Unusual Dynamics of Alanine Residues in Polyalanine Regions with Staggered Packing Structure of Samia cynthia ricini Silk Fiber in Dry and Hydrated States Studied by 13C Solid-State NMR and Molecular Dynamics Simulation.

Recently, the wild silkworm and spider dragline silks have been paid considerable attention as potentially valuable biomedical materials. Samia cynthia ricini is one of the wild silkworms and the primary structure of the silk fibroin (SF) consists of tandemly repeated polyalanine (poly-A:(A)12,13). Here, we report the unusual dynamical character observed in Ala Cβ groups in the poly-A region which forms an antiparallel-β-sheet structure with a staggered packing arrangement. The 13C spin-lattice relaxation ( T1's) and spin-spin relaxation times ( T2's) of Ala Cβ peaks in S. c. ricini SF fibers were observed in dry and hydrated states. The lowest field peak in Ala Cβ of the poly-A region showed 2 times longer T1 value and shorter correlation time than the other Ala Cβ peaks of the staggered packing structure, suggesting unusually fast hopping in methyl groups. Molecular dynamics simulations indicated that two of the Ala Cβ carbons out of eight existing in the unit cell of the staggered packing structure exhibited the fastest hopping motion in spite of the shortest Cβ-Cβ distance, indicating a geared hopping motion. T2 values of the hydrated and dry Ala Cβ peaks showed a similar value, indicating that the backbone motion of S. c. ricini SF fiber is not significantly affected by hydration.