- [3H][D-Ser2(O-tert-butyl),Leu5]enkephalyl-Thr6 and [D-Ser2(O-tert-butyl),Leu5]enkephalyl-Thr6(O-tert-butyl). Two new enkephalin analogs with both a good selectivity and a high affinity toward delta-opioid binding sites. Delay-Goyet, P., Seguin, C., Gacel, G., Roques, B.P. J. Biol. Chem. (1988)