The Structure of α-Keratin

Known structural principles (close packing, maximum hydrogen bonding, the tendency of like groups to be surrounded in like manner, and the approximate constancy of interatomic distances and bond angles) are used, with meridional and equatorial x-ray data, to deduce and check the structure pattern for alpha-keratin. Internally hydrogen-bonded polypeptide helices are grouped into "3-stacks", in which each chain is rotated and shifted vertically a distance equal to the helix pitch (5.15 A, average), relative to the other two. This shift accounts simply for the meridional x-ray reflection at this spacing. The 3-stack structure repeats after three turns, except for differences in the R groups and a slight twist, required to give satisfaction of both intrachain and interchain forces. The 3-stacks are grouped into 9-stacks and these into 27-stacks (all twisting), giving a crystallographic unit containing 81 chains, with the chain axes spaced approximately like those of close-packed cylinders. When the twisting reaches the limit of stability for good interchain contacting and cross-linking, the residue/turn ratio in each chain helix shifts to another, with twisting in the opposite direction. The twisting reversal mechanism keeps all the helix axes approximately straight, parallel, and in a close-packed arrangement. Interchain distances and orientations are suitable for cystine cross-linking. The dimensions of the 27-stacks agree well with estimates of the "effective radius" of microfibrils. X-ray measurements of spacing changes during fiber extension are explained as due to alternation of zones with much cross-linking and zones with few cross-links.