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 C-13 Solid-State NMR and Molecular Dynamics Simulation

A Naito and Y Tasei and A Nishimura and T Asakura, JOURNAL OF PHYSICAL CHEMISTRY B, 122, 6511-6520 (2018).

DOI: 10.1021/acs.jpcb.8b03509

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 C beta groups in the poly-A region which forms an antiparallel-beta-sheet structure with a staggered packing arrangement. The C-13 spin-lattice relaxation (T-1's) and spin-spin relaxation times (T-2's) of Ala C beta peaks in S. c. ricini SF fibers were observed in dry and hydrated states. The lowest field peak in Ala C beta of the poly-A region showed 2 times longer T-1 value and shorter correlation time than the other Ala C beta peaks of the staggered packing structure, suggesting unusually fast hopping in methyl groups. Molecular dynamics simulations indicated that two of the Ala C beta carbons out of eight existing in the unit cell of the staggered packing structure exhibited the fastest hopping motion in spite of the shortest C beta-C beta distance, indicating a geared hopping motion. T-2 values of the hydrated and dry Ala C beta peaks showed a similar value, indicating that the backbone motion of S. c. ricini SF fiber is not significantly affected by hydration.

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