Dynamic Light Scattering of Native Silk Fibroin Solution Extracted from Different Parts of the Middle Division of the Silk Gland of the Bombyx mori Silkworm

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Abstract
Dynamic light scattering (DLS) measurements were performed on aqueous solutions of native silk fibroin extracted from three parts, the posterior (MP), the middle (MM), and the anterior parts (MA), of the middle division (M) of the silk gland of the Bombyxmori silkworm to study the dynamics and aggregation properties of silk fibroin. In the MP part, fibroin molecules are present as aggregates (or clusters) being composed of several large protein complexes or elementary unit (EU), which are further associated to make a large assembly connected via divalent metallic ions. In the MM part, such clusters of EU take more compact structure, and finally in the MA part, clusters disappear, but EUs are more or less aligned to keep the assembly, and the EU takes the conformation of wormlike cylinder capped with hemispheres at both ends. The overall conformational change in solution structure was interpreted as being due to the change in ionic environment in the solution. DLS study was also performed on regenerated silk fibroin solutions, which revealed that fibroin is present as a single molecule dominantly and their association behavior seems completely different from that of native samples and does not depend on types and concentration of added metallic ions.