Synthesis of two S-(methyl-3H)-labelled enkephalins and S-(methyl-14C)substance P

The synthesis of 3H‐labelled Met‐enkephalin and Tyr‐D‐Ala‐Gly‐Phe‐Met‐NH2 (DALA) and 14C‐labelled Substance P (SP) from previously described, fully protected intermediates is reported. The labelled peptides were prepared by methylation with (3H)‐ or (14C)methyl iodide of the sulphide anions formed on deprotection of the corresponding S‐benzyl‐homocysteine precursors with sodium in liquid ammonia. After purification by LC, the labelled peptides were obtained in radiochemical yields in the range of 9 to 24 % with a radiochemical purity higher than 97%. The specific radioactivities of the 3H‐ and 14C‐labelled products, corresponding to the labelled methyl iodides used, were 80 mCi/μmol and 60 μCi/‐μmol, respectively.