A New Model for Multiply Charged Adduct Formation Between Peptides and Anions in Electrospray Mass Spectrometry |
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Authors: | Xiaohua Liu Richard B Cole |
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Institution: | (1) Department of Chemistry, University of New Orleans, 2000 Lakeshore Dr., New Orleans, LA 70148, USA; |
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Abstract: | A new model has been developed to account for adduct formation on multiply charged peptides observed in negative ion electrospray
mass spectrometry. To obtain a stable adduct, the model necessitates an approximate matching of apparent gas-phase basicity
(GBapp) of a given proton bearing site on the peptide with the gas-phase basicity (GB) of the anion attaching at that site. Evidence
supporting the model is derived from the fact that for Glu] Fibrinopeptide B, higher GB anions dominated in adducts observed
at higher negative charge states, whereas lower GB anions appeared predominately in lower charge state adducts. Singly charged
adducts were only observed for lower GB anions: HSO4–, I–, CF3COO–. Ions that have medium GBs (NO3–, Br–, H2PO4–) only form adducts having −2 charge states, whereas Cl– (higher GB) can form adducts having −3 charge states. The model portends that (1) carboxylate groups are much more basic
than available amino groups; (2) apparent GBs of the various carboxylate groups on peptides do not vary substantially from
one another; and (3) apparent GBs of the individual carboxylate and amino sites do not behave independently. This model was
developed for negative ion attachment but an analogous mechanism is also proposed for the positive ion mode wherein (1) binding
of a neutral at an amino site polarizes this amino group, but hardly affects apparent GBs of other sites; (2) proton addition
(charge state augmentation) at one site can decrease the instrinsic GBs of other potential protonation sites and lower their
apparent GBs. |
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