Solid State NMR Investigation of the Molecular Dynamics of Cocoon Silks Produced by Different Bombyx mori (Lepidoptera) Strains

Cocoons produced by different strains of Bombyx mori larvae were investigated by a combination of several high- and low-resolution ¹H and ¹³C solid-state NMR techniques in order to characterize and compare their dynamic behavior at a molecular level. A detailed interpretation in terms of molecular motions in these very complex systems was possible thanks to the integrated analysis of different relaxation measurements and high-resolution selective experiments. Untreated cocoons of all strains were found to be mainly constituted by two different types of rigid domains and by a third one, more mobile, due to physisorbed water molecules. Dynamic processes in the MHz and kHz ranges were characterized by means of different ¹H and ¹³C relaxation times. Cocoons arising from different strains exhibit a different content of physisorbed water and also slightly different dynamic behavior, especially in the MHz regime.