Charge inversion of polypeptide anions using protein and dendrimer cations as charge inversion reagents

The conversion of deprotonated bradykinin, as well as anions derived from other polypeptides, to protonated species has been demonstrated via ion/ion reactions with multiply protonated polypropylenimine diaminobutane (DAB) dendrimers and proteins in a linear ion trap. The charge inversion characteristics of both the proteins and the dendrimers were examined with emphasis on the extent of charging of the analyte and the tendency for adduct formation. Multiply protonated proteins, for example, tended to result in significant adduct formation whereas the multiply protonated amino-terminated dendrimers showed essentially no adduct formation. Ions derived from five dendrimer generations were examined as charge inversion reagents with deprotonated bradykinin serving as a peptide model. Both the size and the charge state of the dendrimer reagent ion played a role in determining the number of protons transferred from the reagent cation to the peptide anion. For a given dendrimer generation, the tendency is for increasing numbers of protons to transfer with increasing dendrimer charge. For a given charge state, the numbers of protons transferred tends to be inversely related to dendrimer size. All of the observed data are consistent with charge inversion taking place via a long-lived intermediate complex with the ultimate products being determined by the fate of the complex (i.e., stabilization of the complex versus break-up into one of several competitive dissociation channels).