Design, Synthesis, Biology, and Conformations of Bicyclic α-Melanotropin Analogues

Seven side chain-constrained bicyclic α-melanotropin (α-MSH) analogues were designed and synthesized, their conformations analyzed, and their biological properties examined in the frog skin and lizard skin bioassays. The structure of these analogues is based on the central sequence [formula omitted] (Xaa⁵ = Asp or Glu) and has been extended on the N-terminal with the amino acids Ser¹-Tyr²-Ser³ and on the C-terminal with Pro¹²-Val¹³ to more closely resemble the native hormone α-MSH. The analogue Ac-Cys⁴-[formula omitted] also was synthesized, and its conformational and biological properties were examined. Design of these analogues was based upon the previously identified superpotent monocyclic peptides [formula omitted] and [formula omitted] with the rationale of increasing conformational constraints to restrict the available backbone conformations as a means to identify the conformations that facilitate biological activity. Computer-assisted conformational analysis of the central tetrapeptide residues 6-9 identified β-turns which varied with respect to the residue in the i + 1 position. Each highly constrained peptide contains D-Phe⁷ and a 23-membered ring which has previously been identified as crucial to produce prolonged acting peptides with superagonistic activities. The bicyclic peptides reported in this study are full agonists and are 25-400-fold less potent than α-MSH in the frog and lizard skin bioassays.