10 research outputs found

    Experimental Binding Energies in Supramolecular Complexes

    No full text
    On the basis of many literature measurements, a critical overview is given on essential noncovalent interactions in synthetic supramolecular complexes, accompanied by analyses with selected proteins. The methods, which can be applied to derive binding increments for single noncovalent interactions, start with the evaluation of consistency and additivity with a sufficiently large number of different host–guest complexes by applying linear free energy relations. Other strategies involve the use of double mutant cycles, of molecular balances, of dynamic combinatorial libraries, and of crystal structures. Promises and limitations of these strategies are discussed. Most of the analyses stem from solution studies, but a few also from gas phase. The empirically derived interactions are then presented on the basis of selected complexes with respect to ion pairing, hydrogen bonding, electrostatic contributions, halogen bonding, π–π-stacking, dispersive forces, cation−π and anion−π interactions, and contributions from the hydrophobic effect. Cooperativity in host–guest complexes as well as in self-assembly, and entropy factors are briefly highlighted. Tables with typical values for single noncovalent free energies and polarity parameters are in the Supporting Information

    Cucurbit[8]uril Mediated Donor–Acceptor Ternary Complexes: A Model System for Studying Charge-Transfer Interactions

    No full text
    A supramolecular self-assembly approach is described which allows for the convenient preparation of a wide range of charge-transfer (CT) donor–acceptor complexes in aqueous solutions. When one equiv of the macrocyclic host cucurbit[8]­uril (CB[8]) is added to an aqueous donor and acceptor solution, a heteroternary complex forms inside the host’s cavity with a well-defined face-to-face π–π-stacking geometry of the donor and acceptor. This heteroternary, CB[8]-mediated complex offers the opportunity to study the CT phenomena at low concentrations and free from complications arising from any donor–donor and acceptor–acceptor interactions as a result of the large binding affinities and the very high selectivity over the formation of these homoternary complexes. Thus, this supramolocular self-assembly strategy is a practical donor–acceptor mix-and-match approach with synthetic advantages over much more cumbersome tethering schemes. While the characteristic UV/vis features of a few CB[8] ternary systems had been described as a CT band, we present for the first time systematic evidence for the existence of CT interactions between several donor–acceptor pairs that are mediated by the host CB[8]. Correlation of the experimentally obtained CT λ<sub>max</sub> to computed HOMO–LUMO energies demonstrated that the CT process in the host’s cavity can be described by the Mulliken model. Furthermore, the literature claim of a “CT driving force” for the formation of CB[8] ternary complexes was scrutinized and evaluated by calorimetric (ITC) and ESI-MS measurements. The findings indicated that neither in the aqueous medium nor in the “gas-phase” is CT of energetic relevance to the Gibbs free binding energy. In contrast, electrostatic considerations combined with solvation effects are much better suited to rationalize the observed trends in binding affinities. Additionally, the CT λ<sub>max</sub> was found to be much more red-shifted (≥75 nm) inside the CB[8] cavity than in any polar organic solvents or water, indicating a significant stabilization of the CT excited state within the host cavity, further demonstrating the unique electrostatic, polar properties of the host cavity

    Quantitative SERS Using the Sequestration of Small Molecules Inside Precise Plasmonic Nanoconstructs

    No full text
    We show how the macrocyclic host, cucurbit[8]­uril (CB[8]), creates precise subnanometer junctions between gold nanoparticles while its cavity simultaneously traps small molecules; this enables their reproducible surface-enhanced Raman spectroscopy (SERS) detection. Explicit shifts in the SERS frequencies of CB[8] on complexation with guest molecules provides a direct strategy for absolute quantification of a range of molecules down to 10<sup>–11</sup> M levels. This provides a new analytical paradigm for quantitative SERS of small molecules

    Synthesis and Photophysics of Fully π‑Conjugated Heterobis-Functionalized Polymeric Molecular Wires via Suzuki Chain-Growth Polymerization

    No full text
    We present a fast and efficient <i>in situ</i> synthetic approach to obtain fully π-conjugated polymers with degrees of polymerization up to 23 and near quantitative (>95%) heterobis-functionalization. The synthesis relies on the key advantages of controlled Suzuki chain-growth polymerization: control over molecular weight, narrow polydispersity, and ability to define polymer end groups. The first end group is introduced through the initiator metal complex <i>t</i>Bu<sub>3</sub>PPd­(X)­Br, while the second end group is added by quenching of the chain-growth polymerization with the desired boronic esters. In all cases, polymers obtained at 50% conversion showed excellent end group fidelity and high purity following a simple workup procedure, as determined by MALDI-TOF, GPC, and <sup>1</sup>H and 2D NMR. End group functionalization altered the optoelectronic properties of the bridge polymer. Building on a common fluorene backbone, and guided by DFT calculations, we introduced donor and acceptor end groups to create polymeric molecular wires exhibiting charge transfer and energy transfer as characterized by fluorescence, absorption, and transient absorption spectroscopy as well as by fluorescence lifetime measurements

    Release of High-Energy Water as an Essential Driving Force for the High-Affinity Binding of Cucurbit[<i>n</i>]urils

    No full text
    Molecular dynamics simulations and isothermal titration calorimetry (ITC) experiments with neutral guests illustrate that the release of high-energy water from the cavity of cucurbit­[<i>n</i>]­uril (CB<i>n</i>) macrocycles is a major determinant for guest binding in aqueous solutions. The energy of the individual encapsulated water molecules decreases with increasing cavity size, because larger cavities allow for the formation of more stable H-bonded networks. Conversely, the total energy of internal water increases with the cavity size because the absolute number of water molecules increases. For CB7, which has emerged as an ultrahigh affinity binder, these counteracting effects result in a maximum energy gain through a complete removal of water molecules from the cavity. A new design criterion for aqueous synthetic receptors has therefore emerged, which is the optimization of the size of cavities and binding pockets with respect to the energy and number of residing water molecules

    Nanomolar Binding of Steroids to Cucurbit[<i>n</i>]urils: Selectivity and Applications

    No full text
    Cucurbit­[<i>n</i>]­urils (CB<i>n</i>, <i>n</i> = 7, 8) serve as artificial receptors for steroids (21 tested), including the hormones testosterone and estradiol as well as steroidal drugs. Fluorescence displacement titrations and isothermal titration calorimetry (ITC) provided up to nanomolar binding affinities in aqueous solution for these hydrophobic target molecules, exceeding the values of known synthetic receptors. Remarkable binding selectivities, even for homologous steroid pairs, were investigated in detail by NMR, X-ray crystal diffraction, ITC, and quantum chemical calculations. Notably, the CB<i>n</i>•steroid complexes are stable in water and buffers, in artificial gastric acid, and even in blood serum. Numerous applications have been demonstrated, which range from the solubility enhancement of the steroids in the presence of the macrocycles (up to 100 times, for drug delivery) and the principal component analysis of the fluorescence responses of different CB<i>n</i>•reporter dye combinations (for differential sensing of steroids) to the real-time monitoring of chemical conversions of steroids as substrates (for enzyme assays)

    Nanomolar Binding of Steroids to Cucurbit[<i>n</i>]urils: Selectivity and Applications

    No full text
    Cucurbit­[<i>n</i>]­urils (CB<i>n</i>, <i>n</i> = 7, 8) serve as artificial receptors for steroids (21 tested), including the hormones testosterone and estradiol as well as steroidal drugs. Fluorescence displacement titrations and isothermal titration calorimetry (ITC) provided up to nanomolar binding affinities in aqueous solution for these hydrophobic target molecules, exceeding the values of known synthetic receptors. Remarkable binding selectivities, even for homologous steroid pairs, were investigated in detail by NMR, X-ray crystal diffraction, ITC, and quantum chemical calculations. Notably, the CB<i>n</i>•steroid complexes are stable in water and buffers, in artificial gastric acid, and even in blood serum. Numerous applications have been demonstrated, which range from the solubility enhancement of the steroids in the presence of the macrocycles (up to 100 times, for drug delivery) and the principal component analysis of the fluorescence responses of different CB<i>n</i>•reporter dye combinations (for differential sensing of steroids) to the real-time monitoring of chemical conversions of steroids as substrates (for enzyme assays)

    In Situ SERS Monitoring of Photochemistry within a Nanojunction Reactor

    No full text
    We demonstrate a powerful SERS-nanoreactor concept composed of self-assembled gold nanoparticles (AuNP) linked by the sub-nm macrocycle cucurbit­[<i>n</i>]­uril (CB­[<i>n</i>]). The CB­[<i>n</i>] functions simultaneously as a nanoscale reaction vessel, sequestering and templating a photoreaction within, and also as a powerful SERS-transducer through the large field enhancements generated within the nanojunctions that CB­[<i>n</i>]­s define. Through the enhanced Raman fingerprint, the real-time SERS-monitoring of a prototypical stilbene photoreaction is demonstrated. By choosing the appropriate CB­[<i>n</i>] nanoreactor, selective photoisomerism or photodimerization is monitored in situ from within the AuNP-CB­[<i>n</i>] nanogap

    Ultrahigh-Water-Content Supramolecular Hydrogels Exhibiting Multistimuli Responsiveness

    No full text
    Hydrogels are three-dimensional networked materials that are similar to soft biological tissues and have highly variable mechanical properties, making them increasingly important in a variety of biomedical and industrial applications. Herein we report the preparation of extremely high water content hydrogels (up to 99.7% water by weight) driven by strong host–guest complexation with cucurbit[8]­uril (CB[8]). Cellulosic derivatives and commodity polymers such as poly­(vinyl alcohol) were modified with strongly binding guests for CB[8] ternary complex formation (<i>K</i><sub>eq</sub> = 10<sup>12</sup> M<sup>–2</sup>). When these polymers were mixed in the presence of CB[8], whereby the overall solid content was 90% cellulosic, a lightly colored, transparent hydrogel was formed instantaneously. The supramolecular nature of these hydrogels affords them with highly tunable mechanical properties, and the dynamics of the CB[8] ternary complex cross-links allows for rapid self-healing of the materials after damage caused by deformation. Moreover, these hydrogels display responsivity to a multitude of external stimuli, including temperature, chemical potential, and competing guests. These materials are easily processed, and the simplicity of their preparation, their availability from inexpensive renewable resources, and the tunability of their properties are distinguishing features for many important water-based applications

    Efficient Host–Guest Energy Transfer in Polycationic Cyclophane–Perylene Diimide Complexes in Water

    No full text
    We report the self-assembly of a series of highly charged supramolecular complexes in aqueous media composed of cyclobis­(4,4′-(1,4-phenylene)­bispyridine-<i>p</i>-phenylene)­tetrakis­(chloride) (ExBox) and three dicationic perylene diimides (PDIs). Efficient energy transfer (ET) is observed between the host and guests. Additionally, we show that our hexacationic complexes are capable of further complexation with neutral cucurbit[7]­uril (CB[7]), producing a 3-polypseudorotaxane via the self-assembly of orthogonal recognition moieties. ExBox serves as the central ring, complexing to the PDI core, while two CB[7]­s behave as supramolecular stoppers, binding to the two outer quaternary ammonium motifs. The formation of the 3-polypseudorotaxane results in far superior photophysical properties of the central PDI unit relative to the binary complexes at stoichiometric ratios. Lastly, we also demonstrate the ability of our binary complexes to act as a highly selective chemosensing ensemble for the neurotransmitter melatonin
    corecore