Synthesis and conformational analysis of apamin analogues with natural and non-natural cystine/selenocystine connectivities

By replacing two cysteine residues in apamin with selenocysteine, the three possible isomers related to the side-chain connectivities of a bis-cystinyl-peptide were synthesized in regioselective manner exploiting the low redox potential of the diselenide bond. Nuclear magnetic resonance conformational analysis of monoselenocystine analogue apamin with the natural diselenide/disulfide network confirmed the highly isomorphous character of the sulfur replacement with selenium despite its slightly larger atomic radius and increased bond lengths. The comparative conformational analysis of the apamin analogues containing the non-natural side-chain links with wild type apamin clearly revealed retention of the main structural fold and thus the high propensity of these small molecules to adopt the secondary structure elements present in natural apamin. These findings offered interesting hints for a better understanding of the oxidative refolding pathway of the bis-cystinyl peptide that leads exclusively to the correct natural isomer.