Environmental Effects Dominate the Folding of Oligocholates in Solution, Surfactant Micelles, and Lipid Membranes

Environmental Modulation of Chiral Prolinamide Catalysts for Stereodivergent Conjugate Addition

Synthetic chiral catalysts generally rely on proximal functional groups or ligands for chiral induction. Enzymes often employ environmental chirality to achieve stereoselectivity. Environmentally controlled catalysis has benefits such as size and shape selectivity but is underexplored by chemists. We here report molecularly imprinted nanoparticles (MINPs) that utilized their environmental chirality to either augment or reverse the intrinsic selectivity of a chiral prolinamide cofactor. The latter ability allowed the catalyst to produce products otherwise disfavored in the conjugate addition of aldehyde to nitroalkene. The catalysis occurred in water at room temperature and afforded γ-nitroaldehydes with excellent yields (up to 94%) and ee (>90% in most cases). Up to 25:1 syn/anti and 1:6 syn/anti ratios were achieved through a combination of catalyst-derived and environmentally enabled selectivity. The high enantioselectivity of the MINP also made it possible for racemic catalysts to perform asymmetric catalysis, with up to 80% ee for the conjugate addition.

Aromatically functionalized pseudo-crown ethers with unusual solvent response and enhanced binding properties

Conformational flexibility in the host's structure is often considered detrimental to its binding. Flexible pseudo-crown ethers with aromatic donor/acceptor groups at the chain ends, however, displayed enhanced binding affinity and selectivity, particularly when the direct binding interactions were compromised by unfavorable solvents.

Tuning surface-cross-linking of molecularly imprinted cross-linked micelles for molecular recognition in water

Molecular recognition in water is an important challenge in supramolecular chemistry. Surface-core double cross-linking of template-containing surfactant micelles by the click reaction and free radical polymerization yields molecularly imprinted nanoparticles (MINPs) with guest-complementary binding sites. An important property of MINP-based receptors is the surface-cross-linking between the propargyl groups of the surfactants and a diazide cross-linker. Decreasing the number of carbons in between the two azides enhanced the binding affinity of the MINPs, possibly by keeping the imprinted binding site more open prior to the guest binding. The depth of the binding pocket can be controlled by the distribution of the hydrophilic/hydrophobic groups of the template and was found to influence the binding in addition to electrostatic interactions between oppositely charged MINPs and guests. Cross-linkers with an alkoxyamine group enabled two-stage double surface-cross-linking that strengthened the binding constants by an order of magnitude, possibly by expanding the binding pocket of the MINP into the polar region. The binding selectivity among very similar isomeric structures also improved.

Surface Ligands in the Imprinting and Binding of Molecularly Imprinted Cross-Linked Micelles

Molecular recognition in water is challenging but water-soluble molecularly imprinted nanoparticle (MINP) receptors were produced readily by double cross-linking of surfactant micelles in the presence of suitable template molecules. When the micellar surface was decorated with different polyhydroxylated ligands, significant interactions could be introduced between the surface ligands and the template. Flexible surface ligands worked better than rigid ones to interact with the polar moiety of the template, especially for those template molecules whose water-exposed surface is not properly solvated by water. The importance of these hydrophilic interactions was examined in the context of different substrates, density of the surface ligands, and surface-cross-linking density of the MINP. Together with the hydrophobic interactions in the core, the surface hydrophilic interactions can be used to enhance the binding of guest molecules in water.

Selective Binding of Folic Acid and Derivatives by Imprinted Nanoparticle Receptors in Water

Folate receptors are overexpressed on cancer cells and frequently used for targeted delivery. Creation of synthetic receptors to bind folic acid and its analogues in water, however, is challenging because of its complex hydrogen-bonding patterns and competition for hydrogen bonds from the solvent. Micellar imprinting within cross-linkable surfactants circumvented these problems because the nonpolar micellar environment strengthened the hydrogen bonds between the amide group in the surfactant and the template molecule. Incorporation of polymerizable thiouronium functional monomers further enhanced the binding through hydrogen-bond-reinforced ion pairs with the glutamate moiety of the template. The resulting imprinted micelles were able to bind folate and their analogues with submicromolar affinity and distinguish small changes in the hydrogen-bonding patterns as well as the number/position of carboxylic acids. The binding constant obtained was 2-3 orders of magnitude higher than those reported for small-molecule synthetic receptors. Our binding study also revealed interesting details in the binding. For example, the relative contributions of different segments of the molecule to the binding followed the order of carboxylates > pyrimidine ring > pyrazine ring.

Intramolecularly enhanced molecular tweezers with unusually strong binding for aromatic guests in unfavorable solvents

Molecular tweezers using aromatic interactions for binding normally work best in polar instead of nonpolar solvents due to the strong solvophobic effect in the binding.

Environmental Engineering of Pd Nanoparticle Catalysts for Catalytic Hydrogenation of CO2 and Bicarbonate

The extraordinary catalytic properties of enzymes are derived not only from their catalytic groups but also the unique properties of the active site. Tuning the microenvironment of synthetic catalysts is expected to enhance their performance if effective strategies can be developed. Interfacially cross-linked reverse micelles were prepared from three different cross-linkable surfactants. Pd nanoparticles were deposited in the core of the micelle for the catalytic hydrogenation of bicarbonate and CO₂. The catalytic performance was found to depend heavily on the nature of the headgroup of the surfactant. Quaternary ammonium-based surfactants through ion exchange could bring bicarbonate to the catalytic center, whereas tertiary amine-based surfactants worked particularly well in CO₂ hydrogenation, with turnover numbers an order of magnitude higher than that of commercially available Pd/C. The amines were proposed to bring CO₂ to the proximity of the catalysts through reversible formation of carbamate, in the nanospace of the hydrophilic core of the cross-linked reverse micelle. In the bicarbonate reduction, additional improvement of the catalysts was achieved through localized sol-gel synthesis that introduced metal oxide near the catalytic metal.

Reversible photo-gated transmembrane channel assembled from an acylhydrazone-containing crown ether triad

Ion channel transport could be reversibly regulated by irradiation of the lipid bilayer with alternating 320/365 nm light.

Conformationally switchable water-soluble fluorescent bischolate foldamers as membrane-curvature sensors

Membrane curvature is an important parameter in biological processes such as cellular movement, division, and vesicle fusion and budding. Traditionally, only proteins and protein-derived peptides have been used as sensors for membrane curvature. Three water-soluble bischolate foldamers were synthesized, all labeled with an environmentally sensitive fluorophore to report their binding with lipid membranes. The orientation and ionic nature of the fluorescent label were found to be particularly important in their performance as membrane-curvature sensors. The bischolate with an NBD group in the hydrophilic α-face of the cholate outperformed the other two analogues as a membrane-curvature sensor and responded additionally to the lipid composition including the amounts of cholesterol and anionic lipids in the membranes.

Conformationally controlled oligocholate membrane transporters: learning through water play

Controlled translocation of molecules and ions across lipid membranes is the basis of numerous biological functions. Because synthetic systems can help researchers understand the more complex biological ones, many chemists have developed synthetic mimics of biological transporters. Both systems need to deal with similar fundamental challenges. In addition to providing mechanistic insights into transport mechanisms, synthetic transporters are useful in a number of applications including separation, sensing, drug delivery, and catalysis. In this Account, we present several classes of membrane transporters constructed in our laboratory from a facially amphiphilic building block, cholic acid. Our "molecular baskets" can selectively shuttle glucose across lipid membranes without transporting smaller sodium ions. We have also built oligocholate foldamers that transiently fold into helices with internal hydrophilic binding pockets to transport polar guests. Lastly, we describe amphiphilic macrocycles, which form transmembrane nanopores in lipid bilayers through the strong associative interactions of encapsulated water molecules. In addition to presenting the different transport properties of these oligocholate transporters, we illustrate how fundamental studies of molecular behavior in solution facilitate the creation of new and useful membrane transporters, despite the large difference between the two environments. We highlight the strong conformational effect of transporters. Because the conformation of a molecule often alters its size and shape, and the distribution of functional groups, conformational control can be used rationally to tune the property of a transporter. Finally, we emphasize that, whenever water is the solvent, its unique properties--small size, strong solvation for ionic functionalities, and an extraordinary cohesive energy density (i.e., total intermolecular interactions per unit volume)--tend to become critical factors to be considered. Purposeful exploitation of these solvent properties may be essential to the success of the supramolecular process involved--this is also the reason for the "learning through water play" in the title of this Account.

Cooperatively enhanced receptors for biomimetic molecular recognition

The concept of preorganization suggests that organizing a receptor around its guest during binding is detrimental, because the cost of conformational change is assumed to be paid out of the binding energy. Although this concept has historically guided the synthesis of a great many synthetic hosts, in recent years, chemists have begun to synthesize receptors that resemble proteins in their cooperative conformational changes. Such changes could enhance the host-guest interactions, in particular if the binding of the guest triggers previously unengaged noncovalent interactions within the host. These hosts, referred to as cooperatively enhanced receptors, corroborate with their biological counterparts to support the approach of creating high-affinity receptors through the combined strategies of cooperativity and preorganization. Solvents, often the invisible participants of any solution-based supramolecular process, should be properly considered in the design of synthetic receptors, whether preorganized or cooperatively enhanced.

Effects of amphiphile topology on the aggregation of oligocholates in lipid membranes: macrocyclic versus linear amphiphiles

A macrocyclic and a linear trimer of a facially amphiphilic cholate building block were labeled with a fluorescent dansyl group. The environmentally sensitive fluorophore enabled the aggregation of the two oligocholates in lipid membranes to be studied by fluorescence spectroscopy. Concentration-dependent emission wavelength and intensity revealed a higher concentration of water for the cyclic compound. Both compounds were shown by the red-edge excitation shift (REES) to be located near the membrane/water interface at low concentrations, but the cyclic trimer was better able to migrate into the hydrophobic core of the membrane than the linear trimer. Fluorescent quenching by a water-soluble (NaI) and a lipid-soluble (TEMPO) quencher indicated that the cyclic trimer penetrated into the hydrophobic region of the membrane more readily than the linear trimer, which preferred to stay close to the membrane surface. The fluorescent data corroborated with the previous leakage assays that suggested the stacking of the macrocyclic cholate trimer into transmembrane nanopores, driven by the strong associative interactions of water molecules inside the macrocycles in a nonpolar environment.

Enhancing binding affinity by the cooperativity between host conformation and host-guest interactions

Glutamate-functionalized oligocholate foldamers bound Zn(OAc)(2), guanidine, and even amine compounds with surprisingly high affinities. The conformational change of the hosts during binding was crucial to the enhanced binding affinity. The strongest cooperativity between the conformation and guest-binding occurred when the hosts were unfolded but near the folding-unfolding transition. These results suggest that high binding affinity in molecular recognition may be more easily obtained from large hosts capable of strong cooperative conformational changes instead of those with rigid, preorganized structures.

Translocation of hydrophilic molecules across lipid bilayers by salt-bridged oligocholates

Macrocyclic oligocholates were found in a previous work (Cho, H.; Widanapathirana, L.; Zhao, Y. J. Am. Chem. Soc.2011, 133, 141-147) to stack on top of one another in lipid membranes to form nanopores. Pore formation was driven by a strong tendency of the water molecules in the interior of the amphiphilic macrocycles to aggregate in a nonpolar environment. In this work, cholate oligomers terminated with guanidinium and carboxylate groups were found to cause efflux of hydrophilic molecules such as glucose, maltotriose, and carboxyfluorescein (CF) from POPC/POPG liposomes. The cholate trimer outperformed other oligomers in the transport. Lipid-mixing assays and dynamic light scattering ruled out fusion as the cause of leakage. The strong dependence on chain length argues against random intermolecular aggregates as the active transporters. The efflux of glucose triggered by these compounds increased significantly when the bilayers contained 30 mol% cholesterol. Hill analysis suggested that the active transporter consisted of four molecules. The oligocholates were proposed to fold into "noncovalent macrocycles" by the guanidinium-carboxylate salt bridge and stack on top of one another to form similar transmembrane pores as their covalent counterparts.