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

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Abstract
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.