Structural Investigation of Silk I Form Prepared from Crystalline Fraction of Silk Fibroin

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The structure of the silk fibroin crystal, silk I, was studied by using X-ray and electron diffraction methods. Samples used in this investigation were prepared by dialysis of a fraction of B. moli silk fibroin precipitates by Chymotripsin and of the periodic co-polypeptide, poly(L-Ala-Gly).The following results were newly obtained from this project.(1) The precipitates with silk I form were obtained only when the solution was dialized under the condition of pH below 5.0.(2) In contrast to the studies carried out previously, the unit cell dimensions and the molecular conformation of the fractionated silk fibroin were quite similar to those of the model polypeptide. The orthorhombic unit cell with dimensions of a=4.65, b=12.24 and c=8.88A consists of two polymer chains aligned in an antiparallel fashion. Each chain exhibits a 2/1-helical symmetry, in which one helical turn contains two dipeptide units, L-Ala-Gly.