Establishing Regiocontrol of Disulfide Bond Isomers of Alpha-conotoxin ImI Via the Synthesis of N-to-C Cyclic Analogs

alpha-Conotoxins are multiple disulfide bond containing peptides that are isolated from venomous marine cone snails. They display remarkable selectivity for different subtypes of nicotinic acetylcholine receptors (nAChRs). While alpha-conotoxins display poor resistance to in vivo degradation by proteases, which limits their use as drug leads, N-to-C cyclization via an oligopeptide spacer unit has been previously shown to improve stability. However, the effect of N-to-C cyclization on the formation of the disulfide bond framework is not fully understood. Four N-to-C cyclic analogs of alpha-conotoxin ImI; cImI-A, cImI-betaA, cImI-AG, and cImI-AGG were synthesized to evaluate the effect of oligopeptide spacer length on disulfide bond selectivity and stability to proteolysis. Different ratios of disulfide bond isomers were obtained for each analog using a nonselective random disulfide bond forming strategy, which was dependent on the length of the spacer. To identify each isomer obtained using the random strategy, and to gain access to disulfide bond isomers otherwise unattainable using the random strategy, both the native (globular) and ribbon isomers were synthesized in good yield and purity using a selective orthogonal cysteine protecting group strategy. As such, a random oxidation strategy showed a clear preference for the ribbon isomer in cImI-A. The cyclic globular isomers showed a high resistance to enzymatic degradation compared to the ribbon isomers, with the cImI-A and cImI-AG globular isomers demonstrating the highest stability. These results suggest that cyclization can improve the biochemical stability of conotoxins with potential applications in the development of drugs.