Feather keratin in Pavo cristatus: A tentative structure
The filament of the F-keratin polymer is an alternating arrangement of two tetrameric sequence segments, the "N-block" made of four strands AA 1-52, a twisted parallelepiped and the "C-block", a sandwich of four strands AA 81-100. The N-blocks have 89{degrees}internal rotation within eight levels of {beta}-sandwiches strengthened by three disulfide bonds per monomer. The C-blocks contain 5 aromatic residues, they provide resilience, like vertebral discs in a spinal column. The pitch of an N+C-block octamer is 10 nm. Solidification of F-keratin may involve the "C-blocks" to temporarily mold into "C-wedges" of 18{degrees} tilt, which align the polymer filaments into laterally amorphous fiber-reinforced composites of 9.5 nm axial periodicity. This distance corresponds to the length of the fully stretched AA 53-80 matrix segment. The "spinal column" is deformed like in scoliosis and unwinds under compression when F-keratin filaments perfectly align horizontally and form stacked sheets in the solid state.