Synthesis of [D-Ala2]Leu-enkephalin and [D-Ala2, D-Leu5]Leu-enkephalin with high specific tritiated activity in the leucine residue

Abstract

[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.