Conformational Study of Silk-Like Peptides Containing the Calcium-Binding Sequence from Calbindin D_9k Using ¹³C CP/MAS NMR Spectroscopy

The calcium-binding sites of calbindin D_9k have a helix−loop−helix motif. In this study, the helix motifs were replaced by several Ala-Gly repeating regions designed on the basis of the primary sequences of several silk fibroins. The synthesized peptides were treated with several organic solvents to modify the secondary structure of the Ala-Gly repeating regions. The local structures of the Ala-Gly repeating regions, as well as the calcium-binding motif, D_9k-loop (D_9kL), were determined by ¹³C CP/MAS NMR spectroscopy. In the four peptides containing D_9kL synthesized, the poly(Ala) domains retain the ability to undergo a conformational transition from α-helical to β-sheet in (A)₁₂-D_9kL despite the presence of the D_9kL domain at the center of the peptide molecule, but the presence of this domain in the other model peptides synthesized has a marked effect on the conformation of the added silk-like domains. The results showed that the structures of the Ala-Gly repeating regions can be controlled by the choice of both the organic solvent and the amino acid sequence of the Ala-Gly repeating regions without disrupting the secondary structure of D_9kL suggesting that it may retain its ability to bind calcium ions.