Negative ion fragmentations of deprotonated peptides. The unusual case of isoAsp: a joint experimental and theoretical study. Comparison with positive ion cleavages.

The following peptides have been examined in this study: GLDFG(OH), caeridin 1.1 [GLLDGLLGLGGL(NH(2))], 11 Ala citropin 1.1 [GLFDVIKKVAAVIGGL(NH(2))], Crinia angiotensin [APGDRIYVHPF(OH)] and their isoAsp isomers. It is not possible to differentiate between Asp- and isoAsp-containing peptides (used in this study) using negative ion electrospray mass spectrometry. This is because the isoAsp residue cleaves to give the same fragment anions as those formed by delta and gamma backbone cleavage of Asp. The isoAsp fragmentations are as follows: RNHCH(CO(2)H)(-)CHCONHR' --> [RNH(-)(HO(2)CCH=CHCONHR')] --> RNH(-)+HO(2)CCH=CHCONHR' and RNHCH(CO(2)H)(-)CHCONHR' --> [RNH(-)(HO(2)CCH=CHCONHR'] --> (-)O(2)CCH=CHCONHR'+RNH(2). Calculations at the HF/6-31+G(d)//AM1 level of theory indicate that the first of these isoAsp cleavage processes is endothermic (by +115 kJ mol(-1)), while the second is exothermic (-85 kJ mol(-1)). The barrier to the highest transition state is 42 kJ mol(-1). No diagnostic cleavage cations were observed in the electrospray mass spectra of the MH(+) ion of the Asp- and isoAsp-containing peptides (used in this study) to allow differentiation between these two amino acid residues.