Lamellar Structure in Alanine-Glycine Copolypeptides Studied by Solid-State NMR Spectroscopy: A Model for the Crystalline Domain of Bombyx mori Silk Fibroin in Silk II Form.

Bombyx mori silk fibroin (SF) fibers with excellent mechanical properties have attracted widespread attention as new biomaterials. However, the structural details are still not conclusive. Here, we propose a lamellar structure for the crystalline domain of the SF fiber based on structural analyses of the Ala Cß peaks in the 13C cross-polarization/magic angle spinning NMR spectra of (Ala-Gly)m (m = 9, 12, 15, and 25) and 13C selectively labeled (Ala-Gly)15 model peptides. Namely, three Ala Cß peaks with relative intensities of 1:2:1 obtained by deconvolution were assigned to two kinds of ß-sheet and a ß-turn, which are interpreted as a lamellar structure formed by repetitive folding using ß-turns every eighth amino acid, for which the basic structure is (Ala-Gly)4 in an antipolar arrangement. The dynamics and intermolecular arrangement were further studied using 13C solid-state spin-lattice relaxation time observations and the rotational echo double resonance experiments, respectively.

Determination of the torsion angles of alanine and glycine residues of model compounds of spider silk (AGG)(10) using solid-state NMR methods.

Spiders synthesize several kinds of silk fibers. In the primary structure of spider silk, one of the major ampullate (dragline, frame) silks, spidroin 1, and flagelliform silk (core fibers of adhesive spiral), there are common repeated X-Gly-Gly (X = Ala, Leu, Pro, Tyr, Glu, and Arg) sequences, which are considered to be related to the elastic character of these fibers. In this paper, two dimensional spin diffusion solid-state NMR under off magic angle spinning (OMAS), (13)C chemical shift contour plots, and Rotational Echo DOuble Resonance (REDOR) were applied to determine the torsion angles of one Ala and two kinds of Gly residues in the Ala-Gly-Gly sequence of (13)C=O isotope-labeled (Ala-Gly-Gly)(10). The torsion angles were determined to be (phi, psi) = (-90 degrees, 150 degrees ) within an experimental error of +/-10 degrees for each residue. This conformation is characterized as 3(1) helix which is in agreement with the structure proposed from the X-ray powder diffraction pattern of poly(Ala-Gly-Gly). The 3(1) helix of (Ala-Gly-Gly)(10) does not change by formic acid treatment although (Ala-Gly)(15) easily changes from the silk I conformation (the structure of Bombyx mori silk fibroin before spinning in the solid state) to silk II conformation (the structure of the silk fiber after spinning) by such treatment. Thus, the 3(1) helix conformation of (Ala-Gly-Gly)(10) is considered very stable. Furthermore, the torsion angles of the 16th Leu residue of (Leu-Gly-Gly)(10) were also determined as (phi, psi) = (-90 degrees, 150 degrees ) and this peptide is also considered to take 3(1) helix conformation.