Regioselective formation of the three disulfide bonds of a 35-residue insect peptide

PMP-D2, a 35-residue peptide containing three disulfide bonds, was synthesized on solid-phase using 9-fluorenylmethoxy-carbonyl (Fmoc) as alpha-NH2 protection and simultaneous air oxidation of the six cysteines for formation of its disulfide bonds. The overall yield was 13%. As very little research has been done on the regioselective formation of three disulfide bonds, we decided to investigate different strategies using either trityl (Trt), acetamidomethyl (Acm) and methoxybenzyl (Mob), or methoxytrityl (Mmt), trityl and acetamidomethyl, as cysteine-protecting groups and Fmoc as alpha-NH2 protection. In the first strategy, the first disulfide bond was formed by air oxidation and the second was formed by iodine oxidation of the Cys(Acm). Then, the Cys (Mob) was deprotected using TFMSA/TFA treatment for formation of the third disulfide bond. This last step was poorly reproducible on a large scale. The overall yield was 2.5%. In the second strategy, the first disulfide was formed on the resin after removal of the methoxytrityl group, and the two remaining disulfide bonds were formed classically in solution. The overall yield was 2%. From the overall yields using these strategies, it appears clear that simultaneous oxidation of the six cysteines is particularly appropriate for the synthesis of PMP D2.