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

Abstract

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)). 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 C spin-lattice relaxation ( T's) and spin-spin relaxation times ( T'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 T 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. T 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.

Structure and Dynamics of Spider Silk Studied with Solid-State Nuclear Magnetic Resonance and Molecular Dynamics Simulation

“…The origin of these long T 1 values in the staggered packing structure was already studied from the 13 C solid-state NMR relaxation work of S. c. ricini silk fibroin fiber as shown in Figure 16 [110][111][112][113]. Especially, the MD simulation was performed to interpret the relationship between the packing structure and the dynamical behavior of these Ala Cβ groups in the poly-A region of S. c. ricini silk fibers [110].…”

“…Figure16. The packing structure and the dynamical behavior of these Ala Cβ groups in the poly-A region of S. c. ricini silk fibers studied by the MD simulation[110]. 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[110].…”

Chain-folded lamellar structure and dynamics of the crystalline fraction of Bombyx mori silk fibroin and of (Ala-Gly-Ser-Gly-Ala-Gly)n model peptides

“…Moreover, the dynamics of the Ala C carbons in the different environments of the Cp fraction and their model peptides, [3- 13 C]Ala-(AGSGAG)5 were studied in detail by the determination of 13 C solid-state spin-lattice relaxation time (T1) values for the three components of the Ala 13 Cpeaks [39][40][41].…”

Dynamics of Alanine Methyl Groups in Alanine Oligopeptides and Spider Dragline Silks with Different Packing Structures As Studied by 13C Solid-State NMR Relaxation

“… a Reference . b The error ranges are obtained as standard deviation of the relaxation times in the fitting processes.The assignment of each peak is shown in the text. …”

“…This is based on the fact that the methyl hopping motion is well characterized and dominates the 13 C spin–lattice relaxation mechanism of Ala Cβ carbons of poly-A region in the τ c values of 10 –9 –10 –11 s. ,, We also observed 13 C T 1 values of Ala Cβ peaks in the 13 C CP/MAS NMR spectra of S. c. ricini SF fibers to clarify the dynamical behavior of Ala residues in the dry and hydrated states. Unusual dynamic character was observed in Ala Cβ carbon in the poly-A region of silk fibroin fiber of S. c. ricini in staggered packing structure from 13 C solid-state T 1 observations …”