Isoleucine and threonine are doubly substituted at the beta carbon, so that side chain loss can give rise to two different ion structures. These pairs are designated d and d' or w and w' . A useful feature of d and w ions is that they enable the isobaric residues leucine and isoleucine to be differentiated. Satellite ions from side chain cleavage are not observed. All of the ion series described above are observed in high energy collision spectra.
Relative abundances are composition dependent.
Unlike low energy CID, ions do not readily lose ammonia or water. All ion series can be accompanied by composition dependent satellites due to loss of ammonia or water. When a peptide contains an internal proline, strong ion series due to internal cleavage are observed, extending from the proline in the direction of the C terminus. 2000 Matrix Science Ltd. All Rights Reserved.