Overview of Silk Fibroin

Silk fibroin is a water-insoluble fibrin forming protein that constitutes the main body of the filament. The various amino acids of silk fibroin are connected by peptide bonds to form peptide chains, and then the peptide chains constitute proteins. Silk fibroin is the main body of cocoon silk. After raw silk or silk fabric is refined to remove most of the sericin, the remaining part is mainly silk fibroin. Its properties directly determine the performance of silk fabrics.

The molecular weight of silk fibroin

The molecular weight of silk fibroin varies greatly, from 10×10 ³ to 1,000×10 ³ , mainly due to the different conditions of degumming and hydrolysis. According to Sasaki Zhouyu (1974), it was determined by the sedimentation method that the large subunit was 280,000 and the small subunit was 26,000. Kensuke Shimura (1980) used ammonium sulfate for precipitation, the large subunit was 67,000, the middle subunit was 45,000, and the small subunit was 25,000. It is generally believed that the molecular weight of large subunits is about 280,000 to 350,000, and there may be several types of small subunits, roughly 20,000 to 50,000.

The spatial configuration of silk fibroin molecules

There are type I (α type) and type II (β type). Type Ⅰ is the unstable type of random coils, and type Ⅱ is the stable type of folded sheet. According to experimental data, liquid silk fibroin crystallizes into type I at 0-40°C, and becomes type II above 50°C, depending on the initial concentration between 40°C and 50°C. Below 0℃～-20℃, it becomes type II, below -20℃ it becomes type I. In addition, dry heat treatment is often type II, and type I and type II coexist during wet heat treatment. For the amorphous α-crystalline silk fibroin film, it will crystallize into type I when it is stretched 1.5 times, and it will become type II when it exceeds 4 times.

Crystalline and amorphous regions of silk fibroin

If the microfibers are given a longitudinal section, it is generally believed that there are two kinds of regions, one is the crystalline region where the peptide chains are arranged more neatly and densely; the other is the amorphous region where the peptide chains are arranged more irregularly and loosely. The two are alternately distributed. Each crystalline region or non-crystalline region will have several peptide chain segments, and each peptide chain must pass through several crystalline regions and non-crystalline regions. The crystal region consists of amino acids with small side chains, tyrosine, alanine, and serine residues, roughly composed of 3:2:1, sometimes including tyrosine, arranged in a compact, neat, and orderly structure. The amorphous region is composed of all 18 kinds of amino acids, including amino acids with large side chains, with a large number of active groups, which aggregate into a loose, disordered, and disordered structure. It plays a major role in the excellent softness, moisture absorption and dyeability of silk. The two are about 40-45: 60-55.

The chemical structure of silk fibroin

The primary structure of silk fibroin is the sequence of amino acids. Strydom D J. et al. (1977) proposed that the molecular segment of the crystal region is:

G—A—G—A—G—S—A—A—G—S—G—(A—G)n] 8—T n=2

Regarding the amorphous region, Lucas (F. Lucas1962, 1969) separated 3 peptides in 40% of the amorphous part, namely

G—A—G—A—G—A—G—T G—Tyrosine

G—A—G—V—G—A—G—T A—Alanine

G—A—G—T S—Serine

T-tyrosine

V—Valine

There are four components in the amorphous region, which account for 32%, 4%, 46%, and 17% respectively. Small subunits including 18 amino acids.

The physical and chemical properties of silk fibroin

Full of luster. Specific gravity of 1.25 g / cm ³ . The specific heat is 0.276～0.291 cal/g·degree, and the isoelectric point is pH 2～3. The expansion degree is 1.160～1.694%. With blue-violet fluorescence. When organic acids increase the gloss, the more the metal valence of the salt, the greater the absorption of silk fibroin. Ultraviolet rays have a serious embrittlement effect on silk fibroin, and it has a protective effect after absorbing tannin. Basic dyes are easy to dye during dyeing.