Ion trap collisional activation of protonated poly(propylene imine) dendrimers: generations 1–5

Abstract

Ions derived from electrospray ionization of poly(propylene imine) dendrimers (generations 1–5, synthesized from a 1,4-diaminobutane core) were subjected to ion trap tandem mass spectrometry. In some cases, ion/ion proton transfer reactions were used to form low charge state parent ions from higher charge state ions generated by electrospray. In addition, ion/ion proton transfer reactions performed on product ions formed by ion trap collisional activation of multiply charged parent ions facilitated interpretation of tandem mass spectrometry (MS/MS) spectra. Almost all of the products derived from dendrimer parent ions could be rationalized based on a set of dissociation processes involving a previously noted intramolecular nucleophilic attack by a nearest-neighbor nitrogen atom on a carbon alpha to a protonation site. For a given protonation site, attack can occur either from the adjacent nitrogen closer to the interior or closer to the exterior of the dendrimer. The processes that lead to the dominant products are highly dependent upon parent ion charge state. Singly charged ions fragment primarily by processes directed by protonation at a diaminobutane nitrogen whereas highly charged parent ions fragment largely by processes directed by protonation closer to the periphery of the dendrimer. MS/MS data for singly charged ions of series of synthesis failure products in the fourth and fifth generation dendrimers were collected. The results show that ion trap tandem mass spectrometry can provide information about the composition of mixtures of isomeric products resulting from side reactions that occur during the course of the multistep syntheses of the dendrimers. The data show, for example, that synthesis failures tend to occur on one side of the dendrimer.