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Mechanistic insights into an unusual side chain mediated N-Cα bond cleavage under collision induced dissociation conditions in the disulfide containing peptide conopressin.

Conopressin, a nonapeptide disulfide CFIRNCPKG amide, present in cone snail venom undergoes a facile cleavage at the Cys(6)-Pro(7) peptide bond to yield a disulfide bridged b6 ion. Analysis of the mass spectral fragmentation pattern reveals the presence of a major fragment ion, which is unambiguously assigned as the tripeptide sequence IRN amide. The sequence dependence of this unusual fragmentation process has been investigated by comparing with the fragmentation patterns of related peptides, oxytocin (CYIQNCPLG amide), Lys-vasopressin (CYFQNCPKG amide) and a series of synthetic analogs. The results establish the role of the Arg(4) residue in facilitating the unusual N-Cα bond cleavage at Cys(6). Structures are proposed for a modified disulfide bridged fragment containing the Cys(1) and Cys(6) residues. Gas phase molecular dynamics simulations provide evidence for the occurrence of conformational states that permit close approach of the Arg(4) side chain to the Cys(6) Cβ methylene protons.