The natural silk spinning process

The spinning mechanism of natural silk has been an open issue. In this study, both the conformation transition from random coil to β sheet and the β sheet aggregation growth of silk fibroin are identified in the B. mori regenerated silk fibroin aqueous solution by circular dichroism (CD) spectroscopy. A nucleation‐dependent aggregation mechanism, similar to that found in prion protein, amyloid β (Aβ) protein, and α‐synuclein protein with the conformation transition from a soluble protein to a neurotoxic, insoluble β sheet containing aggregate, is a novel suggestion for the silk spinning process. We present evidence that two steps are involved in this mechanism: (a) nucleation, a rate‐limiting step involving the conversion of the soluble random coil to insoluble β sheet and subsequently a series of thermodynamically unfavorable association of β sheet unit, i.e. the formation of a nucleus or seed; (b) once the nucleus forms, further growth of the β sheet unit becomes thermodynamically favorable, resulting a rapid extension of β sheet aggregation. The aggregation growth follows a first order kinetic process with respect to the random coil fibroin concentration. The increase of temperature accelerates the β sheet aggregation growth if the β sheet seed is introduced into the random coil fibroin solution. This work enhances our understanding of the natural silk spinning process in vivo.