Conformation of Oligocholate Foldamers with 4-Aminobutyroyl Spacers

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.

Tracking nucleic acid nanocapsule assembly, cellular uptake and disassembly using a novel fluorescently labeled surfactant

Intracellular trafficking and delivery of nucleic acids is an area of growing interest, particularly as it relates to therapeutic applications. Spectroscopic methods have been used to observe and quantitatively measure the delivery of oligonucleotides both in vitro and in vivo. Herein we demonstrate the use of a new fluorophore labeled surfactant presenting a solvatochromatic chromophore for tracking the assembly and degradation of a hybrid biomaterial we refer to as a nucleic acid nanocapsule (NAN). We show that the surfactant enables critical micelle concentration determination, monitoring of NAN disassembly in vitro, and the ability to track the cellular movement and activity of surfactant–oligonucleotide conjugates in cells when coupled with quantitative PCR analysis.

By introducing a solvatochromatic chromophore to a DNA–surfactant conjugate, the dynamics of its assembly and disassembly can be monitored using fluorescence spectroscopy.

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.

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.

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.

Protein-mimetic, molecularly imprinted nanoparticles for selective binding of bile salt derivatives in water

A tripropargylammonium surfactant with a methacrylate-terminated hydrophobic tail was combined with a bile salt derivative, divinyl benzene (DVB), and a photo-cross-linker above its critical micelle concentration (CMC). Surface-cross-linking with a diazide, surface-functionalization with an azido sugar derivative, and free-radical-core-cross-linking under UV irradiation yielded molecularly imprinted nanoparticles (MINPs) with template-specific binding pockets. The MINPs resemble protein receptors in size, complete water-solubility, and tailored binding sites in their hydrophobic cores. Strong and selective binding of bile salt derivatives was obtained, depending on the cross-linking density of the system.

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.