Metallopeptides and metallopeptide combinatorial libraries specific for opioids receptors are provided, for use in biological, pharmaceutical and related applications. The metallopeptides and combinatorial libraries are made of peptides, peptidomimetics and peptide-like constructs, in which the peptide, peptidomimetic or construct is conformationally fixed in a biologically active configuration specific for one or more opioid receptors on complexation of a metal ion-binding portion thereof with a metal ion.