13C solid-state NMR study of structural heterogeneity in peptides containing both polyalanine and repeated GGA sequences as a local structural model of Nephila clavipes dragline silk (Spidroin 1)

To avoid potential ambiguity in the structural determination of native spider silk, we prepared both a nonlabeled peptide with a sequence containing both the polyalanine and the repeated GGA regions, QGAG(A)(6)GGAGA(GGA)(3)GAGRGGLGG (I), and the C-13-labeled peptides QGAGAAA[1-C-13]A(8)AAGG[2-C-13]A(13)GAGGAG[2-C-13]G(20)[3-C-13]A(21)GGAGAGRGGLGG (Ia) and QGAGAAAAAAGGAGAG-GAG [1-C-13]G(20)[1-C-13]A(21)GGAGAGRGGLGG (II) as a local structural model of spidroin 1 (MaSpl) protein in spider dragline silk of Nephila clavipes. Solvent treatments prior to the NMR measurements induce a structural change of these model peptides and provide a model to reproduce the structure of the silk fiber. Conformation-dependent C-13 NMR chemical shifts were mainly used to determine the local structure, including the evaluation of the fraction of several conformations. The characteristic structure, 65% beta-sheet for the Ala(8) residue in the poly-Ala region, and 70% 3(1)-helix for the Ala(21) residue and mainly 3(1)-helix for the Gly(20) residue in the GG(20)A(21) sequence was observed after peptides Ia and II were dissolved in 9 M LiBr followed by dialysis against water. The 2D spin diffusion 13C solid-state NMR spectrum of the Ala(21) residue of peptide II after this treatment was also reproduced by 70% 3(1)-helix (phi, rho = -90 degrees, 120 degrees) and 30% beta-sheet (phi, rho = -150 degrees, 150 degrees) structure. However, the Ala C-beta peak assigned to the 31-helix in the spectrum of Ia is broad, implying that the torsion angles of the Ala(21) residue are distributed, but with an average that corresponds approximately to the torsion angles of the 31-helix. An increase in the fraction of beta-sheet in both the poly-Ala and GG(20)A(21) regions was observed for la after it was dissolved in formic acid and then dried in air. Moreover, after Ia was dissolved in formic acid and then precipitated in methanol, the spectrum showed a tightly packed beta-sheet structure with a further increase in the fraction of beta-sheet although 15% 3(1)-helix still remained in the GG(20)A(21) region. The beta-sheet structure of the poly-Ala region and both 3(1)-helix and beta-sheet structures in the repeated GGA sequence are in agreement with the structural model for the native spider dragline silk fiber from N. clavipes from a previous NMR study (van Beek, J. D.; et al. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 10266-10271). On the other hand, the a-helical conformation was found to be dominant for the peptide treated with trifluoroacetic acid together with a significant contribution from other structures. The fraction of the other structures was 20-40% depending on the position of the C-13-labeled Ala residue in the chain.