Gas-Phase Host-Guest Chemistry of Dendritic Viologens and Molecular Tweezers: A Remarkably Strong Effect on Dication Stability

Encapsulation in Charged Droplets Generates Distorted Host-Guest Complexes

The ability of various hydrogen-bonded resorcinarene-based capsules to bind α,ω-alkylbisDABCOnium (DnD) guests of different lengths was investigated in solution and in the gas-phase. While no host-guest interactions were detected in solution, encapsulation could be achieved in the charged droplets formed during electrospray ionisation (ESI). This included guests, which are far too long in their most stable conformation to fit inside the cavity of the capsules. A combination of three mass spectrometric techniques, namely, collision-induced dissociation, hydrogen/deuterium exchange, and ion-mobility mass spectrometry, together with computational modelling allow us to determine the binding mode of the DnD guests inside the cavity of the capsules. Significant distortions of the guest into horseshoe-like arrangements are required to optimise cation-π interactions with the host, which also adopt distorted geometries with partially open hydrogen-bonding seams when binding longer guests. Such quasi "spring-loaded" capsules can form in the charged droplets during the ESI process as there is no competition between guest encapsulation and ion pair formation with the counterions that preclude encapsulation in solution. The encapsulation complexes are sufficiently stable in the gas-phase - even when strained - because non-covalent interactions significantly strengthen in the absence of solvent.

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.

Electrocatalytic Reduction of Oxygen on Platinum Nanoparticles in the Presence and Absence of Interactions with the Electrode Surface

We report that ultraviolet/ozone (UV/O3) treatment can be used to remove sixth-generation, hydroxyl-terminated poly(amidoamine) (PAMAM) dendrimers from dendrimer-encapsulated Pt nanoparticles (Pt DENs) previously immobilized onto a pyrolyzed photoresist film (PPF) electrode. Results from X-ray photoelectron spectroscopy, scanning transmission electron microscopy, and electrochemical experiments indicate that removal of the dendrimer proceeds without changes to the size, shape, or electrocatalytic properties of the encapsulated nanoparticles. The UV/O3 treatment did not damage the PPF electrode. The electrocatalytic properties of the DENs before and after removal of the dendrimer were nearly identical.

Competing noncovalent host-guest interactions and H/D exchange: reactions of benzyloxycarbonyl-proline glycine dipeptide variants with ND3

A combination of density functional theory calculations, hydrogen/deuterium exchange (HDX) reactions, ion mobility-mass spectrometry, and isotope labeling tandem mass spectrometry was used to study gas-phase "host-guest" type interactions of a benzyloxycarbonyl (Z)-capped proline (P) glycine (G) model dipeptide (i.e., Z-PG) and its various structural analogues with ND3. It is shown that in a solvent-free environment, structural differences between protonated and alkali metal ion (Na(+), K(+), or Cs(+))-complexed species of Z-PG affect ND3 adduct formation. Specifically, [Z-PG + H](+) and [Z-PG-OCH3 + H](+) formed gas-phase ND3 adducts ([Z-PG (or Z-PG-OCH3) + H + ND3](+)) but no ND3 adducts were observed for [Z-PG + alkali metal](+) or [Z-PG + H - CO2](+). Experimentally measured and theoretically calculated collision cross sections (CCSs) of protonated and alkali metal ion-complexed Z-PG species showed similar trends that agreed with the observed structural differences from molecular modeling results. Moreover, results from theoretical ND3 affinity calculations were consistent with experimental HDX observations, indicating a more stable ND3 adduct for [Z-PG + H](+) compared to [Z-PG + alkali metal](+) species. Molecular modeling and experimental MS results for [Z-PG + H](+) and [Z-PG + alkali metal](+) suggest that optimized cation-π and hydrogen bonding interactions of carbonyl groups in final products are important for ND3 adduct formation. Graphical Abstract ᅟ.

Investigation of dendriplexes by ion mobility-mass spectrometry

Highly branched polyamidoamine (PAMAM) dendrimers presenting biological activities have been envisaged as non-viral gene delivery vectors. They are known to associate with nucleic acid (DNA) in non-covalent complexes via electrostatic interactions. Although their transfection efficiency has been proved, PAMAMs present a significant cytotoxicity due to their cationic surface. To overcome such a drawback, different chemical modifications of the PAMAM surface have been reported such as the attachment of hydrophobic residues. In the present work, we studied the complexation of DNA duplexes with different low-generation PAMAM; ammonia-cored G0(N) and G1(N) PAMAM, native or chemically modified with aromatic residues, i.e., phenyl-modified-PAMAM G0(N) and phenylalanine-modified-PAMAM G1(N). To investigate the interactions involved in the PAMAM/DNA complexes, also called dendriplexes, we used electrospray ionization (ESI) coupled to ion mobility spectrometry-mass-spectrometry (IM-MS). ESI is known to allow the study of non-covalent complexes in native conditions while IM-MS is a bidimensional separation technique particularly useful for the characterization of complex mixtures. IM-MS allows the separation of the expected complexes, possible additional non-specific complexes and the free ligands. Tandem mass spectrometry (MS/MS) was also used for the structural characterization. This work highlights the contribution of IM-MS and MS/MS for the study of small dendriplexes. The stoichiometries of the complexes and the equilibrium dissociation constants were determined. The [DNA/native PAMAM] and [DNA/modified-PAMAM] dendriplexes were compared.

Supramolecular reactivity in the gas phase: investigating the intrinsic properties of non-covalent complexes

The high vacuum inside a mass spectrometer offers unique conditions to broaden our view on the reactivity of supramolecules. Because dynamic exchange processes between complexes are efficiently suppressed, the intrinsic and intramolecular reactivity of the complexes of interest is observed. Besides this, the significantly higher strength of non-covalent interactions in the absence of competing solvent allows processes to occur that are unable to compete in solution. The present review highlights a series of examples illustrating different aspects of supramolecular gas-phase reactivity ranging from the dissociation and formation of covalent bonds in non-covalent complexes through the reactivity in the restricted inner phase of container molecules and step-by-step mechanistic studies of organocatalytic reaction cycles to cage contraction reactions, processes induced by electron capture, and finally dynamic molecular motion within non-covalent complexes as unravelled by hydrogen-deuterium exchange processes performed in the gas phase.

Benzylic viologen dendrimers: a review of their synthesis, properties and applications

Dendrimers containing benzylic viologen branching units, their guest complexation, photophysical and biological applications has been reviewed.

Structural and spectroscopic properties of the second generation phosphorus-viologen "molecular asterisk"

The FTIR and FT Raman spectra of the second generation phosphorus-viologen "molecular asterisk" G2 built from cyclotriphosphazene core with 12 viologen units and 6 terminal phosphonate groups have been recorded and analyzed. The experimental X-ray data of 1,1-bis(4-formylbenzyl)-4,4'-bipyridinium bis(hexaflurophosphate) was used in molecular modeling studies. The optimization of isolated 1,1-bis(4-formylbenzyl)-4,4'-bipyridinium (BFBP) molecule without counter ions PF6(-) does not lead to significant changes of dihedral angles, thus the molecular conformation does not depend on interactions with the counter ions. The structural optimization and normal mode analysis were performed for G2 on the basis of the density functional theory (DFT). The calculated geometrical parameters and harmonic vibrational frequencies are predicted in a good agreement with the experimental data. It was found that G2 has a kind of "egg timer" structure with planar OC6H4CHNN(CH3) fragments and slightly non-planar cyclotriphosphazene core. The experimental IR and Raman spectra of G2 were interpreted by means of potential energy distribution.

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.

Aromatic interactions by molecular tweezers and clips in chemical and biological systems

Noncovalent interactions involving aromatic rings, such as π-stacking and CH-π, occur throughout a range of fundamental processes including self-assembly and (bio)catalysis. Molecular clips and tweezers possess a central parallel or torus-shaped cavity with a surrounding belt of convergent aromatic rings; hence these structures exploit multiple aromatic interactions in a positively cooperative manner. Both clips and tweezers demonstrate selective binding of cationic or neutral guests that bear acceptor groups. The electrostatic surface potentials (ESP) explain this unexpected behavior: calculated ESPs were highly negative inside the tweezer or clip cavity, providing complementary profiles to the positive ESP plots of their preferred guest molecules. This Account presents more complex systems that use aromatic clips and tweezers to alter the reactivities of included guest species, to distinguish between guest enantiomers, and to interfere with biological processes such as enzymatic activity and protein aggregation. Napthalene tweezers show potential applications in organocatalysis. When pyridinium moieties are bound within the spacious cavity of naphthyl-spaced tweezers, the resulting complex significantly influences the first step of single-electron reductions of (bi)pyridinium salts. In addition, the environment within the tweezer cavity strongly accelerates the Menshutkin reaction (the alkylation of pyridine derivatives). Introduction of phosphonate, phosphate, or sulfate anions into the central aromatic bridge renders clips and tweezers water-soluble. Larger systems form extremely tight intertwined dimers that rely on the nonclassical hydrophobic effect for their stability. Smaller clips and tweezers with a simple benzene bridge remain monomeric in buffered aqueous solution and display a complementary binding profile. While the clips with parallel sidewalls prefer flat aromatic cations such as pyridinium salts, the torus-shaped tweezers bind to basic amino acids lysine and arginine via a threading process. These mutually exclusive binding modes make water-soluble clips and tweezers valuable tools for probing critical biological interactions with positively charged amino acid side chains and cofactors. Molecular clips and tweezers can be employed for the complete inhibition of dehydrogenases. The clip extracts NAD(+) from its Rossman fold, while the tweezer complexes access strategic lysine residues around the active site. Our new enzyme inhibitors recognize the protein surface and thus offer additional targets for medicinal chemistry. Finally, the ability of molecular tweezers to cap critical lysine residues can be used to interfere with the pathology of protein misfolding diseases such as Alzheimer's disease, because many of them involve noncovalent interactions with these critical residues during their early stages. When the key protein produces a β-sheet-rich nucleus, this structure undergoes spontaneous polymerization into highly toxic oligomers, ultimately leading to mature fibrils. The benzene-spaced phosphate tweezer forms a specific complex with lysine residues 16 and 28 in Aβ42 and thus prevents the formation of misfolded oligomers rich in β-sheets. This entirely new process-specific mechanism that prevents pathologic protein aggregation also operates in many other related amyloidogenic proteins.

Encapsulation of luminescent homoleptic [Ru(dpp)3](2+)-type chromophores within an amphiphilic dendritic environment

A new series of homoleptic metallodendrimers has been synthesized through ruthenium-metal complexation by dendritically modified bathophenanthroline ligands. The presence of hydrophilic oligo(ethylene glycol) groups on the surface of the monodisperse metal complexes enabled the solubilization of all of the fractal species in a wide range of solvents, including water. The specific properties of all of these compounds have been systematically investigated by using photophysical techniques as a function of the generation number. Accordingly, the encapsulation of the highly luminescent [Ru(dpp)(3)](2+)-type (dpp=4,7-diphenyl-1,10-phenanthroline) core unit within a dendritic microenvironment creates a powerful means to shield the center from dioxygen quenching. This shielding effect, as exerted on the phosphorescent ruthenium-derived center, is reflected by enhanced emission intensities and extended excited-state lifetimes that are close to the highest values reported so far, even in an air-equilibrated aqueous medium. Interestingly, when inspecting the largest dendritic assembly, that is, the third-generation assembly, significant drops in emission quantum yields and lifetimes are observed. This anomalous behavior has been attributed to the folding of the branches towards the luminescent core.

Tandem mass spectrometry for the analysis of self-sorted pseudorotaxanes: the effects of Coulomb interactions

The increasing complexity of self-assembled supramolecules generates the need for analytical techniques that can accurately elucidate their structures. Here, we explore the ability of tandem mass spectrometry to deliver structural information on a series of self-sorted crown ether/ammonium pseudorotaxanes. Of these intertwined molecules, different charge states are accessible and the effects of Coulomb interactions on the fragmentation pattern can be examined. Three different cases can be distinguished: (1) one or more counterions are present in the complex and compete with the crown for binding to the ammonium ion. This destabilizes the supramolecular bond. (2) In multiply charged complexes, charge repulsion significantly alters the fragmentation behavior as compared with singly charged ions. (3) If guest and host are both charged, the supramolecular bond becomes very weak. The different charge states provide different pieces of information about the supramolecules under study. Although singly charged complexes provide data on the building block connectivity, the doubly charged analogs are more reliable with respect to complex stoichiometry. As there are several factors which may cause differences in the gas phase and solution behavior of supramolecules (the presence and absence of solvation, changes in the strength of non-covalent interactions upon ionization), it is important to establish well understood correlations between the complexes' gas-phase behavior and their solution structures. A more detailed understanding will help to characterize the structures of even more complex supramolecular architectures by mass spectrometry.

Structure of Molecular Tweezer Complexes in the Solid State: NMR Experiments, X-ray Investigations, and Quantum Chemical Calculations

The structure of supramolecular complexes formed by a naphthalene-spaced tweezer molecule as host and 1,4-dicyanobenzene (DCNB), 1,2,4,5-tetracyanobenzene (TCNB), and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) as aromatic, electron-deficient guests is investigated by solid-state NMR and X-ray diffraction measurements. Quantum chemical calculations using linear scaling methods are applied to predict and to assign the 1H NMR chemical shifts of the complexes. By combining experiment and theory, insights into intra- and intermolecular effects influencing the proton chemical shifts of the host-guest system are provided in the solid state.

Triple-Ion Interactions for the Construction of Supramolecular Capsules

A novel type of [2+4] capsules based on triple-ion interactions was obtained. Four monovalent anions (bromide, nitrate, acetate, and tosylate) bring together two tetrakis(pyridiniummethyl)tetramethyl cavitands by pyridinium-anion-pyridinium interactions. ESI-MS experiments have confirmed the capsule structure due to different fragmentation pathways of triple ions, cations, and ion-pairs. Each capsule encapsulates one or two anions, depending on its size. The capsules exist in equilibrium with hemicapsules containing three walls. The latter form complexes with phenols and anilines to give new unsymmetrical capsules containing both pyridinium-anion-pyridinium and pyridinium-guest-pyridinium walls.

Mass spectrometric studies of non-covalent compounds: why supramolecular chemistry in the gas phase?

Supramolecular chemistry has progressed quite a long way in recent decades. The examination of non-covalent bonds became the focus of research once the paradigm that the observed properties of a molecule are due to the molecule itself was revised, and researchers became aware of the often quite significant influence of the environment. Mass spectrometry and gas-phase chemistry are ideally suited to study the intrinsic properties of a molecule or a complex without interfering effects from the environment, such as solvation and the effects of counterions present in solution. A comparison of data from the gas phase, i.e. the intrinsic properties, with results from condensed phase, i.e. the properties influenced by the surroundings of the molecule, can consequently contribute significantly to the understanding of non-covalent bonds. This review provides insight into the often-underestimated power of mass spectrometry for the investigation of supramolecules. Through example studies, several aspects are discussed, including determination of structure in solution and the gas phase, ion mobility studies to reveal the formation of zwitterionic structures, stereochemical issues, analysis of reactivity of supramolecular compounds in the condensed and in the gas phase, and the determination of thermochemical data.

Host−Guest Complexes between an Aromatic Molecular Tweezer and Symmetric and Unsymmetric Dendrimers with a 4,4‘-Bipyridinium Core

We have investigated the spectroscopic and electrochemical behavior of symmetric and unsymmetric first-, second-, and third-generation dendrimers comprising an electron-acceptor 4,4'-bipyridinium core (viologen type) and electron-donor 1,3-dimethyleneoxybenzene (Fréchet-type) dendrons. The quite strong fluorescence of the symmetrically and unsymmetrically disubstituted 1,3-dimethyleneoxybenzene units of the dendrons is completely quenched as a result of donor-acceptor interactions that are also evidenced by a low-energy tail in the absorption spectrum. In dichloromethane solution, the 4,4'-bipyridinium cores of the investigated dendrimers are hosted by a molecular tweezer comprising a naphthalene and four benzene components bridged by four methylene units. Host-guest formation causes the quenching of the tweezer fluorescence. The association constants, as measured from fluorescence and (1)H NMR titration plots, (i) are of the order of 10(4) M(-1), (ii) decrease on increasing dendrimer generation, and (iii) are slightly larger for the unsymmetric than for the symmetric dendrimer of the same generation. The analysis of the complexation-induced shifts of the temperature-dependent (1)H NMR signals of the host and guest protons confirms that the bipyridinium core is positioned inside the tweezer cavity and allows the conclusions that (i) shuttling of the tweezer from one to the other pyridinium ring is fast (DeltaG < 10 kcal/mol), (ii) in the case of the unsymmetric dendrimers, the less substituted pyridinium ring is preferentially complexed in apolar solvents, and (iii) complexation of the 4,4'-bipyridinium core proceeds by clipping for the symmetric dendrimers and by threading in the case of unsymmetric ones. Host-guest formation causes a displacement of the first reduction wave of the 4,4'-bipyridinium unit toward more negative potential values, whereas the second reduction wave is unaffected. These results show that the host-guest complexes between the tweezer and the dendrimers are stabilized by electron donor-acceptor interactions and can be reversibly assembled/disassembled by electrochemical stimulation.

Bicyclo[2.2.2]octene-Based “Crab-like” Molecules: Synthesis, Complexation, Luminescence Properties, and Solid-State Structures

[structure, reaction: see text] The U-shaped, multifunctionalized tetraetheno-bridged dicyclopenta[b,i]anthracenediol 10 was synthesized to serve as a platform molecule. The molecule was prepared from the Diels-Alder adduct 5a of tricycloundecatriene 3 and bicyclo[2.2.2]octene-fused p-benzoquinone 4. Functionalization of 10 to construct crab-like molecules was achieved via the base-promoted bis-O-alkylation of two endo-oriented hydroxyl groups at termini in 10 with the following alkyl halides: allyl, propagyl, and benzyl bromides; 1-bromo- and 1-iodo-4-(bromomethyl)benzene; 9-(bromomethyl)anthracene; 1-(bromomethyl)pyrene; and isomeric bromomethylpyridines. Single-crystal X-ray structures were obtained for bis-phenyl (21) and bis-pyrenyl (25) crabs, and for the silver(I) complex (32 and 33) crabs. The silver(I) complex 32 from bis-o-pyridyl crab 30 is a [2+2] dimeric dimetallocyclophane, and 33 from bis-m-pyridyl crab 29 is a [1+1] metallo-bridged cyclophane. The self-assembled intramolecular pi-stacking of pyrenyl rings in 25 with an interplanar distance of 3.40 A and the consequent pi-pi interactions were revealed by the X-ray crystal structure and its luminescence property.

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.