Several insects and spiders produce silk substance that contains a fibrous protein, fibroin. In fibroin, which is considered to be a β-keratin, the polypeptide chains are arranged in anti-parallel β-pleated sheet confirmations because the pleated sheets are loosely bonded to each other (van der Waals force) as shown in Figure 3.18. They slide over each other easily. This arrangement gives silk fibers their flexibility.

Figure 3.18 Structure of Silk Fibroin