Cosolvent-assisted oxidative folding of a bicyclic α-conotoxin ImI



α-Conotoxin ImI is a 12-amino acid peptide, found in the venom of the marine snail Conus imperialis. This conotoxin is a selective antagonist of α7 nicotinic acetylcholine receptors. To produce biologically active α-ImI, disulfide bonds must be formed between Cys2–Cys8 and Cys3–Cys12. Oxidative folding of bicyclic conotoxins, such as α-ImI, has been traditionally achieved using two-step oxidation protocols with orthogonal protection on two native pairs of cysteines. In this work, two alternative oxidation protocols were explored: (1) the recently described one-pot oxidation of t-butyl/4-methylbenzyl protected Cys pairs and (2) direct oxidative folding. In contrast to the first method, the latter one resulted in high yields of correctly folded α-ImI. The addition of organic cosolvents, such as methanol, ethanol or isopropanol into the folding mixture significantly increased the accumulation of the native peptide. This effect was also observed for another conotoxin, α-PnIA. It is suggested that cosolvent-assisted direct oxidation might be of general use for other bicyclic α-conotoxins, but efficiency should be assessed on a case-by-case basis. Copyright © 2003 European Peptide Society and John Wiley & Sons, Ltd.