Special feature: Commentary - Five-membered ring formation in unimolecularreactions of peptides: a key structural element controlling low-energy collision-induced dissociation of peptides

Unimolecular fragmentation reactions of peptides in low-energy collision-in duced dissociation are reviewed in the mechanistic context of five-membered ring formation. This structure of intermediates or of fragment ions is rec ognized as a key element that governs unimolecular peptide fragmentation wi thin the structural framework determined by the peptide backbone and its si de-chains. A collection of collision-induced dissociation reactions is pres ented covering (i) b-ion formation, (ii) the fragmentation of N -terminally acylated peptides, (iii) neutral loss of the C-terminal amino acid in alka li or silver cationized peptides, (iv) the fragmentation of isoAsp-containi ng peptides and (v) the fragmentation of negatively charged Asp- or Glu-con taining peptides, It appears that for all possible nucleophile-electrophile interactions leading to a five-membered ring structure an associated unimo lecular peptide fragmentation reaction can be observed. Copyright (C) 2000 John Wiley & Sons, Ltd.