CRYSTAL-STRUCTURE OF SILK OF BOMBYX-MORI

The crystal structure of silk fibroin (Bombyx mori) reexamined by using newly collected intensity data. molecules pass through a rectangular unit cell with parameters, a = 9.38 A, b = 9.49 A and c (fiber axis) = 6.98 A, and the space group P2(1)-C-2(2). The sheet formed by hydrogen bond is arranged parallel to the ac-plane. The molecular conformation is essentially the same pleated-sheet conformation as Marsh, Corey, and Pauling (1) proposed, but the sheet formed by hydrogen bonds assumes the antipolar-antiparallel structure in which the methyl groups of the alanine residues alternately point to both sides of the sheet along the hydrogen bonding direction, differing from the polar-antiparallel structure proposed by Marsh, Corey, and Pauling in which the methyl groups are all on the one side of the sheet. In the crystal structure of silk, two antipolar-antiparallel sheets with different orientations statistically occupied a crystal site with different probabilities. The crystalline region of silk is composed of rather irregular stacking of the antipolar-antiparallel sheets with different orientations.