Synthesis and conformational analysis of apamin analogues with natural andnon-natural cystine/selenosystine connectivities

By replacing two cysteine residues in apamin with selenocysteine, the three possible isomers related to the side-chain connectivities of a bis-cystiny l-peptide were synthesized in regioselective manner exploiting the late red ox potential of the diselenide bond. Nuclear magnetic resonance conformatio nal analysis of monoselenocystine analogue apamin with the natural diseleni de/disulfide network confirmed the highly isomorphous character of the sulf ur replacement with selenium despite its slightly larger atomic radius and increased bond lengths. The comparative conformational analysis of the apam in analogues containing the non-natural side-chain links with wild type apa min clearly revealed retention of the main structural fold and thus the hig h propensity of these small molecules to adopt the secondary secondary elem ents present in natural apamin. These findings offered interesting hints fo r a better understanding of the oxidative refolding pathway of he bis-cysti nyl peptide that leads exclusively to the correct natural isomer. (C) 2000 John Wiley & Sons, Inc.