Electrospray Mass Spectrometry Studies of Poly(Propylene Imine) Dendrimers: Probing Reactivity in the Gas Phase

Mass spectrometry of dendrimers

Mass spectrometry (MS) has become an essential technique to characterize dendrimers as it proved efficient at tackling analytical challenges raised by their peculiar onion-like structure. Owing to their chemical diversity, this review covers benefits of MS methods as a function of dendrimer classes, discussing advantages and limitations of ionization techniques, tandem mass spectrometry (MS/MS) strategies to determine the structure of defective species, as well as most recently demonstrated capabilities of ion mobility spectrometry (IMS) in the field. Complementarily, the well-defined structure of these macromolecules offers major advantages in the development of MS-based method, as reported in a second section reviewing uses of dendrimers as MS and IMS calibration standards and as multifunctional charge inversion reagents in gas phase ion/ion reactions.

Sulfonimide-Based Dendrimers: Progress in Synthesis, Characterization, and Potential Applications

There are more than 50 families of dendrimers, and some of which, such as polyamidoamine PAMAM, are well studied, and some are just starting to attract the attention of researchers. One promising type of dendrimers is sulfonimide-based dendrimers (SBDs). To date, SBDs are used in organic synthesis as starting reagents for the convergent synthesis of higher generations dendrimers, in materials science as alternative electrolyte solutions for fuel cells, and in medicinal chemistry as potential substances for drug transfer procedures. Despite the fact that most dendrimers are amorphous substances among the SBDs, several structures are distinguished that are prone to the formation of crystalline solids with melting points in the range of 120-250 °C. Similar to those of other dendrimers, the chemical and physical properties of SBDs depend on their outer shell, which is formed by functional groups. To date, SBDs decorated with end groups such as naphthyl, nitro, methyl, and methoxy have been successfully synthesized, and each of these groups gives the dendrimers specific properties. Analysis of the structure of SBD, their synthesis methods, and applications currently available in the literature reveals that these dendrimers have not yet been fully explored.

Design, Synthesis, and Characterization of Fully Zwitterionic, Functionalized Dendrimers

Dendrimers are interesting candidates for various applications because of the high level of control over their architecture, the presence of internal cavities, and the possibility for multivalent interactions. More specifically, zwitterionic dendrimers modified with an equal number of oppositely charged groups have found use in in vivo biomedical applications. However, the design and control over the synthesis of these dendrimers remains challenging, in particular with respect to achieving full modification of the dendrimer. In this work, we show the design and subsequent synthesis of dendrimers that are highly charged while having zero net charge, that is zwitterionic dendrimers that are potential candidates for biomedical applications. First, we designed and fully optimized the synthesis of charge-neutral carboxybetaine and sulfobetaine zwitterionic dendrimers. Following their synthesis, the various zwitterionic dendrimers were extensively characterized. In this study, we also report for the first time the use of X-ray photoelectron spectroscopy as an easy-to-use and quantitative tool for the compositional analysis of this type of macromolecules that can complement techniques such as nuclear magnetic resonance and gel permeation chromatography. Finally, we designed and synthesized zwitterionic dendrimers that contain a variable number of alkyne and azide groups that allow straightforward (bio)functionalization via click chemistry.

Fragmentation of organic ions bearing fixed multiple charges observed in MALDI MS

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS) was used to analyze a series of synthetic organic ions bearing fixed multiple charges. Despite the multiple intrinsic charges, only singly charged ions were recorded in each case. In addition to the pseudo-molecular ions formed by counterion adduction, deprotonation and electron capture, a number of fragment ions were also observed. Charge splitting by fragmentation was found to be a viable route for charge reduction leading to the formation of the observed singly charged fragment ions. Unlike multivalent metal ions, organic ions can rearrange and/or fragment during charge reduction. This fragmentation process will evidently complicate the interpretation of the MALDI MS spectrum. Because MALDI MS is usually considered as a soft ionization technique, the fragment ion peaks can easily be erroneously interpreted as impurities. Therefore, the awareness and understanding of the underlying MALDI-induced fragmentation pathways is essential for a proper interpretation of the corresponding mass spectra. Due to the fragment ions generated during charge reduction, special care should be taken in the MALDI MS analysis of multiply charged ions. In this work, the possible mechanisms by which the organic ions bearing fixed multiple charges fragment are investigated. With an improved understanding of the fragmentation mechanisms, MALDI TOF MS should still be a useful technique for the characterization of organic ions with fixed multiple charges.

Multivalency in the gas phase: H/D exchange reactions unravel the dynamic "rock 'n' roll" motion in dendrimer-dendrimer complexes

Noncovalent dendrimer-dendrimer complexes were successfully ionized by electrospray ionization of partly protonated amino-terminated polypropylene amine (POPAM) and POPAM dendrimers fully functionalized with benzo[21]crown-7 on all branches. Hydrogen/deuterium exchange (HDX) experiments conducted on dendrimer-dendrimer complexes in the high vacuum of a mass spectrometer give rise to a complete exchange of all labile NH hydrogen atoms. As crown ethers represent noncovalent protective groups against HDX reactions on the ammonium group to which they are coordinated, this result provides evidence for a very dynamic binding situation: each crown is mobile enough to move from one ammonium binding site to another. Schematically, one might compare this motion with two rock 'n' roll dancers that swirl around each other without completely losing all contact at any time. Although the multivalent attachment certainly increases the overall affinity, the "microdynamics" of individual site binding and dissociation remains fast.

A pitfall of using 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile as a matrix in MALDI TOF MS: chemical adduction of matrix to analyte amino groups

2-[(2E)-3-(4-tert-Butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB) has been considered as an excellent matrix for matrix-assisted laser desorption/ionization (MALDI) of many types of synthetic compounds. However, it might provide troublesome results for compounds containing aliphatic primary or secondary amino groups. For these compounds, strong extra ion peaks with a mass difference of 184.1 Da were usually observed, which might falsely indicate the presence of some unknown impurities that were not detected by other matrices. On the basis of the possible mechanisms proposed, these extra ions are the products of nucleophilic reactions between analyte amino groups and DCTB molecules or radical cations. In these reactions, an amino group replaces the dicyanomethylene group of DCTB forming a matrix adduct via a -C=N-bond. An aliphatic primary amine could react easily with DCTB and the reaction could start once they are mixed in a MALDI solution. For an aliphatic secondary amine, on the other hand, the reaction most likely occurs in the gas phase. Protonation of amino groups by adding acid seems to be a useful way to stop DCTB adduction for compounds with one single amino group, but not for compounds with multiple amino groups. Unlike aliphatic primary or secondary amines, aliphatic tertiary amines and aromatic amines do not yield DCTB adducts. This is because tertiary amines do not have the required transferrable H-(N) atom to form an extra -C=N-bond, while aromatic amines are not sufficiently nucleophilic to attack DCTB. In view of the possible matrix adduction, care should be taken in MALDI time-of-flight mass spectrometry (TOF MS) when DCTB is used as the matrix for compounds containing amino group(s).

Structural characterization of poly(amino)ester dendrimers and related impurities by electrospray tandem mass spectrometry

An acid-terminated poly(amino)ester dendrimer was studied by electrospray ionization tandem mass spectrometry to establish its fragmentation pathways, with the aim of using them to investigate the structure of any defective molecules generated during the dendrimer synthesis. This poly(amino)ester dendrimer could be ionized in both polarities but the most structurally relevant dissociation pathways were found from the deprotonated molecule in negative ion mode. The dissociation pattern of this dendrimer is fully described and supported by accurate mass measurements. The main dissociation reactions of the negatively charged polyacidic dendrimer were shown to consist of (i) the release of carbon dioxide and ethene within a branch, which proceeds as many times as intact neutral branches are available; and (ii) the elimination of an entire dendrimer arm. Monitoring the occurrence of these reactions together with any deviation from these two main routes allowed six major dendritic impurities to be structurally characterized.

Characterization of self-assembled metallodendrimers in solution, in the gas phase, and at air/solid interfaces

It is found that 4,4'-bipyridines functionalized in their 3,3'-positions with Fréchet dendrons of 0th to 3rd generation self-assemble with (dppp)M(II) triflates (dppp: bis-(diphenylphosphino)propane; M = Pd, Pt) into metallo-supramolecular squares. They bear a nanometer-sized cavity inside an unpolar dendritic shell. A total of eight amide groups decorate the rims of the cavity connecting the dendrons to the square. Evidence for their formation up to the third generation comes from ESI-FTICR mass spectrometry and NMR experiments. Based on these results, the presence of significant amounts of other polygons or open-chain oligomers can be excluded. Exchange processes have been studied by variable-temperature NMR spectroscopy and by following the ligand exchanges between different squares by mass spectrometry. The ligand exchange is much slower for the Pt(II) squares as compared to their Pd(II) analogs. Visualization of films of these dendrimers using atomic force microscopy (AFM) provides information on their molecular dimensions. After deposition of a square monolayer on the surface, a slow reorganization within this layer is observed which leads to the formation of "tower-like" aggregates and multi-layer formation. The interplay of interactions between the dendrimers and the surface and interactions between different dendrimers are invoked to rationalize the observations.

Dehydrogenation of tertiary amines in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

In the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS) analysis of various compounds synthesized in our laboratory, strong [M - H]+ ion peaks were often observed for the molecules with tertiary amino groups. In this work, the MALDI TOF MS behavior of two groups of compounds that incorporate tertiary amino moieties was investigated. One group is bisurea dimethylanilines (BUDMAs) prepared for the study of molecular recognition in thermoplastic elastomers, and the other group is the poly(propylene imine) diaminobutane dendrimers. The results clearly demonstrate the appearance of the [M - H]+ ions. In order to understand the possible mechanisms for the generation of these ions, a series of model compounds, ranging from primary to tertiary amines, were investigated. Unlike the tertiary amines, no [M - H]+ ion peaks were recorded for the primary amines, and only barely detectable ones, if any, for some secondary amines. It appears that the tertiary amino groups play an important role in the formation of these ions. In addition to MALDI TOF MS analysis, these samples were also applied to electrospray ionization (ESI) MS where no [M - H]+ ions were observed. The results indicate that the generation of [M - H]+ ion is due to the unique MALDI conditions and is likely to be formed via dehydrogenation of a protonated tertiary amine resulting in an N=C double bond. The absence of [M - H]+ ion peaks for the primary and secondary amines is probably because upon their formation these ions could easily transfer one proton to the corresponding amines in the MALDI gas-phase plume, yielding neutral imines that cannot be detected by MS.

Gas-phase fragmentation of half- and first-generation polyamidoamine dendrimers by electrospray mass spectrometry using a quadrupole ion trap

Polyamidoamine (PAMAM) dendrimers are nanopolymers that can bind with biomolecules such as DNA, drugs or proteins. In order to study these complexes, we first fragmented half- and first-generation PAMAM, G0.5 and G1, respectively, using a quadrupole ion trap (QIT) equipped with an electrospray ionisation source. For both G0.5 and G1 we observed a series of impurities that only can stem from synthesis defects and that are principally due to missing branches and intramolecular cyclisations. Fragmentations of G1 showed regularity in the product ions. These ions result from the loss of 60 Da, obtained by an intramolecular cyclisation, and from the loss of 114 Da, obtained by a four-centred hydrogen transfer or a retro-Michael reaction. The fragmentations stemmed either from competitive or from consecutive reactions, even though resonant fragmentation QIT was used. It is shown that the principal fragmentation reaction is a retro-Michael rearrangement for both G1 and G0.5. In addition, by fragmenting totally deuterated [G1-d28]Na+ we were able to establish fragmentation pathways.

Investigation of silver binding to polyamidoamine (PAMAM) dendrimers by ESI tandem mass spectrometry

Electrospray ionization tandem mass spectrometry (ESI-MS/MS) was used to probe the binding of silver ions and reduced silver species with polyamidoamine generation 1 amine-terminated (PAMAMG1NH2) and generation 2 hydroxyl-terminated (PAMAMG2OH) dendrimers. At Ag(+)/PAMAMG2OH molar ratios of </=1, 1:1 complexes are observed, while at ratios >1, 2:1 and low abundance 3:1 complexes emerge. Similar results were observed for PAMAMG1NH2. The collisional activated dissociation (CAD) patterns of the dendrimer ions are characterized by losses of amidoamine branches resulting largely from hydrogen migration and cleavage reactions. Ag+/dendrimer complexes are characterized by the loss of a dendrimer branch from the complex, with the silver ion remaining bound to a dendrimer fragment. When the Ag+-bound dendrimer complexes are reduced by hydrazine, low abundance complexes, whose m/z values are consistent with ones containing zerovalent silver species, are observed in the mass spectra. Complexes with three silver atoms are observed in the spectrum containing PAMAMG1NH2, and complexes with four and five silver atoms are observed with PAMAMG2OH. The CAD fragmentation patterns of the complexes formed after the silver reduction are different than those observed for complexes containing one silver ion and are characterized by the ejection of all silver species, possibly as a cluster, leaving the intact dendrimer ion. Experiments with Cu+, Cu2+, and Pt2+ binding to PAMAMG2OH were also done, but reduced metal clusters were not observed in the mass spectra after the addition of hydrazine.

A Combined ESI- and MALDI-MS(/MS) Study of Peripherally Persulfonylated Dendrimers: False Negative Results by MALDI-MS and Analysis of Defects

Mass spectrometry, in particular MALDI-MS, has often been used as a valuable means to characterize dendritic molecules with respect to their molecular masses. Also, it is a valuable tool for analyzing potential defects in their structure which result from incomplete synthetic steps. This article presents a comparison of ESI and MALDI mass spectrometric experiments on dendrimers persulfonylated at their periphery. While the ESI mass spectra easily permit impurities and defects to be identified and thus provide evidence for sample purity, reactions with acidic matrices occur during the MALDI process. The resulting defects are identical to those expected from incomplete substitution. Thus, in these cases, MALDI-MS yields false negative results. With mass-selected, ESI-generated ions, collision experiments were performed in an FT-ICR mass spectrometer cell to provide detailed insight into the fragmentation patterns of the various dendrimers. Different fragmentation patterns are observed depending on the exact structure of the dendrimer. Also, the nature of the charge is important. The fragmentation reactions for protonated species differ much from those binding a sodium or potassium ion. These differences can be traced back to different sites for binding H+ versus Na+ or K+. Tandem MS experiments on mass-selected dendrimer ions with defects can be used to distinguish different types of defects. A concise structural assignment can thus be made on the basis of these experiments. Even mixtures of two isobaric defect variants with the same elemental composition can be identified.

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.

Application of capillary zone electrophoresis to the separation and characterization of poly(amidoamine) dendrimers with an ethylenediamine core

Generations 0 through 5 of ethylenediamine-core poly(amidoamine) dendrimers were synthesized and capillary zone electrophoresis has been applied to the separation of different generations of synthesized dendrimers and for the characterization of individual generations.

Quaternary ammonium functionalized poly(propylene imine) dendrimers as effective antimicrobials: structure-activity studies

Quaternary ammonium functionalized poly(propyleneimine) dendrimers were synthesized and their antibacterial properties were evaluated using a bioluminescence method. These quaternary ammonium dendrimers are very potent biocides. The antibacterial properties depend on the size of the dendrimer, the length of hydrophobic chains in the quaternary ammonium groups, and the counteranion. Since these dendrimers are well characterized and monodisperse, they also serve as an effective system to study the structure-activity relationship. The antimicrobial properties of these dendrimer biocides have a parabolic dependence on molecular weight, which is different from the bell-shaped molecular weight dependence of conventional polymer biocides. The dependence on the hydrophobic chain of the quaternary ammonium structure is similar to conventional polymer biocides, and shows a parabolic relationship with dendrimer biocides carrying C10 hydrophobes the most potent. The antimicrobial properties of these novel biocides with bromide anions are more potent than those with chloride anions. Biocides derived from hyperbranched polymers were also synthesized and found to possess somewhat lower effectiveness.

Molecular recognition and supramolecular chemistry in the gas phase

Supramolecular chemistry, in particular, the fields of molecular recognition and self-assembly, profit much from the development of soft ionization techniques and advanced methods for mass analysis and gas-phase chemistry. Vice versa, weakly bonded architectures and host-guest complexes represent a veritable challenge for the mass spectrometrist, leading to further development of methods and techniques. This review describes the state-of-the-art in this field, and includes topics such as the effects of solvation on meta binding to crown ethers, chiral discrimination of guests by chiral hosts, the elucidation of the secondary structure of self assembled complexes, and the mechanistic pathways of self assembly or the fragmentations of supramolecular complexes in the gas phase.