Probing opioid receptor–ligand interactions by employment of indolizidin-9-one amino acid as a constrained Gly[sup 2]-Gly[sup 3] surrogate in a leucine-enkephalin mimic: Authors' affiliations:. - Gosselin, F.; Lubell, W.D.; Tourwé, D.; Ceusters, M.; Meert, T.; Heylen, L.; Jurzak, M. (Journal of Peptide Research, 2001)