Synthesis of [D-Ala2]Leu-enkephalin and [D-Ala2, D-Leu5]Leu-enkephalin with high specific tritiated activity in the leucine residue
[D-Ala2]Leu-enkephalin [DALE] and [D-Ala2, D-Leu5]Leu-enkephalin (DADLE) labelled with tritium in the leucine residue have been prepared. Synthesis of the precursor peptides, [D-Ala2, 4,5-didehydro-L-Leu5]Leu-enkephalin and [D-Ala2, 4,5-didehydro-D-Leu5]Leu enkephalin, was carried out by solid-phase synthesis using Fmoc amino acid derivatives, followed by diastereoisomeric separation on HPLC. These peptides were tritiated catalytically to yield [3H]DALE with a specific activity of 5.35 TBq mmol–1 and [3H]DADLE with that of 5.43 TBq mmol–1, respectively. The distribution of tritium label was investigated by HPLC with a radioisotope detector following acidic hydrolysis, which confirmed that the tritium label in both labelled peptides was exclusively located at the leucine residue.