Two ion/ion charge inversion steps to form a doubly protonated peptide from a singly protonated peptide in the gas phase.

An approach is described to increase the degree of protonation of a polypeptide ion in the gas phase. Sequential charge inversion reactions involving the reactions of oppositely charged ions are used to yield a net increase in ion charge. The approach is illustrated here with the conversion of singly protonated bradykinin to doubly protonated bradykinin. The first step involves conversion of the singly protonated peptide to the singly deprotonated peptide via reactions with multiply charged anions derived from carboxylate-terminated dendrimers. Some of the singly deprotonated peptide was then converted to doubly protonated peptide via reactions with multiply charged cations derived from amino-terminated dendrimers. The overall approach is illustrative of a general strategy for increasing the absolute charge states of large ions in the gas phase.