The spinning of silk, 1. Molecular weight, subunit structure, and molecular shape of bombyx mori fibroin

Abstract

The molecular structure of Bombyx mori fibroin is still subjected to much controversy; there are discordant reports in the literature on both the value of the molecular weight and the subunit structure of the molecule. The method reported in the present paper allows the molecular weight of Na,CO, extracted fibroin samples to be unambiguously determined, and investigations on the composition and the molecular weight of the peptides arising from the degradation process succeeded in establishing some overall aspects of the structure of the subunits. The main results are: 1) the molecular weight of undegraded fibrion is 4 . 1 6 ; 2) two small subunits are disulfide linked to the main chain; 3) the subunits are located near the terminals of the main chain; 4) in agreement with previous partial literature data, the small subunits are rich in bulky and acid amino acids; our work further shows that they possess a three-block structure, consisting of a central hydrophobic core bearing two strongly acid sequences at the terminals. The mechanism of degradation of the fiber appears to be directed by structural, rather than chemical, factors, and its detailed investigation may represent a valuable method to determine the molecular architecture of the fiber.

Effects of Silk Degumming Process on Physicochemical, Tensile, and Optical Properties of Regenerated Silk Fibroin

“…Increasing degumming time using sodium carbonate on the other hand substantially affected SF integrity. In agreement with previous reports, average molecular weight of SF was reduced, and molecular weight distribution increased with increasing degumming time (Figure A) . Interestingly, incubation in sodium carbonate also resulted in a shift toward less acidic charge variants with increasing degumming time (Figure C), suggesting predominant degradation of the light subunit and/or C‐terminal region of the heavy chain of SF .…”

“…In agreement with previous reports, average molecular weight of SF was reduced, and molecular weight distribution increased with increasing degumming time ( Figure 3A). [11,12,21,27,28] Interestingly, incubation in sodium carbonate also resulted in a shift toward less acidic charge variants with increasing degumming time ( Figure 3C), suggesting predominant degradation of the light subunit and/or C-terminal region of the heavy chain of SF. [29] This conclusion corresponds with the previous finding that beta-sheet content of SF is not significantly affected by degumming time.…”