Mass spectrometric evidence for mechanisms of fragmentation of charge-derivatized peptides

Mass spectrometry of charged derivatives of peptides has been a growing are a of interest in the past decade. Fragmentation of charged derivatives of p eptides is believed to be different from than that of protonated peptides w hen analyzed by collisionally activated dissociation-tandem mass spectromet ry (CAD-MS/MS). The charged derivatives fragment by charge-remote fragmenta tion mechanisms, which are usually classified as high-energy (HE)-CAD proce sses. Our objective in the present study is to investigate the mechanism of fragmentation of charged derivatives of peptides when analyzed by matrix-a ssisted laser desorption/ionization-post-source decay-mass spectrometry (MA LDI-PSD-MS) and electrospray ionization (ESI)-CAD-MS/MS (ion trap), which i nvolve low-energy processes. Three major types of hydrogens (alpha, beta, a nd amide) are available for migration during the formation of the *a(n) ion s (the predominant ion series produced from these charged derivatives). To pinpoint which of the three hydrogens is involved in the formation of the * a(n) ions, deuterium-labeled peptide derivatives with labels at specific si tes were synthesized and analyzed by MALDI-PSD-MS and ESI-CAD-MS/MS. Our re sults suggest that the amide hydrogen of the residue at which the cleavage occurs shifts during the formation of *a(n); this observation serves as evi dence for the mechanism proposed earlier by Liao et al. for fragmentation o f such charged derivatives. The results also help elucidate the structure o f the *a(n) ions, *b(n) ions, and others formed during cleavage at the prol ine residue, as well as the ions formed during loss of the C-terminal resid ue from these charged-derivatives. (C) 2001 American Society for Mass Spect rometry.