Determination of binding selectivities in host-guest complexation by electrospray/quadrupole ion trap mass spectrometry

AbstractThe quantifiable relationship between the equilibrium solution composition and electrospray (ESI) mass spectral peak intensities of simple host–guest complexes was investigated. Specifically, host–guest complexes of simple crown ethers or glymes with alkali metals and ammonium ions were studied. Comparisons were made between the theoretical concentrations of host–guest complexes derived in solution from known stability constants and the peak intensities for the complexes observed by ESI mass spectrometry (ESI-MS). Two types of complexation experiments were undertaken. First, complexation of a single guest ion, such as an alkali metal, and two crown ethers was studied to evaluate the determination of binding selectivities. Second, complexation of two different guest ions by a single polyether host was also examined. In general, solvation was found to play an integral part in the ability to quantify binding selectivities by ESI-MS. The more similar the solvation energies of the two complexes in the mixture, the more quantifiable their binding selectivities by ESI-MS. In some cases, excellent correlation was obtained between the theoretically predicted selectivity ratios and the ESI mass spectral ratios, in particular when the ESI ratios were adjusted based on evaluation of ESI response factors for the various host–guest complexes

The reactive and dissociative behavior of size-selected clusters and ion complexes in the gas phase

Molecular complexation properties of several types of size-selected ions are examined in the gas phase by mass spectrometric techniques. Transition metal oxide clusters of (MoO₃)[subscript n]⁻ where n = 2-12 and (WO₃)[subscript n]⁻ extending to n = 8 are generated by field and laser desorption, and electron capture in the negative ionization mode. The structures of these clusters are characterized by low energy collision induced dissociation (CID). The clusters dissociate by the neutral loss of metal trioxide units. Reactions of ligands such as ethylene oxide and cyclohexene sulfide are examined to distinguish the structural variations of the clusters. The systematic evaluation of the size effect in molecular recognition processes is examined by association reactions of crown ether macrocyclic hosts with alkali metal ions and primary amine substrates. Ion complexes of crown ethers with two different alkali metals are formed by liquid secondary ion mass spectrometry. These complexes produce crown ether/alkali metal adduct ions upon gas-phase isolation and high energy dissociation. The abundance ratios of product ions are correlated to the alkali metal ion selectivites of the crown ether. The selectivity of 18-crown-6 follows the order of Na⁺ ≥ K⁺ > Li⁺ > Rb⁺ > Cs⁺, and the concept of 'maximum contact point' (MCP) describes this order. In contrast, the high energy activation of alkali metal ion adducts of crown ethers results in homolytic cleavages of C-C or C-O bonds. The metastable ion dissociation of hydrogen bonded complexes of crown ethers with ammonium or methyl-ammonium shows a stability order of 21-crown-7 > 18-crown-6 > 15-crown-5. High energy CID of the ion complexes proceeds by cavity size dependent intramolecular ring opening reactions of the host. Complexes of crown ethers with methyl- and tosyl-hydrazinium ions dissociate by covalent bond cleavages of the N-N or N-S bonds of the guest substrates. Finally, reactions of perfluorinated cyclic or bicyclic ether anions with O₂ are examined as models of oxygen transport by perfluorinated compounds. These complexes show binding strengths that reflect both the topological and chemical nature of the hosts.Chemistr

Structural Evidence for the Tetrameric Assembly of Chemokine CCL11 and the Glycosaminoglycan Arixtra™.

Understanding chemokine interactions with glycosaminoglycans (GAG) is critical as these interactions have been linked to a number of inflammatory medical conditions, such as arthritis and asthma. To better characterize in vivo protein function, comprehensive knowledge of multimeric species, formed by chemokines under native conditions, is necessary. Herein is the first report of a tetrameric assembly of the human chemokine CCL11, which was shown bound to the GAG Arixtra™. Isothermal titration calorimetry data indicated that CCL11 interacts with Arixtra, and ion mobility mass spectrometry (IM-MS) was used to identify ions corresponding to the CCL11 tetrameric species bound to Arixtra. Collisional cross sections (CCS) of the CCL11 tetramer-Arixtra noncovalent complex were compared to theoretical CCS values calculated using a preliminary structure of the complex deduced using X-ray crystallography. Experimental CCS values were in agreement with theoretical values, strengthening the IM-MS evidence for the formation of the noncovalent complex. Tandem mass spectrometry data of the complex indicated that the tetramer-GAG complex dissociates into a monomer and a trimer-GAG species, suggesting that two CC-like dimers are bridged by Arixtra. As development of chemokine inhibitors is of utmost importance to treatment of medical inflammatory conditions, these results provide vital insights into chemokine-GAG interactions

Synthesis and recognition properties of α-d-glucose-based fluorescent crown ethers incorporating an acridine unit

Two new chiral glucopyranoside-based crown ethers incorporating acridine fluorescent signalling units, 15-membered ligand 1 and 21-membered ligand 2 were synthesized. Their complexation properties toward alkali and alkali earth metal ions, and their enantioselectivity towards chiral ammonium salts were studied by absorption and fluorescence spectroscopic experiments. Macrocycle 1 formed 1:1 complexes with all the metal ions selected and the stability constants were low (lg K < 2.3). The cavity-size of 2 allowed only the complexaton of organic ammonium ions. Crown 2 showed chiral discrimination in case of all the four ammonium salts used as model guest compounds; the highest enantioselectivity (K(R)/K(S) ~3) was observed for the enantiomers of phenylethyl ammonium perchlorate. Ligand 2 forms much more stable complexes with metal ions; the highest stability constant was obtained for the Ca2+ complex (lg K = 6.15). The coordination of metal ions by ligand 2 was accompanied by marked fluorescence enhancement, whereas the binding of ammonium ions by the same species resulted in significant fluorescence quenching. © 2014 Springer Science+Business Media Dordrecht

Tandem Mass Spectrometric and Ion Mobility Studies of Supramolecular Complexes

Synthetic supramolecular systems share many similarities with natural biological assemblies, especially when considering that the structure and guest binding are typically governed by non-covalent interactions. As such, the defining characteristic is that only comparably weak forces define the shape of a synthetic supramolecule or the tertiary structure of a protein, so that the resulting dynamic binding mode makes structure elucidation challenging. One of the major advances in recent analytical chemistry has been the development of ion mobility-mass spectrometry (IM-MS) to tackle the challenging problems faced in proteomics, glycomics, metabolomics, and lipidomics. By analogy, the prospects of applying IM-MS to supramolecules are bright and it is to be expected that unprecedented analytical insights into diverse systems such as host-guest complexes, molecular devices, self-assemblies and metallosupramolecular complexes will be obtained. The recurrent theme throughout this dissertation is that both structure (differentiation of diastereomers, photoisomers, mechanoisomers) and non-covalent interactions (hydrogen bonding, $TTF^{n+}/TTF^{n+}$-charge repulsion, dispersive interactions) can be investigated by a combination of the three methods of ion-mobility mass spectrometry (IM-MS), collision-induced dissociation (CID) and gas-phase H/D-exchange (GP-HDX). In the study of the gas-phase chiral recognition of crown-ether ammonium complexes, the importance of a single hydrogen bond for the enantiodifferentiation was revealed. Similarly, in an azobenzene model a hydrogen bonding interaction led to an increased stability of the (Z)-photoisomer. This surprising observation illustrates an important aspect, namely that there can be significant differences between the gas-phase and the solution environment. In the absence of solvent, both the stabilization of charged sites and the Coulomb repulsion of nearby charges are accentuated. In a way, the conundrum of supramolecular mass spectrometry revolves around the problem that ions are easily manipulated in the gas-phase where a high analytic resolution power is available, to then face the question if the obtained results still reflect the solution environment. Therefore, it is very convincing to see that in three of the five presented studies, the solution environment is reflected in a quantitative fashion: In the quantification of the enantiomeric excess (first study), the quantification of photoisomer content (second study), and the quantitative determination of equilibrium constants for redox-controlled dethreading (third study). Together with these five studies, and the detailed description in the subsequent chapters, I expect the treatment to be useful also from the practitioner's point of view. It is my hope that the performance, speed, and reliability with which measurements can be performed with modern instrumentation will make IM-MS a routine analytical tool in the repertoire of the working supramolecular chemist

Untersuchung von nicht-kovalenten Wechselwirkungen zur Konstruktion von supramolekularen Schalter

Molecular switches can interconvert between two stable states controlled by an external stimulus. They are applied in adaptive materials, molecular electronics, and are needed to develop artificial molecular machines. Molecular switches can be assembled and operated based on noncovalent interactions. A detailed understanding of the involved noncovalent interactions is fundamental to develop new switching functions and operation modes. In this thesis, the redox-switchable units tetrathiafulvalene (TTF) and naphthalene diimide (NDI) are incorporated into crown ethers. The thermodynamic binding properties of crown ethers and sec-ammonium ions were investigated through isothermal titration calorimetry (ITC). The effects of the counterion as well as the structural changes both in the macrocycle and in the axle on the noncovalent interactions in the pseudorotaxanes were identified. Additionally, an organic cage was developed, which can adopt a self-inclusion complex. The cage showed a very strong enthalpy driven binding (Ka > 109 M-1) of cations such as acetylcholine. Interestingly, the binding was also entropically favoured. Based on a toolbox of molecular components of redox-switchable crown ethers and sec-ammonium axles, different functions of molecular switches were constructed using the concepts of self-assembly, self-sorting, mechanical bonding, and multivalency. Switching of a kinetically hindered pseudorotaxane revealed differences in the disassembly timeframe depending on the type of the applied stimulus. Five redox-switchable monovalent rotaxanes with different switching modes were assembled. In two of these rotaxanes a shuttling motion of the wheel along the axle was observed upon switching. In a planar chiral rotaxane, redox-switching resulted in a sign inversion in the electronic circular dichroism spectrum. In a homo[3]rotaxane, attractive noncovalent interactions between two extended TTF units which are incorporated in both wheels were identified. When both TTFs were oxidised to TTF2+, Coulomb repulsion triggers a 180° rotation of the wheels in relation to each other. By applying integrative self-sorting, two different wheels were combined on one axle to form hetero[3]pseudorotaxanes. The self-sorting equilibrium was studied and it revealed a major impact of the smaller second wheel on the hetero[3]pseudorotaxane properties. The construction of four hetero[3]rotaxanes was achieved. The formation of a divalent pseudorotaxane containing TTF and NDI showed positive chelate cooperativity and an emerging intramolecular charge-transfer band confirmed the donor-acceptor interaction between TTF and NDI. The investigation of the redox-switching of a divalent donor-acceptor rotaxane revealed interesting optoelectronic properties.Molekulare Schalter können reversibel zwischen zwei stabilen Zuständen wechseln und werden dabei von einem externen Stimulus kontrolliert. Sie finden Anwendung in adaptiven Materialen, in molekularer Elektronik und sind notwendig in der Entwicklung von künstlichen molekularen Maschinen. Nicht-kovalente Wechselwirkungen können zum Zusammenbau von molekularen Schaltern verwendet werden und diese haben einen Einfluss auf ihren Betrieb. Ein genaues Verständnis der zugrunde liegenden nicht-kovalenten Wechselwirkungen ist fundamental, um neue Schalterfunktionen zu entwickeln. In dieser Dissertation wurden die elektrochemisch schaltbaren molekularen Einheiten Tetrathiafulvalen (TTF) und Naphtalendiimid (NDI) in Kronenether eingebaut. Die thermodynamischen Bindungseigenschaften dieser Kronenether zu sec-Ammoniumionen wurden durch isothermale Titrationskalorimetrie (ITC) detailliert untersucht. Die Effekte von Gegenionen und von strukturellen Änderungen im Makrozyklus, sowie in der Ammoniumachse auf die nicht-kovalenten Wechselwirkungen im Pseudorotaxan wurden offengelegt. Zusätzlich wurde ein organischer Käfig entwickelt, der einen selbst-Einschlusskomplex bilden kann. Es wurde gezeigt, dass die starke Bindung des Käfigs von Kationen, wie Acetylcholin, (Ka > 109 M-1) Enthalpie-getrieben ist. Interessanterweise ist diese Bindung aber auch entopisch günstig. Ausgehend von den molekularen Komponenten, bestehend aus elektrochemisch schaltbaren Kronenethern und sec-Ammoniumachsen, wurde durch die Anwendung der Konzepte der Selbstassemblierung, der Selbstsortierung, der Multivalenz und der mechanischen Bindung eine Vielzahl von molekularen Schaltern mit verschiedenen Funktionen konstruiert. Bei der Schaltung eines kinetisch gehemmten Pseudorotaxans mit zwei unterschiedlichen Arten von Stimuli zeigten sich Unterschiede beim Zeitraum, in dem der Reif von der Achse abfädelte. Es wurden fünf elektrochemisch schaltbare Rotaxane mit unterschiedlichen Schaltungsmechanismen zusammengebaut. In zwei dieser Rotaxane konnte eine Translationsbewegung des Makrozyklus entlang der Achse festgestellt werden, die durch die Schaltung ausgelöst wurde. In einem planar chiralen Rotaxan führte die elektrochemische Schaltung zu einem Vorzeichenwechsel des elektronischen Zirkulardichroismus. In einem Homo[3]rotaxan konnten anziehende, nicht-kovalente Wechselwirkung zwischen zwei verlängerten TTF-Einheiten, die sich jeweils in den beiden Reifen befanden, nachgewiesen werden. Sobald beide TTF-Einheiten zum Dikation oxidiert wurden, löste die Coulomb-Abstoßung eine 180° Rotation der beiden Reifen zueinander aus. Durch die Anwendung von integraler Selbstsortierung konnten zwei verschiedene Reifen auf eine Achse gefädelt werden und so Hetero[3]pseudorotaxane gebildet werden. Das Selbstsortierungsgleichgewicht wurde untersucht und es konnte gezeigt werden, dass der kleinere zweite Reif den größeren Einfluss auf die Eigenschaften des Hetero[3]pseudorotaxane hatte als der größere Reif. Davonausgehend wurden vier Hetero[3]rotaxane hergestellt. Die Bildung eines divalenten Pseudorotaxans, das TTF- und NDI-Einheiten enthält zeigte positive Chelatkooperativität und eine intramolekulare Ladungstransferbande entstand, welche die Bildung eines Donor-Akzeptor-Komplexes zwischen TTF und NDI belegte. Die elektrochemische Schaltung eines daraus hergestellten Donor-Akzpetor-Rotaxans zeigte interessante optoelektronische Eigenschaften

FLUORESCENT PROBE INVESTIGATIONS OF MICROENVIRONMENTS OF ANALYTICAL INTEREST (REVERSED-PHASE, POLYETHYLENIMINE, POLARIZATION)

Fluorescent probes were applied to investigate three systems. Reverse phase chromatographic surfaces were studied using ion pairs. Variables were cation reagent structure and concentration, bonded phase (methyl, octyl, octadecyl, and phenyl), and solvent (water or methanol). Emission wavelength shifts for the anionic polarity probe, ANS, (8-anilino-napthalene-1-sulfonate) reflect the nature and extent of lipophilic interactions. Tetramethylammonium promoted ANS penetration into surface structure. Tetrabutylammonium overcame aqueous surface alkyl aggregation, which greatly enhanced ANS-surface interaction of C18. For the other phases at high cation concentrations there was lipophilic interaction between ANS and cation. High concentrations of small cations excluded ANS from the surface, as did all levels of trimethylmyristylamine cation. Methanol solvation reduced lipophilic interactions with alkyl surfaces. Pi-pi interactions were important with the phenyl surface. Results are consistent with the ion interaction retention model for ion pairing chromatography, which is based on double layer formation on a dynamic surface. Polyelectrolyte-counterion binding strength and proximity were studied using a three component system: metals bound to polyethylenimine (PEI) and pyrenesulfonate counterion probes. Metals altered rates of excited state processes and defined binding environment. Variables were net charge on metal and probe and metal-amine complex properties. Cu(II)-PEI efficiently and selectively quenched probes. Ground state dimerization in Zn(II)-PEI implied territorial binding involving lipophilic interactions with PEI and between probes was important. Quenching and excimer formation in Ag(I)-PEI were due to more than net charge since protonated sites did not alter emission. Fluorescence polarization was used to detect intramolecular energy transfer between equivalent fluorophors in crown ethers, metal complexes, and simple organic molecules. Energy transfer randomizes the transition moment of emission relative to that of excitation, thereby decreasing polarization. In dilute glycerol solutions intermolecular depolarization is eliminated. A simple model based on Forster energy transfer theory was developed to distinguish molecules with different numbers of fluorophors and to obtain average angles between fluorophors, based on the extent of polarization differences