Crystalline Nanoparticles of Water-Soluble Covalent Basket Cages (CBCs) for Encapsulation of Anticancer Drugs

We herein describe the preparation, assembly, recognition characteristics, and biocompatibility of novel covalent basket cage CBC-11, composed of four molecular baskets linked to four trivalent aromatic amines through amide groups. The cage is tetrahedral in shape and similar in size to small proteins (Mw =8637 g/mol) with a spacious nonpolar interior for accommodating multiple guests. While 24 carboxylates at the outer surface of CBC-11 render it soluble in aqueous phosphate buffer (PBS) at pH=7.0, the amphiphilic nature prompts its assembly into nanoparticles (d=250 nm, DLS). Cryo-TEM examination of nanoparticles revealed their crystalline nature with wafer-like shapes and hexagonally arranged cages. Nanoparticulate CBC-11 traps anticancer drugs irinotecan and doxorubicin, with each cage binding up to four drug molecules in a non-cooperative manner. The inclusion complexation resulted in nanoparticles growing in size and precipitating. In media containing mammalian cells (HCT 116, human colon carcinoma), the IC50 value of CBC-11 was above 100 μM. While this work presents the first example of a large covalent organic cage operating in water at the physiological pH and forming crystalline nanoparticles, it also demonstrates its biocompatibility and potential to act as a polyvalent binder of drugs for their sequestration or delivery.

Surface supramolecular assemblies tailored by chemical/physical and synergistic stimuli: a scanning tunneling microscopy study

Supramolecular self-assemblies formed by various non-covalent interactions can produce diverse functional networks on solid surfaces. These networks have recently attracted much interest from both fundamental and application points of view. Unlike covalent organic frameworks (COFs), the properties of the assemblies differ from each other depending on the constituent motifs. These various motifs may find diverse applications such as in crystal engineering, surface modification, and molecular electronics. Significantly, these interactions between/among the molecular tectonics are relatively weak and reversible, which makes them responsive to external stimuli. Moreover, for a liquid-solid-interface environment, the dynamic processes are amenable to in situ observation using scanning tunneling microscopy (STM). In the literature, most review articles focus on supramolecular self-assembly interactions. This review summarizes the recent literature in which stimulation sources, including chemical, physical, and their combined stimuli, cooperatively tailor supramolecular assemblies on surfaces. The appropriate design and synthesis of functional molecules that can be integrated on different surfaces permits the use of nanostructured materials and devices for bottom-up nanotechnology. Finally, we discuss synergic effect on materials science.

Photoinduced interruption of interannular cooperativity for delivery of cationic guests in water

Photoinduced decarboxylation of two hexaanionic baskets, surrounding a divalent cationic guest, reduced the interannular cooperativity (i.e. multivalency) holding the complex together to result in the release of guests.

On the encapsulation and assembly of anticancer drugs in a cooperative fashion

In this study, we report the remarkable recognition and assembly characteristics of D 3h symmetric basket 1 6- containing two adjoining and nonpolar cavities with six biocompatible GABA residues at their northern and southern termini. From the results of experimental (1H NMR, fluorescence and UV-Vis spectroscopies) and computational (MM-MC/OPLS3e) investigations, we deduced that hexaanionic 1 6- captured two molecules of anticancer drug doxorubicin 2 + in water and accommodated them in its two deep cavities. The formation of stable 1 6-⊂2 2 2+ (K a = 3 × 1012 M-2) was accompanied by the exceptional homotopic cooperativity (α = 4K 2/K 1 = 112) in which K 1 = 3.2 ± 0.8 × 105 M-1 and K 2 = 9 ± 1 × 106 M-1. Furthermore, bolaamphiphilic 1 6-⊂2 2 2+ assembled into spherical nanoparticles (DLS, cryo-TEM and TEM) possessing 41% drug loading. The preorganization of abiotic receptor 1 6- and its complementarity to 2 + have been proposed to play a part in the positive cooperativity in which ten favorable noncovalent contacts (i.e. hydrogen bonds, salt bridges, C-H···π and π-π contacts) are formed between doxorubicin and the dual-cavity host. In the case of topotecan 3 +, however, the absence of multiple and favorable basket⊂drug interactions resulted in the predominant formation of a binary 1 6- ⊂ 3 + complex (K 1 = 2.12 ± 0.01 × 104 M-1) and the negative homotopic allostery (α ≪ 1). To summarize, our study lays out a roadmap for creating a family of novel, accessible and multivalent hosts capable of complexing anticancer agents in a cooperative manner. As basket⊂drug complexes organize into highly loaded nanoparticles, the reported soft material is amenable to the bottom-up construction of stimuli-responsive nanomedicine capable of effective scavenging and/or delivery of drugs.

End-to-end conformational communication through a synthetic purinergic receptor by ligand-induced helicity switching

The long-range communication of information, exemplified by signal transduction through membrane-bound receptors, is a central biochemical function. Reversible binding of a messenger ligand induces a local conformational change that is relayed through the receptor, inducing a chemical effect typically several nanometres from the binding site. We report a synthetic receptor mimic that transmits structural information from a boron-based ligand binding site to a spectroscopic reporter located more than 2 nm away. Reversible binding of a diol ligand to the N-terminal binding site induces a screw-sense preference in a helical oligo(aminoisobutyric acid) foldamer, which is relayed to a reporter group at the remote C-terminus, communicating information about the structure and stereochemistry of the ligand. The reversible nature of boronate esterification was exploited to switch the receptor sequentially between left- and right-handed helices, while the exquisite conformational sensitivity of the helical relay allowed the reporter to differentiate even between purine and pyrimidine nucleosides as ligands.

Conformationally dynamic π-conjugation: probing structure-property relationships of fluorescent tris(N-salicylideneaniline)s

We recently reported the design and synthesis of a series of conformationally dynamic chromophores that are built on the C(3)-symmetric tris(N-salicylideneaniline) platform. This system utilizes cooperative structural folding-unfolding motions for fluorescence switching, which is driven by the assembly and disassembly of hydrogen bonds between the rigid core and rotatable peripheral part of the molecule. Here, we report detailed time-resolved spectroscopic studies to investigate the structure-property relationships of a series of functionalized tris(N-salicylideneaniline)s. Time-resolved fluorescence decay spectroscopy was applied to determine the main relaxation mechanisms of these π-extended fluorophores, and to address the effects of hydrogen bonding, steric constraints, and extension of the π-conjugation on their relaxation dynamics. Our results agree well with the conformational switching model that was previously suggested from steady-state experiments. Notably, extension of the π-conjugation from peripheral aryl groups resulted in the stabilization of the excited states, as evidenced by longer lifetimes and lower nonradiative decay constants. As a consequence, an increase in the fluorescence quantum yields was observed, which could be explained by the suppression of the torsional motions about the C-N bonds from an overall increase in the quinoid character of the excited states. A combination of time-resolved and steady-state techniques also revealed intermolecular interactions through π-π stacking at higher concentrations, which provide additional de-excitation pathways that become more pronounced in solid samples.

Dendritic molecular switch: chiral folding and helicity inversion

Appropriately designed chemical architectures can fold to adopt well-defined secondary structures without the need for structural motifs of biological origin. We have designed tris(N-salicylideneaniline)-based hyperbranched molecules that spontaneously collapse to compact three-blade propeller geometry of either (P)- or (M)-handedness. For a homologous series of compounds, a direct correlation was established between the absolute screw sense, either (P)- or (M)-, of this helical folding and the absolute configuration, either (R)- or (S)-, of the chiral alcohol groups introducing local asymmetric bias to the conformationally restricted molecular backbone. 1H NMR and CD spectroscopic studies provided significant insights into structural folding and unfolding of these chiral molecules in solution, which proceed via reversible assembly and disassembly of the C3-symmetric hydrogen-bonding network. Notably, solvents profoundly influenced this dynamic process. A strong correlation between the solvent donor number (DN) or solvent basicity (SB) parameters and the change in the Cotton effects pointed toward specific O-H...solvent interactions that drive structural unfolding and eventual refolding to apparently opposite helicity. This unusual chirality inversion process could also be induced by installation of chemical protecting groups that simulate specific solvent-solute interactions. Removal of this covalent mimic of the solvent shell restored the original screw sense of the parent molecule, thus establishing the feasibility of covalently triggered helicity inversion as a new mode of operation for chiroptical molecular switches.