APPARENT GAS-PHASE ACIDITIES OF MULTIPLY PROTONATED PEPTIDE IONS - UBIQUITIN, INSULIN-B, AND RENIN SUBSTRATE

The gas-phase deprotonation reactions of multiply protonated bovine ub iquitin, insulin chain B, and renin substrate tetradecapeptide ions ha ve been studied in a Fourier transform ion cyclotron resonance mass sp ectrometer coupled with an external electrospray source. Rate constant s were measured for the reactions of these peptide ions with a series of reference compounds of known gas-phase basicities ranging from 195. 6 to 232.6 kcal/mol. The apparent gas-phase acidities (GA(app)) of the multiply protonated peptide ions [M + nH](n+) were determined with de protonation reactions. The deduced values of GA(app) show a strong dep endence on the charge states of the multiply protonated peptide ions. In general, the values decrease as the charge states of the peptide io ns increase. For ubiquitin ions, the determined GA(app)s values decrea se from > 232.6 to 205.0 kcal/mol for n = 4-13; for GA(app)s decrease from > 232.6 to 198.2 kcal/mol for n = 2-5; for renin insulin B ions, the GA(app)s decrease from > 232.6 to 198.2 kcal/mol for n = 2-5; for renin substrate ions, the GA(app)s decrease from 221.6 to < 195.6 kcal /mol for n = 2-4. Interestingly, at a given mass-to-charge ratio, the GA(app)s of these peptide ions agree within 10 kcal/mol despite large differences in their mass and charge. The ubiquitin and insulin B ions generated under the present conditions reveal multiple isomers at cer tain charge states, n = 4, 5, 6, 12 for ubiquitin and n = 4, 5 for ins ulin B, as evidenced by the fact that the isomers display distinctivel y different deprotonation reaction rates with certain reference compou nds. (C) 1996 American Society for Mass Spectrometry.