Three-Layered Sericins around the Silk Fibroin Fiber from <i>Bombyx mori</i> Cocoon and their Amino Acid Composition

The sericin coated around silk fibroin fiber is a natural protein-based polymer and presents a layered structure. According to the solubility in water under different temperatures and pressures, the sericin is divided into three parts of outer, middle and inner sericin layers accounting for 15%, 10.5% and 4.5% to the total silk protein (included sericin layers and fibroin fiber) in terms of the mass respectively. The partition within the sericin layer presents the relative proportions of 50%, 35% and 15% from the outer to inner layer. Furthermore, the differences of three layers in the amino acid composition are very significant. The non-polar amino acids in the outer, middle and inner sericins are gradually increased, in particular for alanine, which are accounted for 5.20, 6.13 and 11.58 mol% respectively; while its content jumps to 33.38 mol% in the silk fibroin fiber. Nevertheless, the polar amino acids, especially the neutral ones, are gradually decreased accounting for 39.34, 38.62 and 23.82 mol% respectively. Concentration of Serine drops most prominently, i.e. 28.00, 25.57 and 13.32 mol% respectively; while its content goes to 7.65 mol% in the silk fibroin fiber. On the other hand, the hydrophobic amino acids gradually increase and the hydrophilic amino acids go to oppsite way. These results indicate that the amino acid compositions of the outer and middle sericins are similar to each other, but those of the inner sericin are different from the others. The closer the sericin layer is to the silk fibroin fiber, the closer its amino acid composition is to that of the fibroin fiber. These obtained results are of important reference value for processing of silk sericin peptides and their applications in cosmetics, cell culture, healthy food and other areas.