Molecular weight and secondary structure change in eri silk during alkali degumming and powdering

Abstract

Changes in molecular weight and secondary structure of eri silk during alkali degumming and silk powdering were studied. An increase in silk degumming intensity, through increased alkali concentration, treatment temperature, and time, reduced the fibroin molecular weight and, therefore, the fiber tenacity, but at the same time, increased the b-sheet fraction. These changes reduced the time required to mill the degummed silk fibers into fine powders. Mechanical forces used in wet attritor and air jet milling disturbed intermolecular packing along the direction of side chains, but the conformation remained essentially bsheet even in the sub-micron silk particles. Dry milling drastically reduced molecular weight and changed the conformation of the fibroin chains. The rate of the spontaneous conformation transition in regenerated fibroin solution prepared from fibers and powders increased with a decrease in fibroin molecular weight, affecting the time fibroin solutions could be stored before gelling. Overall, the study showed that molecular weight and secondary structure of silk powders could be manipulated by suitably changing the degumming and milling conditions. It also suggests that wet media milling and air jet milling are better than dry media milling to prepare less degraded and more crystalline ultrafine silk particles.

Milled non-mulberry silk fibroin microparticles as biomaterial for biomedical applications

“…Also, the reformation of original microstructure of fiber is difficult in regenerated silk biomaterials and therefore they are unlikely to be stable like native fibers. For example, the degradation of regenerated silk is faster than native silk fibers and also their mechanical and thermal properties are inferior [45][46][47]. Furthermore, no chemical agents are used in our top-down strategy other than alkali utilized during the degumming process.…”

Relationship between processing, surface energy and bulk properties of ultrafine silk particles

“…Low surface area of the ID-AM particles is likely to have resulted from the increase in density after intensive degumming. Intensive degumming causes substantial reduction in the amorphous content and as a result fibres become more crystalline [13]. We found that intensive alkaline treatment during degumming resulted in 37% weight loss, whereas only 15% weight reduction was found due to the removal of sericin (gummy protein in silk) during normal degumming of eri silk.…”

“…ID-AM particles are more crystalline due to hydrolysis and loss of amorphous domains and therefore are expected to have higher true density [13]. ID-AM particles are also less porous as shown in Fig.…”

“…However, as viscoelastic silk fibres are difficult to mill into fine particles, pre-treatments such as chemical hydrolysis, exposure to thermal or radiation energy are often needed to reduce fibre strength and impart brittleness to facilitate milling. In contrast, we have used a combined wet milling/spray drying approach and demonstrated that ultrafine silk particles could be produced without pre-treatments and the particles retained much of the original composition and structures of parent fibres [12,13].…”

Sustainable Processing of Luxury Textiles

“…Various enzymes also have been investigated for degumming eri, muga and tussah silk fibers [24]. It was found that alkaline degumming caused molecular weight and secondary structure change in eri silk [25]. Sarma et al [26] investigated a biosurfactant (reetha) for the degumming of muga silk, which provided acceptable levels of degumming.…”