Secondary structures in β-casein peptide 1–42: A two dimensional nuclear magnetic resonance study

Two dimensional NMR spectroscopy was used to study the structure of a peptide composed of the N-terminal 42 amino acid residues of beta-casein. The peptide was obtained by enzymic cleavage using endoproteinase Asp-N. Complete sequence-specific ¹H NMR assignment was performed for the peptide at three Ca²⁺ concentrations (0, 22 and 37 mM). The NMR results show that the peptide was highly flexible and adopted multiple conformations. No stable secondary structures were present; however, the peptide had some regions with non-random structure. The region between residues Leu¹⁶ and Asn²⁷ adopted conformations with an increased contribution of alpha-helical structure, a so-called nascent helix. Two regions, Glu¹¹-SerP¹⁵ and Lys²⁹-Phe³³ showed an increased population of conformations with extended structures. Addition of Ca²⁺ induced chemical shift changes for the backbone amide protons, especially around the phosphoserine region and around the suggested alpha-helical structure, indicating that the addition of Ca²⁺ stabilized the structure already present in the apo form of the peptide.