Circular dichroism (CD) studies of antagonists derived from parathyroid hormone‐related protein

We have undertaken a study of the structure of antagonist peptides derived from the parathyroid hormone‐related protein (PTHrP) in the presence of amphiphiles using circular dichroism (CD). The results were used to gain knowledge about bioactive conformations of the peptide when bound to a membrane. The substitutions within the PTHrP‐(7‐34)amide sequence resulted in differences in biological activity. Structural determination by CD showed the presence of an α‐helical structure. The antagonist activity was increased in constrained peptides in which i to (i+ 4) side‐chain to side‐chain cyclization was used to form a lactam,[Lys¹³,Asp¹⁷]PTHrP‐(7‐34)NH₂. This peptide showed increased helicity in the presence of a surfactant. Hydrophobic substitutions Leu and d‐Trp at positions 11 (Lys) and 12 (Gly), respectively, in PTHrP‐(7‐34)NH₂ resulted in increased potency, but the derivatives were not significantly more helical than the un‐substituted peptide in the presence of surfactants. The combination of the hydrophobic substitutions with the constraint of lactam formation were mutually exclusive in terms of their biological activity and their α‐helical content. We conclude that hydrophobic substitutions contribute to an increase in binding affinity by increasing hydrophobic interactions which stabilize receptor‐ligand complexes. Structural rigidification, on the other hand, increases the α‐helical content, which is important for attaining a conformation recognized by the receptor.