Selective cation removal from gaseous polypeptide ions: proton vs. sodium ion abstraction via ion/ion reactions

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Abstract
Doubly charged positive ions of the polypeptide bradykinin with an excess proton and sodium ion have been subjected to reactions with a perfluorocarbon anion, C8F15 −, and with hexafluorophosphate, PF6 −. The perfluorocarbon anion reacted exclusively by proton transfer to yield sodium cationized bradykinin with no evidence of adduct formation. Reaction with PF6 −, yielded proton transfer, sodium ion transfer, and adduct products. Collisional activation studies of the adduct species showed both loss of NaPF6 and loss of HPF6. The relative proportions of these reaction channels were dependent upon ion activation conditions. Collisional activation experiments in a quadrupole ion trap mass spectrometer showed evidence for at least two adduct ion structures that yield different relative contributions from NaPF6 and HPF6 loss. Loss of NaPF6 from the complex was favored under conditions that gave rise to relatively high dissociation rates. Calculations of the cation affinities and stabilities of various species relevant to these experiments suggest that proton transfer is much more thermodynamically favorable for the perfluorocarbon anion. The calculated ion affinities are also consistent with sodium ion transfer being competitive with proton transfer during reactions with PF6 −. Calculation of the stabilities of model ion/ion reaction intermediate structures show that the intermediate containing the perfluorocarbon anion is much less strongly bound than that containing the PF6 − ion. This difference likely underlies the observation that no adduct formation is observed with the perfluorocarbon ion whereas relatively extensive adduct formation is noted for PF6 −.