Determination of disulfide functionality in underivatised peptides using negative ion mass spectrometry: An aid to structure determination

This review outlines the mass spectrometric methods used to identify disulfide functionality in peptides, particularly cleavages from peptide (M-H)(-) parent anions. A brief introduction to characteristic negative ion fragmentations of peptides is given, including (i) the alpha and beta cleavages which provide data analogous to that provided by Y+2 and B cleavages of MH+ ions, (ii) characteristic side chain cleavages, and (iii) gamma backbone cleavages initiated from anion sites on the side chains of certain residues (e.g. Ser, Thr, Cys, Asp, Asn, Glu and Gln). The cystine disulfide functionality is difficult to identify from positive ion fragmentations of an MH+ parent cation of an underivatised peptide. However, the -S-S- group is readily identified by the diagnostic loss of the elements of H2S2 from the (M-H)- anion of the peptide. This process is anion directed (from that cystine backbone enolate anion closest to the N-terminal end of the peptide). The stepwise process is exothermic (by 4.6 kcal mol(-1); calculations at the HF/6-31G(d)//AM1 level of theory) with a barrier of 9.1 kcal mol(-1) to the highest energy transition state. If one of the cystine residues is C-terminal with a terminal CO2H moiety, the presence of a pronounced peak corresponding to the fragmentation sequence [(M-H)(-) - (H2S2 + CO2)](-) identifies this functionality. The studied fragmentation processes are charge initiated; there is no evidence for the operation of any charge-remote processes.