Fibroin

Insoluble, white, fibrous protein comprising the filaments of raw silk, secreted along with sericin by the insect. Unlike wool, silk is composed mainly of non-sulfur containing amino acids. The primary amino acids, glycine, alanine and serine, are contained in dense, highly crystalline regions of the fiber. The amorphous regions are composed of tryosine, tryptophane, histidine and phenylalanine.