Synthesis and conformational study of silk model polypeptides [Ala-Gly] 12 by solid-state NMR

Abstract

A series of well-defined model polypeptides of Bombyx mori silk fibroin, [Ala-Gly] n ( n=12 and 5–9), [Ala d4Gly] 12 and [Gly d2Ala] 12, (where Ala, Gly, Ala d4 and Gly d2 denote l-alanine, glycine, 2,3,3,3-deuterated l-alanine and 2,2-deuterated glycine residues, respectively), were successfully synthesized by an automatic solid phase peptide synthesizer. Followed by treatment with 9 M lithium bromide (LiBr) solution to convert their conformation from the silk II to the silk I form. We determined the intrinsic 1H, 13C and 15N NMR chemical shifts of Ala and Gly residues, characteristic to the silk I and silk II forms in Bombyx mori, by high-resolution and solid-state 13C CP-MAS, 1H CRAMPS, 1H– 13C HETCOR and 15N CP-MAS NMR measurements. Next, we demonstrated that it is necessary at least 12 amino acid residues ( n=6) in the [Ala-Gly] n-I series for the formation of stable silk I form by 13C CP-MAS NMR. Finally, we found that the intrinsic 1H and 13C chemical shifts of the specific deuterium labeled polypeptides, [Ala d4-Gly] 12 and [Gly d2-Ala] 12, cause a meaningful chemical shift displacement, suggesting that the deuterium interacts with proton and/or carbon atoms in the solid state. Thus, the selective deuterium labeling has a potential to useful for the structural analysis by 1H NMR in the solid state.