TRIGONAL CRYSTAL-STRUCTURE OF BOMBYX-MORI SILK INCORPORATING A THREEFOLD HELICAL CHAIN CONFORMATION FOUND AT THE AIR-WATER-INTERFACE

A new crystalline polymorph of Bombyx mori silk, which forms specifica lly at the air-water interface, has been characterized This new polymo rph has a trigonal crystal structure and is distinctly different from the two previously observed silk crystal structures, silks I and II. O ur identification of this new silk polymorph is based on evidence from transmission electron microscopy and electron diffraction, coupled wi th molecular modeling. Electron diffraction indicates that the crystal structure has a trigonal unit cell. This structure consists of a hexa gonal packing of chains, each of which assumes a three-fold helical co nformation. The resulting crystal structure is found to be similar to that observed for polyglycine II. The sterics of the alanine and serin e residues in the crystallizable segments of silk fibroin strongly fav or a left-handed 3/2 helix over a right-handed 3/1 helix. Electron dif fraction from unoriented samples (powder-type diffraction) provides qu antitative support for a left-handed polyglycine II type of 3/2 helica l conformation for the silk chains in the crystals of this new polymor ph. Single-crystal diffraction patterns and patterns from uniaxially o riented samples are consistent with the proposed crystal structure.