Overview of Silk Sericin

Sericin is a translucent film wrapped on silk fibroin, which has a certain protective effect on silk fibroin, swelling to water, and soluble in hot water.

The utility and distribution of sericin

Sericin-based globular proteins usually have a protective effect against heat, light, enzymes, acids, alkalis and oxidation that damage silk fibroin. When the silkworm is spinning, sericin can bond two monofilaments to form a cocoon silk; as the spinning is arranged crosswise, it can adhere to the cocoon silk to form a cocoon layer; when reeling, sericin can also help the cocoon silk to hold the raw silk. Generally, the outer layer of cocoons contains more sericin, while the inner layer contains less sericin. European silkworm varieties contain more sericin, while Japanese silkworm varieties have less sericin. The content of yellow cocoon sericin is high, while the content of white cocoon sericin is low.

Types of sericin

Due to the complexity of the molecular structure of sericin protein and its variability, people have different opinions on whether sericin is a single substance or composed of several substances. There are also many different types of sericin. For example, according to the sequence of secretion and storage of sericin in the front, middle, and rear regions of the natural division of the central silk gland in the silkworm body, it can be generally divided into three layers in the order of covering the silk fibroin after spinning. According to the difficulty of swelling and dissolving water of sericin, it can be divided into easily soluble A sericin and insoluble B sericin. In 1975, Keichi Komatsu, according to the three turning points in the dissolution curve of cocoon layer sericin, can be divided into 4 types of sericin and 4 layers of distribution. In 1984, Wang Tianyu et al. used γ-rays to cross-cut and stratify the cocoon layer sericin. Scanning observation with electron microscope showed that the natural state of sericin was three-layer distribution. At present, many people hold the view that sericin is composed of 3 or 4 proteins.

The molecular weight of sericin

The gold hero is measured based on the diffusion constant. When the concentration is 0.3%, the molecular weight is about 15,000, and when it is 0.5 to 1.0%, it is 35,000 to 40,000; Lin Shengzai et al. measured it and the molecular weight is 1.6×104 to 3.09×105.

This shows the complexity of sericin, but in a broad sense: the molecular weight is nothing more than ten thousand and one hundred thousand. It is generally believed that around 100,000 to 120,000, 3 to 4 subunits are connected by weak interactions (hydrogen bonds).

The layered structure and molecular morphology of sericin

Sericin was layered. In 1922, Yamanoguchi took a fixed section of the central silk gland and stained it into 3 layers. Jiro Machida (1926) and Haruo Oba (1957) support this statement. Shimizu Zhengde (1941) used hot water to dissolve the silk and X-ray diffraction to divide the sericin into 3 layers (species), the content of which is roughly 2:2:1, which is in line with the natural state of the flexion zone 3 of the central silk gland system.

The three layers of sericin are from the outside to the inside. Sericin I is in an amorphous state, sericin II has crystalline parts, and sericin III contains more crystalline parts, but is arranged in disorder. The molecular structure of sericin is mainly random coils accounting for about 2/3, of which about 10% are alpha helices, followed by 1/3 of beta structures. In the layered molecular morphology, roughly the first layer of sericin in the outer periphery is mainly random coils and contains a small amount of α helices, and the second layer of sericin in the middle has a slightly mixed β structure except for the random coils. To the third layer of sericin at the inner edge, the β structure ratio increases. At the same time, the β structure is the crystalline part of sericin.

Due to the influence of moisture absorption and heating, sericin stretches the curled random coils into a compact β-shaped structure, the water solubility gradually decreases, and the crystallinity of sericin gradually increases.

Under the condition that the denaturation of sericin has the participation of moisture, that is, when the relative humidity is >50%, the transition condition from random coil to β-type is satisfied.

The physical and chemical properties of sericin

The gloss is weak. The specific gravity is 1.2～1.4 g/cm3. The specific heat is 0.389～0.394 cal/g·degree. The maximum suction volume of 1 gram of sericin is about 2.17 ml. The isoelectric point pH is 3.8～4.5. Viscosity: 1.771 centipoise for unwinding, 1.126 centipoise for unwinding, and 1.091 centipoise for poorly unwinding. Gelation temperature: 43.2℃ at 0.8% concentration, 55.6℃ at 1.02% concentration.