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

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Abstract
Two dimensional NMR spectroscopy was used to study the structure of a peptide composed of the N-terminal 42 amino acid residues of β-casein. The peptide was obtained by enzymic cleavage using endoproteinase Asp-N. Complete sequence-specific1H 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 Leu16and Asn27adopted conformations with an increased contribution of α-helical structure, a so-called nascent helix. Two regions, Glu11-SerP15and Lys29-Phe33showed 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 α-helical structure, indicating that the addition of Ca2+stabilized the structure already present in the apo form of the peptide.