Poly(propylene imine) dendrimer conformations in the gas phase: a tandem mass spectrometry study

[M + H](+) ions are produced from a generation three poly(propylene imine) dendrimer (DAB-dendr-(NH2)(16)) by matrix-assisted laser desorption ionization (MALDI) or electrospray ionization (ESI), employing solvents of varying polarity and acidity for sample preparation (in MALDI) or the spraying process (in ESI). Subsequently, the structures of [M + H]+ are investigated by tandem mass spectrometry (MS/MS), via the post-source decay (PSD) and collisionally activated dissociation (CAD) techniques. Upon MALDI-PSD, elimination of the outer branches, i.e., of HN(CH2CH2CH2NH2)2 and HN[CH2CH2CH2N(CH2CH2CH2NH2)(2)](2), is observed with apolar protic but not with a nonpolar solvent. Similarly, the loss of the outer branches upon ESI-CAD is discriminated against when employing a nonpolar instead of a polar protic solvent. These differences are attributed to the existence of distinct conformations for the neutral dendrimer and the [M + H]+ ions formed from it. A nonpolar solvent leads to a folded conformation, i.e., one with the periphery branches pointing into the interior of the dendrimer to enable intramolecular hydrogen bonds. Due to such self-solvation, certain C-N bonds cannot be cleaved efficiently upon MS/MS. On the other hand, a polar protic solvent interacts favorably with the dendrimer through intermolecular hydrogen bonds, promoting the formation of an extended, unfolded conformation from which outer and periphery branches can readily be eliminated. These structural differences disappear in the [M+2H](2+) cations produced by ESI; now, the loss of the outer branches takes place upon MS/MS irrespective of the solvent used in the ion generation step; this result is consistent with a stretched conformation for [M +2H](2+), where the repulsion between the like charges is minimized. (Int J Mass Spectrom. 214 (2002) 75-88) (C) 2002 Elsevier Science B.V. All rights reserved.