THE CRYSTAL-STRUCTURE OF BOMBYX-MORI SILK FIBROIN AT THE AIR-WATER-INTERFACE

A new crystalline polymorph of Bombyx mori silk, which forms al the ai r-water interface, has been characterized A previous study found this structure to be trigonal, and to be distinctly different than the two previously observed silk crystal structures, silk I and silk II. This new structure was named silk III. Identification of this new silk poly morph was based on evidence from transmission electron microscopy and electron diffraction, coupled with molecular modeling. In the current paper, addition! data enables us to refine our model of the silk III s tructure. Some single crystal electron diffraction patterns indicate a deviation in symmetry away from a perfect trigonal unit cell to monoc linic unit cell. The detailed shape of the powder diffraction peaks al so supports a,monoclinic cell. The monoclinic crystal structure has an nonprimitive unit cell incorporating a slightly distorted hexagonal p acking of silk molecular helices. The chains each assume a threefold h elical conformation, resulting in a crystal structure similar to that observed for polyglycine II, but with some additional sheetlike packin g features common to the threefold helical crystalline forms of many g lycine-rich polypeptides. (C) 1997 John Wiley & Sons, Inc.