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 13 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.