Feather keratin inPavo 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°internal rotation within eight levels of β -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° 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.