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

用X射线衍射和电子衍射方法研究了丝素蛋白晶体Silk I的结构。本研究中使用的样品通过透析B级分制备。糜蛋白酶和周期性共多肽poly（L-Ala-Gly）对桑蚕丝素蛋白进行沉淀。(1)只有在pH值低于5.0的条件下透析，才能得到丝素Ⅰ型沉淀。(2)与先前进行的研究相反，分级丝素蛋白的晶胞尺寸和分子构象与模型多肽的那些非常相似。正交晶胞的尺寸为a=4.65，B=12.24和c= 8.88 A，由两个聚合物链以反平行方式排列组成。每条链呈现2/1-螺旋对称性，其中一个螺旋圈含有两个二肽单元，L-Ala-Gly。

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.