Structural characterization of chelator-terminated dendrimers and their synthetic intermediates by mass spectrometry

Herein, we report the structural analysis of a novel family of iron-chelating dendrimers and their synthetic intermediates utilizing matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and electrospray ionization ion trap (ESI IT) MS. These dendrimers share a benzene tricarbonyl core moiety attached to three tripodal branching units, each linking to three terminal groups, ranging from carboxyl, catechol and 3-hydroxy-6-methyl-pyran-4-one moieties and their protected analogs. In order to monitor the progression of dendrimer synthesis, all intermediates and final products were mass analyzed by conventional MALDI-TOF MS with and without alkali metal spiking. For structural characterization, interpretable post-source decay (PSD) and electrospray ionization ion trap MS/MS spectra were obtained from proton, sodium and potassium adducts of the dendrimers. One major route of dendrimer fragmentation was at or adjacent to the amide bonds either of the terminal chelating groups or near the core moiety. Fragmentation in the latter case was primarily at the N-terminal side of the amide bond and was directed by the proximity of a tertiary carbon of the branching unit. Furthermore, it was found that terminal ester, ether and amide linkages to the protecting and chelating groups could be sequentially broken in a single MS/MS spectrum through multiple cleavages resulting in product ions of decreasing intensity. Moreover, such cleavages could also be induced in a stepwise manner in a multistage ion trap MS(n) experiment.