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(NH2)], 11 Ala citropin 1.1 [GLFDVIKKVAAVIGGL(NH2)], 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 δ and γ backbone cleavage of Asp. The isoAsp fragmentations are as follows: RNHCH(CO2H)CHCONHR′ → [RNH(HO2CCH[DOUBLE BOND]CHCONHR′)] → RNH+HO2CCH[DOUBLE BOND]CHCONHR′ and RNHCH(CO2H)CHCONHR′ → [RNH(HO2CCH[DOUBLE BOND]CHCONHR′] → O2CCH[DOUBLE BOND]CHCONHR′+RNH2. 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. Copyright © 2009 John Wiley & Sons, Ltd.