Solid-phase synthesis and biological activity of a thioether analogue of conotoxin G1

A bicyclic thioether analogue of alpha-conotoxin G1, a neurotoxin found in the venom of cone snails, was synthesized on solid phase. Two successive intramolecular on-bead cyclization between a cysteine residue and a chloracetylated reduced peptide bond are the key steps in the synthesis. The first reduced peptide bond was introduced by a reductive alkylation with a 9-fluorenylmethoxy-carbonyl protected amino aldehyde, and the second by coupling of a dipeptide building block containing an allyloxycarbonyl protected reduced peptide bond. The desired bicyclic product was obtained as a mixture of two isomers, which were evaluated for their ability to inhibit the muscular nicotinic acetylcholine receptor expressed in Xenopus oocytes. The two isomers were found to have IC50 values (inhibitory activities) of 144 muM and 48 muM, compared to 0.18 muM for native conotoxin. G1.