Conformational changes induced in Bombyx mori silk fibroin by cyclodextrin inclusion complexation

We have previously demonstrated that the formation of and coalescence from polymercyclodextrin (CD) inclusion compounds (ICs) represents a very useful approach to modify the chain conformations and improve the crystallinity of various bulk polymers. The present work deals, for the first time, with the formation of a gamma-CD IC with a natural protein as guest, i.e., silk fibroin from Bombyx mori silkworm. Formation of the crystalline inclusion compound was verified by wide-angle X-ray diffraction, solid-state NMR, and infrared spectroscopy to have the host gamma-CD molecules arranged in a channel structure, with the isolated silk chains included, at least in large part, in their internal cavities. Removing the gamma-CD host lattice by washing with hot water produced a white coalesced silk sample that was collected and characterized. Unlike the original or precipitated silk fibroin, the coalesced sample shows most of its protein residues in a beta-sheet conformation with an elevated degree of crystallinity.