The synthesis of the neuropeptide Met-enkephalin and two metabolic fragments labelled with 11C in the methionine methyl group.

Starting from the corresponding N-benzyloxycarbonyl S-benzyl homocysteine peptide benzyl esters, Met-enkephalin and two metabolites, Gly-Phe-Met and Phe-Met, have been labelled with 11C for application in positron emission tomography in vivo. All labelling experiments were accomplished in high radiochemical yields within 30-40 min from start of the [11C]methyl iodide synthesis. Alkylations with this reagent were performed in liquid ammonia, using sodium to generate the free peptides with their reactive sulphide anions, essentially as previously described for [methyl-11C]methionine. The products were purified by liquid chromatography (LC) to a radiochemical purity of 98% or better.