arcs and intense 9.8 Å spots. In 1951 Pauling, Corey and Branson51 conjectured that the α-keratin diffraction pattern had arisen from proteins in the fibers that had helical backbones with their axes along the length of the fibers. We now know such helices are a common structural feature of many proteins and indeed are an important feature of the KIFs of human hair. The single α-helix is right-handed, with a helical pitch of 5.4 Å, 3.6 amino acid residues per turn and is stabilized by hydrogen bonds between the turns of the helix. More specifically the hydrogen bonds occur between the carbonyl oxygen atom of each amino acid residue and an imino hydrogen atom three amino acid residues further along the protein chain. An early problem was that whilst 5.4 Å was expected for the pitch of the straight helix, the meridional X-ray reflection from hair that might have corresponded was somewhat less at 5.1 Å. This dilemma was resolved by Crick52 in recognizing that in mammalian keratins the helices occurred in pairs as a two-stranded rope or coiled-coil. Thus the axis of each α-helix in the coiled-coil is everywhere tilted at an angle of approximately 18 degrees to the long axis of the fiber and with a helical pitch in the axial direction of 5.4 cos 18o = 5.1 Å in concordance with the X-ray diffraction observations. In addition, the 9.8 Å equatorial X-ray diffraction maximum was recognized as identifying the transverse distance between the axes of the two helices in the coiled coil. 2-Dimensional polyacrylamide gel electrophoresis of proteins solubilized from mammalian keratin fibers after derivitising them by converting their cystine residues into S-carboxymethyl cysteines, has provided a wealth of information about the proteins in the fibers (c.f. Figure 11). In one direction electrophoresis is undertaken at constant pH that serves to separate the proteins according to their fundamental charge and then electrophoresis in an orthogonal direction in the presence of sodium dodecyl sulphate separates the proteins according to molecular mass. In the resultant “map” a