Synthesis of Tripeptides Containing d‑Trp Substituted at the Indole Ring, Assessment of Opioid Receptor Binding and in Vivo Central Antinociception

The noncationizable tripeptide Ac-d-Trp-Phe-GlyNH₂ was recently proposed as a novel minimal recognition motif for μ-opioid receptor. The introduction of different substituents (methyl, halogens, nitro, etc.) at the indole of d-Trp significantly influenced receptor affinities and resulted in serum stability and in a measurable effect on central antinociception in mice after ip administration.