Structure and Activity of ( 2 , 8 )-Dicarba-( 3 , 12 )-cystino rImI , an r-Conotoxin Containing a Nonreducible Cystine Analogue †

The R-conotoxins are potent and selective antagonists of nicotinic acetylcholine receptors (nAChR). Exploitation of these and other peptides in research and clinical settings has been hampered by the lability of the disulfide bridges that are essential for toxin structure and activity. One solution to this problem is replacement of cystine bridges with nonreducible dicarba linkages. We explore this approach by determining the solution structure and functional characteristics of a dicarba analogue of the R-conotoxin R-ImI, (2,8)dicarba-(3,12)-cystino R-ImI. The structure of the dicarba analogue was similar to that of native R-ImI, with differences attributable to the different covalent geometry of the disulfide and dicarba bridges. DicarbaR-ImI maintained inhibitory activity of nAChR comparable to that of native R-ImI in two in vitro assays. These findings confirm the potential of the dicarba linkage to improve stability while maintaining R-conotoxin function.