D. Podwysocka, a† P. Kosson, b AW Lipkowski b and A. Olma a* b-Amino acids containing a, b-hybrid peptides show great potential as peptidomimetics. In this paper, we describe the synthesis and affinity to m-opioid and d-opioid receptors of a, b-hybrids, analogs of the tetrapeptide Tyr-D-Ala-Phe-Phe-NH2 (TAPP). Each amino acid was replaced with an L-or D- b3-h-amino acid. All a, b-hybrids of TAPP analogs were synthesized in solution and tested ...