A New Model for Multiply Charged Adduct Formation Between Peptides and Anions in Electrospray Mass Spectrometry

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 (GB app ) 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: HSO ) 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.

The Use of Chromium(III) to Supercharge Peptides by Protonation at Low Basicity Sites

“…The identity of the negative counter ion was considered because anions affect protonated ion generation by ESI in ways that relate to their structure and their acid/base properties [54–56]. As the intensity data in Table 1 show, the nitrate salts of chromium(III) and iron(III) produced an [M+2H] 2+ signal for A7 that was almost twice as intense as the signal produced with the corresponding chloride salts.…”

“…As the intensity data in Table 1 show, the nitrate salts of chromium(III) and iron(III) produced an [M+2H] 2+ signal for A7 that was almost twice as intense as the signal produced with the corresponding chloride salts. Chloride anions are more basic than nitrate anions [56], which suggests that chloride anions may be less effective in enhancing peptide protonation. Acetate buffers are often employed in LC-MS experiments due to the volatility of the resulting neutrals and their ability to minimize salt-induced signal suppression in ESI [57,58].…”

What Protein Charging (and Supercharging) Reveal about the Mechanism of Electrospray Ionization

“…Ion pairing effects on charging have been discussed previously [73,74], and can be rationalized (in positive mode) as reflecting the gas phase disparity between analyte basicity for adding an nth proton and the anion's basicity [74]. Ion pairs are typically transferred to the gas phase from solution, where they were governed by solvent properties.…”

Electron Detachment Dissociation of Underivatized Chloride-Adducted Oligosaccharides

“…This seemingly unique behavior for the chlorideadducted precursor ion may be explained by the chloride adduct sharing a hydrogen with the sialic acid carboxylate group rather than with an oligosaccharide hydroxyl group (such carboxylic acid binding is not possible for maltoheptaose and NA2 as they lack sialic acid). This mode of binding, which prevents carboxylic acid deprotonation, was recently proposed by Cole and co-workers for chloride-peptide anionic complexes [66].…”