Hexagonal Packing of Intermediate Filaments (Microfibrils) in α-Keratin Fibers

Electron micrographs of cross sections of highly oriented α-keratin fibers show the presence of intermediate filaments (IF) (microfibrils) packed parallel to each other in hexagonal arrays. Because of the presence of this ordered packing, some form of in terfibrillar linkage or interaction is believed to exist between neighboring IFs in the six hexagonal directions. The possible presence of such linkages has already been sug gested from the mechanical behavior of wool fibers in formic-acid-water solutions up to concentrations of 75% v/v. In water, the IFs are arrayed parallel to each other and . to the axis of the wool fiber. In the formic acid solution, the fibers swell considerably radially, with a smaller longitudinal contraction occurring up to a concentration of 75% formic acid in water. The contraction is a result of the bending or kinking of the IFs to accommodate the diametral swelling of the fiber with fixed interfibrillar linkages. Geometric calculations based on the presence of these linkages in the hexagonal di rections yield a spacing of 46 to 51 nm in the direction of the IFs for linkages between any two neighboring IFs. This result agrees with the observed meridional x-ray dif fraction spacing of 47.0 nm for α-keratin fibers. An IF model is proposed with potential interaction sites for the hexagonal directions. The model is based on the presence of eight tetramers of helices and "tails" in the outer ring of the IF, with the linkage sites in the high sulphur tails. The model concurs that the interfibrillar linkages, which are present in the matrix, are the source of the meridional repeat of 47 nm. This repeat was modified in x-ray diffraction data for human hair taken from diabetic patients, placing the modification of the hair in the labile matrix structure where the linkages exist.