ABSTRACT
This chapter introduces the several strategies for attaining a specific peptide ligand. The modification at Gly 2 or Gly 3 is especially interesting, because two successive glycine residues in Tyr-Gly-Gly-Phe-sequence of endogenous opioid peptides allow many low-energy conformations, which should result in binding to different receptor sites such as μ-and δ-receptors. The weak receptor selectivity of the endogenous short linear peptides can be ascribed to the molecular flexibility, which allows their easy adaptation to the various receptor types. Gacel et al. The number of available conformations of peptides can be enormously reduced by cyclization. Indeed, cyclic peptides have been intensively utilized for conformational analyses and construction of functional peptides such as enzyme and ionophore models. The molecular organization of peptide hormones has been classified by Schwyzer into two types: sychnologic and rhegnylogic.
