EVIDENCE OF CHARGE INVERSION IN THE REACTION OF SINGLY CHARGED ANIONS WITH MULTIPLY CHARGED MACROIONS

Novel gas-phase reactions were studied by mass spectrometry using merged gas streams containing oppositely charged species at near atmospheric pressure. Protonated adenosine 5'-monophosphate (AMP) and fluorescein molecules (M + H)+ are observed as products in the reaction of multiply charged macroions (M' + nH)n+ with species generated in the negative electrospray ionization of AMP or fluorescein. The (M + H)+ products most likely arise from charge inversion of (M - H)- anions in an ion-ion reaction. Protonated fluorescein and AMP were observed as a product of reaction with multiply protonated myoglobin, cytochrome c, and melittin but not with the less highly charged serine and bradykinin ions or other positively charged species of lower proton affinity from an air discharge. These observations suggest that the (M + H)+ products arise from charge inversion of (M - H)- anions in a single-step encounter of the negative ion with a multiply charged cation.