Surface-induced dissociation of singly and multiply protonated polypropylenamine dendrimers

The ease of fragmentation of various charge states of protoned polypropylenamine (POPAM) dendrimers is investigated by surface-induced dissociation. Investigated are the protonated diaminobutane propylenamines [DAB(PA)_n] DAB(PA)₈ (1+ and 2+), DAB(PA)₁₆ (2+ and 3+), and DAB(PA)₃₂ (3+ and 4+). These ions have been proposed to fragment by charge-directed intramolecular nucleophilic substotution (S_Ni) reactions. Differences in relative fragment ion abundances between charge states can be related to the occupation of different protonation sites. These positions can be rationalized based on estimates of Coulomb energies and gas-phase basicities of the protonation/fragmentation sites. The laboratory collision energies at which the frgament ion current is ~50% of the total ion current were found to increase with the size, but to be independent of charge state of the protonated POPAM dendrimers. It is suggested that intramolecular Coulomb repulsion within the multiply protonated POPAM dendrimers selected for activation does not readily result in easier fragmentation, which is in accordance with the proposed fragmentation mechanism.