Ion/Ion Reactions in the Gas Phase:  Proton Transfer Reactions Involving Multiply-Charged Proteins

Multiply-charged cations derived from electrospray of bovine ubiquitin and horse skeletal muscle apomyoglobin have been subjected to reactions with anions derived from glow discharge ionization of perfluoro-1,3-dimethylcyclohexane. The results are compared with data obtained from proton transfer reactions with strong gaseous neutral bases. Ion/ion reaction rates are shown to be linearly related to the square of the charge on the protein ion, as expected based on a simple capture collision model. Cationic products with charge as low as +1 could be readily formed via ion/ion reactions, whereas efforts to produce such low charge states via ion/molecule reactions have proved unsuccessful. Ion/ion proton transfer reactions appear to be an effective means of reducing charge on highly charged proteins to arbitrarily low charge states. In addition to proton transfer, ion/ion recombination has also been observed. The propensity for proton transfer versus anion attachment to the cation is highly dependent upon the identity of the anion.