Structures of the glycine-rich diastereomeric peptides bombinin H2 and H4

Skin secretions of the European frog Bombina variegata contain a family of hydrophobic peptides, called bombinins H, which probably play a role in the defense against microbes. These peptides are rich in glycine (25%), which may allow structural polymorphism. Indeed, there is increasing evidence that bombinin H can, dependent on the environment, adopt different conformations. Moreover, some of these peptides contain a d-amino acid; the bombinins H2 and H4 differ from each other in that they contain either L-Ile or D-allo-Ile at position 2.In this paper we report the solution structure obtained by using NMR techniques of the mainly helical conformation, which these peptides adopt upon binding to the bilayer mimetics SDS and DPC. A glycine ridge is exposed at one side of the helix and may provide a helix-helix interaction site. In this respect, the structure of bombinin H resembles the influenza hemagglutinin fusion peptide and the helical conformer of Alzheimer peptide Aβ(25-40). Neither structure nor orientation of bombinin H are affected by the chiral inversion.Environmental conditions can trigger self-aggregation of bombinin H in solution due to hydrophobic interaction. Under these conditions the stereochemistry of the randomly ordered N-terminal segment modulates the preference to fold into a particular conformation.