Solution Phase Peptide Synthesis: The Case of Biphalin.

Solution phase synthesis was the first developed and the only method for peptide synthesis until the solid phase peptide synthesis (SPPS) introduced by Merrifield revolutionized the way peptides and their analogues are prepared nowadays. However, some peptides because of their chemical structure cannot be synthetized by SPPS, and the "old school" technique is still favorable to make them. Biphalin is a good example. It was first synthesized by Lipkowski almost 40 years ago as a dimeric analogue of enkephalin in which two tetra-amino acid fragments (Tyr-D-Ala-Gly-Phe-) are joined tail to tail by a hydrazide bridge. The synthesis of this octapeptide (Tyr-D-Ala-Gly-Phe-NH-NH ← Phe ← Gly ← D-Ala ← Tyr) and its analogues requires synthesis in solution because routine synthesis on a polymeric support is not possible. Biphalin shows high affinity at both μ and δ opioid receptors and produces a more robust spinal analgesia than morphine after intrathecal administration. Although biphalin and its analogues have been already deeply investigated, a complete description for its analgesic activity is not yet available.Here, we present a detailed procedure for the solution phase synthesis of biphalin.