Self-assembled Supramolecular Host Research Articles

Hierarchically Self-Assembled Supramolecular Host-Guest Delivery System for Drug Resistant Cancer Therapy.

In this report, a new star-like copolymer β-CD- g-(PNIPAAm- b-POEGA) x, consisting of a β-CD core, grafted with temperature-responsive poly( N-isopropylacrylamide) (PNIPAAm) and biocompatible poly(oligo(ethylene glycol) acrylate) (POEGA) in a block copolymer of the arms, was used to deliver chemotherapeutics to drug resistant cancer cells and tumors. The first step of the self-assembly process involves the encapsulation of chemotherapeutics through host-guest inclusion complexation between the β-cyclodextrin cavity and the anticancer drug. Next, the chain interaction of the PNIPAAm segment at elevated temperature drives the drug-loaded β-CD- g-(PNIPAAm- b-POEGA) x/PTX inclusion complex to hierarchically self-assemble into nanosized supramolecular assemblies at 37 °C, whereas the presence of poly(ethylene glycol) (PEG) chains in the distal end of the star-like copolymer arms impart enhanced stability to the self-assembled structure. More interestingly, this supramolecular host-guest nanocomplex promoted the enhanced cellular uptake of chemotherapeutics in MDR-1 up-regulated drug resistant cancer cells and exhibited high therapeutic efficacy for suppressing drug resistant tumor growth in an in vivo mouse model, due to the increased stability, improvement in aqueous solubility, enhanced cellular uptake, and partial membrane pump impairment by taking the advantage of PEGylation and supramolecular complex between this star-like copolymer and chemotherapeutics. This work signifies that temperature-sensitive PEGylated supramolecular nanocarriers with good biocompatibility are effective in combating MDR-1 mediated drug resistance in both in vitro and in vivo models, which is of significant importance for the advanced drug delivery platform designed to combat drug resistant cancer.

Brønsted Acid-Catalyzed Carbonyl-Olefin Metathesis inside a Self-Assembled Supramolecular Host.

Carbonyl-olefin metathesis represents a powerful yet underdeveloped method for the formation of carbon-carbon bonds. So far, no Brønsted acid based method for the catalytic carbonyl-olefin metathesis has been described. Herein, a cocatalytic system based on a simple Brønsted acid (HCl) and a self-assembled supramolecular host is presented. The developed system compares well with the current benchmark catalyst for carbonyl-olefin metathesis in terms of substrate scope and yield of isolated product. Control experiments provide strong evidence that the reaction proceeds inside the cavity of the supramolecular host. A mechanistic probe indicates that a stepwise reaction mechanism is likely.

Origins of Large Rate Enhancements in the Nazarov Cyclization Catalyzed by Supramolecular Encapsulation

The self-assembled supramolecular host [Ga4L6](12-) (1; L=N,N-bis(2,3-dihydroxybenzoyl)-1,5-diaminonaphthalene) catalyzes the Nazarov cyclization of 1,3-pentadienols with extremely high levels of efficiency. The catalyzed reaction proceeds at a rate over a million times faster than that of the background reaction, an increase comparable to those observed in some enzymatic systems. A detailed study was conducted to elucidate the reaction mechanism of both the catalyzed and uncatalyzed Nazarov cyclization of pentadienols. Kinetic analysis and (18)O-exchange experiments implicate a mechanism, in which encapsulation, protonation, and water loss from substrate are reversible, followed by irreversible electrocyclization. Although electrocyclization is rate determining in the uncatalyzed reaction, the barrier for water loss and for electrocyclization are nearly equal in the assembly-catalyzed reaction. Analysis of the energetics of the catalyzed and uncatalyzed reaction revealed that transition-state stabilization contributes significantly to the dramatically enhanced rate of the catalyzed reaction.

1H NMR Chemical Shift Calculations as a Probe of Supramolecular Host–Guest Geometry

The self-assembled supramolecular host [Ga(4)L(6)](12-) (1; L = 1,5-bis[2,3-dihydroxybenzamido]naphthalene) can encapsulate cationic guest molecules within its hydrophobic cavity and catalyze the chemical transformations of bound guests. The cavity of host 1 is lined with aromatic naphthalene groups, which create a magnetically shielded interior environment, resulting in upfield shifted (1-3 ppm) NMR resonances for encapsulated guest molecules. Using gauge independent atomic orbital (GIAO) DFT computations, we show that (1)H NMR chemical shifts for guests encapsulated in 1 can be efficiently and accurately calculated and that valuable structural information is obtained by comparing calculated and experimental chemical shifts. The (1)H NMR chemical shift calculations are used to map the magnetic environment of the interior of 1, discriminate between different host-guest geometries, and explain the unexpected downfield chemical shift observed for a particular guest molecule interacting with host 1.

High-Precision Measurement of Isotope Effects on Noncovalent Host−Guest Interactions

The self-assembled supramolecular host [Ga(4)L(6)](12-) can bind cationic guest molecules to both the interior and exterior of the host assembly through noncovalent interactions. Very small equilibrium isotope effects (EIEs) have been precisely measured for the association of benzyltrimethylphosphonium isotopologues to the host exterior surface by adapting an NMR titration method originally developed by the Perrin group for measuring isotope effects on acidity constants. Deuteration of the phosphonium methyl groups was found to have a larger EIE than deuteration at the ring and benzyl positions, suggesting subtle differences in the noncovalent interactions between the host exterior and different guest C-H/D bonds. The application of this method to the measurement of EIEs on noncovalent host-guest interactions demonstrates the generality of this NMR technique in precisely measuring relative equilibrium constants.

Proton-Mediated Chemistry and Catalysis in a Self-Assembled Supramolecular Host

Synthetic supramolecular host assemblies can impart unique reactivity to encapsulated guest molecules. Synthetic host molecules have been developed to carry out complex reactions within their cavities, despite the fact that they lack the type of specifically tailored functional groups normally located in the analogous active sites of enzymes. Over the past decade, the Raymond group has developed a series of self-assembled supramolecules and the Bergman group has developed and studied a number of catalytic transformations. In this Account, we detail recent collaborative work between these two groups, focusing on chemical catalysis stemming from the encapsulation of protonated guests and expanding to acid catalysis in basic solution. We initially investigated the ability of a water-soluble, self-assembled supramolecular host molecule to encapsulate protonated guests in its hydrophobic core. Our study of encapsulated protonated amines revealed rich host-guest chemistry. We established that self-exchange (that is, in-out guest movement) rates of protonated amines were dependent on the steric bulk of the amine rather than its basicity. The host molecule has purely rotational tetrahedral (T) symmetry, so guests with geminal N-methyl groups (and their attendant mirror plane) were effectively desymmetrized; this allowed for the observation and quantification of the barriers for nitrogen inversion followed by bond rotation. Furthermore, small nitrogen heterocycles, such as N-alkylaziridines, N-alkylazetidines, and N-alkylpyrrolidines, were found to be encapsulated as proton-bound homodimers or homotrimers. We further investigated the thermodynamic stabilization of protonated amines, showing that encapsulation makes the amines more basic in the cavity. Encapsulation raises the effective basicity of protonated amines by up to 4.5 pK(a) units, a difference almost as large as that between the moderate and strong bases carbonate and hydroxide. The thermodynamic stabilization of protonated guests was translated into chemical catalysis by taking advantage of the potential for accelerating reactions that take place via positively charged transition states, which could be potentially stabilized by encapsulation. Orthoformates, generally stable in neutral or basic solution, were found to be suitable substrates for catalytic hydrolysis by the assembly. Orthoformates small enough to undergo encapsulation were readily hydrolyzed by the assembly in basic solution, with rate acceleration factors up to 3900 compared with those of the corresponding uncatalyzed reactions. Furthering the analogy to enzymes that obey Michaelis-Menten kinetics, we observed competitive inhibition with the inhibitor NPr(4)(+), thereby confirming that the interior cavity of the assembly was the active site for catalysis. Mechanistic studies revealed that the assembly is required for catalysis and that the rate-limiting step of the reaction involves proton transfer from hydronium to the encapsulated substrate. Encapsulation in the assembly changes the orthoformate hydrolysis from an A-1 mechanism (in which decomposition of the protonated substrate is the rate-limiting step) to an A-S(E)2 mechanism (in which proton transfer is the rate-limiting step). The study of hydrolysis in the assembly was next extended to acetals, which were also catalytically hydrolyzed by the assembly in basic solution. Acetal hydrolysis changed from the A-1 mechanism in solution to an A-2 mechanism inside the assembly, where attack of water on the protonated substrate is rate limiting. This work provides rare examples of assembly-catalyzed reactions that proceed with substantial rate accelerations despite the absence of functional groups in the cavity and with mechanisms fully elucidated by quantitative kinetic studies.

Encapsulation and characterization of proton-bound amine homodimers in a water-soluble, self-assembled supramolecular host

Cyclic amines can be encapsulated in a water-soluble self-assembled supramolecular host upon protonation. The hydrogen-bonding ability of the cyclic amines, as well as the reduced degrees of rotational freedom, allows for the formation of proton-bound homodimers inside of the assembly that are otherwise not observable in aqueous solution. The generality of homodimer formation was explored with small N-alkyl aziridines, azetidines, pyrrolidines, and piperidines. Proton-bound homodimer formation is observed for N-alkylaziridines (R = methyl, isopropyl, tert-butyl), N-alkylazetidines (R = isopropyl, tert-butyl), and N-methylpyrrolidine. At high concentration, formation of a proton-bound homotrimer is observed in the case of N-methylaziridine. The homodimers stay intact inside the assembly over a large concentration range, thereby suggesting cooperative encapsulation. Both G3(MP2)B3 and G3B3 calculations of the proton-bound homodimers were used to investigate the enthalpy of the hydrogen bond in the proton-bound homodimers and suggest that the enthalpic gain upon formation of the proton-bound homodimers may drive guest encapsulation.

The Acid Hydrolysis Mechanism of Acetals Catalyzed by a Supramolecular Assembly in Basic Solution

A self-assembled supramolecular host catalyzes the hydrolysis of acetals in basic aqueous solution. The mechanism of hydrolysis is consistent with the Michaelis-Menten kinetic model. Further investigation of the rate-limiting step of the reaction revealed a negative entropy of activation (DeltaS(double dagger) = -9 cal mol(-1) K(-1)) and an inverse solvent isotope effect (k(H(2)O)/k(D(2)O) = 0.62). These data suggest that the mechanism of hydrolysis that takes place inside the assembly proceeds through an A-2 mechanism, in contrast to the A-1 mechanism operating in the uncatalyzed reaction. Comparison of the rates of acetal hydrolysis in the assembly with the rate of the reaction of unencapsulated substrates reveals rate accelerations of up to 980 over the background reaction for the substrate 1,1-diethoxyethane.

Supramolecular Catalysis of Orthoformate Hydrolysis in Basic Solution: An Enzyme-Like Mechanism

A water-soluble self-assembled supramolecular host molecule catalyzes the hydrolysis of orthoformates in basic solution. Comparison of the rate constants of the catalyzed and uncatalyzed reactions for hydrolysis displays rate accelerations of up to 3900 for tri- n-propyl orthoformate. Kinetic analysis shows that the mechanism of hydrolysis with the supramolecular host obeys the Michaelis-Menten model. Mechanistic studies, including (13)C-labeling experiments, revealed that the resting state of the catalytic system is the neutral substrate encapsulated in the host. Activation parameters for the k cat step of the reaction revealed that upon substrate encapsulation in the assembly, the entropy of activation becomes more negative in contrast to the uncatalyzed reaction. Furthermore, solvent isotope effects reveal a normal k(H 2O)/ k(D 2O) = 1.6, confirming an A-S E2 mechanism in which proton transfer occurs in the rate-limiting step. This is in contrast with the A1 mechanism of the uncatalyzed reaction in which decomposition of the protonated substrate is rate-limiting.

Simultaneously bound guests and chiral recognition: a chiral self-assembled supramolecular host encapsulates hydrophobic guests

Driven by the hydrophobic effect, a water-soluble, chiral, self-assembled supramolecular host is able to encapsulate hydrophobic organic guests in aqueous solution. Small aromatics can be encapsulated in the supramolecular assembly, and the simultaneous encapsulation of multiple species is observed in many cases. The molecular host assembly is able to recognize different substitutional isomers of disubstituted benzenes with ortho substitution leading to the encapsulation of two guests, but meta or para substitution leading to the encapsulation of only one guest. The scope of hydrophobic guest encapsulation is further explored with chiral natural products. Upon encapsulation of chiral molecules into the racemic host, diastereomeric host–guest complexes are formed with observed diastereoselectivities of up to 78:22 in the case of fenchone.