A pH Switch in Supramolecular Polymeric Capsules

Multilayer capsules made of weak polyelectrolytes: a review on the preparation, functionalization and applications in drug delivery

Multilayer capsules have been of great interest for scientists and medical communities in multidisciplinary fields of research, such as drug delivery, sensing, biomedicine, theranostics and gene therapy. The most essential attributes of a drug delivery system are considered to be multi-functionality and stimuli responsiveness against a range of external and internal stimuli. Apart from the highly explored strong polyelectrolytes, weak polyelectrolytes offer great versatility with a highly controllable architecture, unique stimuli responsiveness and easy tuning of the properties for intracellular delivery of cargo. This review describes the progress in the preparation, functionalization and applications of capsules made of weak polyelectrolytes or their combination with biopolymers. The selection of a sacrificial template for capsule formation, the driving forces involved, the encapsulation of a variety of cargo and release based on different internal and external stimuli have also been addressed. We describe recent perspectives and obstacles of weak polyelectrolyte/biopolymer systems in applications such as therapeutics, biosensing, bioimaging, bioreactors, vaccination, tissue engineering and gene delivery. This review gives an emerging outlook on the advantages and unique responsiveness of weak polyelectrolyte based systems that can enable their widespread use in potential applications.

Microfluidics for real-time direct monitoring of self- and co-assembly biomolecular processes

Molecular self-assembly is a major approach for the fabrication of functional supramolecular nanomaterials. This dynamic, straightforward, bottom-up procedure may result in the formation of various architectures at the nano-scale, with remarkable physical and chemical characteristics. Biological and bio-inspired building blocks are especially attractive due to their intrinsic tendency to assemble into well-organized structures, as well as their inherent biocompatibility. To further expand the morphological diversity, co-assembly methods have been developed, allowing to produce alternative unique architectures, enhanced properties, and improved structural control. However, in many cases, mechanistic understanding of the self- and co-assembly processes is still lacking. Microfluidic techniques offer a set of exclusive tools for real-time monitoring of biomolecular self-organization, which is crucial for the study of such dynamic processes. Assembled nuclei, confined by micron-scale pillars, could be subjected to controlled environments aiming to assess the effect of different conditions on the assembly process. Other microfluidics setups can produce droplets at a rate of over 100 s^-1, with volumes as small as several picoliters. Under these conditions, each droplet can serve as an individual pico/nano-reactor allowing nucleation and assembly. These processes can be monitored, analyzed and imaged, by various techniques including simple bright-field microscopy. Elucidating the mechanism of such molecular events may serve as a conceptual stepping-stone for the rational control of the resulting physicochemical properties.

Readily functionalized AAA-DDD triply hydrogen-bonded motifs

Herein we present a new, readily functionalized AAA-DDD hydrogen bond array. A novel AAA monomeric unit (3a-b) was obtained from a two-step synthetic procedure starting with 2-aminonicotinaldehyde via microwave radiation (overall yield of 52-66%). 1H NMR and fluorescence spectroscopy confirmed the complexation event with a calculated association constant of 1.57 × 107 M-1. Likewise, the usefulness of this triple hydrogen bond motif in supramolecular polymerization was demonstrated through viscosity measurements in a crosslinked supramolecular alternating copolymer.

Ring/Chain Morphology Control in Overall-Neutral, Internally Ion-Paired Supramolecular Polymers

The self-assembly of internally ion-paired, neutral AA/BB-type supramolecular polymers composed of complementary di-ionizable homoditopic pairs of monomers is reported. Host-to-guest double-proton transfer mediates the recognition between bis-calix[5]arenedicarboxylic acids and α,ω-diaminoalkanes to yield cyclic, doughnut-shaped assemblies with morphologies (i.e., cyclic vs. linear) that can be controlled by means of external chemical stimuli. The behavior of these intriguing aggregates, both in solution and on surfaces, has been investigated by a combination of ¹ H and DOSY NMR spectroscopy, light-scattering, and atomic force microscopy.

Supramolecular "Big Bang" in a Single-Ionic Surfactant/Water System Driven by Electrostatic Repulsion: From Vesicles to Micelles

In aqueous solution, dimethyldi-n-octylammonium chloride, [DiC₈][Cl], spontaneously forms dimers at low concentrations (1-10 mM) to decrease the strength of the hydrophobic-water contact. Dimers represent ideal building blocks for the abrupt edification of vesicles at 10 mM. These vesicles are fully characterized by dynamic and static light scattering, self-diffusion nuclear magnetic resonance, and freeze-fracture transmission electron microscopy. An increase in concentration leads to electrostatic repulsion between vesicles that explode into small micelles at 30 mM. These transitions are detected by means of surface tension, conductivity, and solubility of hydrophobic solutes as well as by isothermal titration microcalorimetry. These unusual supramolecular transitions emerge from the surfactant chemical structure that combines two contradictory features: (i) the double-chain structure tending to form low planar aggregates with low water solubility and (ii) the relatively short chains giving high hydrophilicity. The well-balanced hydrophilic-hydrophobic character of [DiC₈][Cl] is then believed to be at the origin of the unusual supramolecular sequence offering new opportunities for drug delivery systems.

Calix[4]arene-based low molecular mass gelators to form gels in organoalkoxysilanes

Organoalkoxysilanes-based molecular gels for melting-free deposition molding, potential smart materials for 3D printing.

Gated molecular baskets

In this review, we describe the construction of gated molecular baskets, discuss their mechanism of action in regulating the exchange of guests and illustrate the potential of these concave hosts to act as catalysts for controlling chemical reactions. Importantly, a number of computational and experimental studies have suggested that gated baskets ought to unfold their gates at the rim for permitting the passage of guests to/from their inner space. These dynamic hosts are therefore offered as useful models for investigating the process of gating in artificial systems. Furthermore, gated baskets should permit examining the benefit of controlling the rate by which reactants access a gated catalyst for promoting chemical reactions occurring in its confined space.

Structure-activity relationship of cyclodextrin/biocidal double-tailed ammonium surfactant host-guest complexes: towards a delivery molecular mechanism?

In aqueous solution, the biocidal double-tailed cationic surfactant, di-n-decyl-dimethyl-ammonium chloride can form inclusion complexes with various cyclodextrins (alpha-CD, beta-CD, gamma-CD, HP-alpha-CD, HP-beta-CD and CM-beta-CD). A physicochemical study has been performed to investigate the association parameters of these host-guest complexes by combining the use of ammonium and chloride selective electrodes, NMR spectroscopy and molecular modeling. stoichiometries, equilibrium constants and geometries were determined by resorting to a specific algorithm. The antimicrobial activity of the encapsulated ammonium surfactant was compared with that of the free ammonium showing three different behaviors depending on the cyclodextrin. The close relationship between the complex structure and the biocidal activity is used to propose a delivery molecular mechanism.

Stepwise aggregation of dimethyl-di-n-octylammonium chloride in aqueous solutions: from dimers to vesicles

The self-aggregation of dimethyl-di-n-octylammonium chloride, in diluted aqueous solutions, was studied with various experimental and theoretical techniques: zetametry, conductimetry, dimethyl-di-n-octylammonium and chloride-selective electrodes, tensiometry, NMR spectroscopy ((1)H and DOSY), and molecular modeling (PM3 and molecular dynamic). The combination of the data obtained by these techniques led us to propose a stepwise aggregation process with increasing concentration: dimers (0.2-10 mM), bilayers (10-30 mM), and finally vesicles (>30 mM).

C-lysine conjugates: pH-controlled light-activated reagents for efficient double-stranded DNA cleavage with implications for cancer therapy

Double-stranded DNA cleavage of light-activated lysine conjugates is strongly enhanced at the slightly acidic pH (<7) suitable for selective targeting of cancer cells. This enhancement stems from the presence of two amino groups of different basicities. The first amino group plays an auxiliary role by enhancing solubility and affinity to DNA, whereas the second amino group, which is positioned next to the light-activated DNA cleaver, undergoes protonation at the desired pH threshold. This protonation results in two synergetic effects which account for the increased DNA-cleaving ability at the lower pH. First, lysine conjugates show tighter binding to DNA at the lower pH, which is consistent with the anticipated higher degree of interaction between two positively charged ammonium groups with the negatively charged phosphate backbone of DNA. Second, the unproductive pathway which quenches the excited state of the photocleaver through intramolecular electron transfer is eliminated once the donor amino group next to the chromophore is protonated. Experiments in the presence of traps for diffusing radicals show that reactive oxygen species do not contribute significantly to the mechanism of DNA cleavage at the lower pH, which is indicative of tighter binding to DNA under these conditions. This feature is valuable not only because many solid tumors are hypoxic but also because cleavage which does not depend on diffusing species is more localized and efficient. Sequence-selectivity experiments suggest combination of PET and base alkylation as the chemical basis for the observed DNA damage. The utility of these molecules for phototherapy of cancer is confirmed by the drastic increase in toxicity of five conjugates against cancer cell lines upon photoactivation.

Calix[4]arene daisychains

Generic calix[4]arenes became readily accessible in the late 70s. With their potential eight anchoring points, their utility for the production of sophisticated, highly functionalised macrocyclic molecules was rapidly recognised. While most studies in calixarene chemistry have focused on monocalixarene derivatives, there is now an increasing interest in developing multicalixarene compounds, especially those made of several linearly-arranged calix[4]arene units, the first examples of which were reported in 1989. This critical review will present the most important synthetic routes to such molecules together with an analysis of the properties that such cavity combinations may induce. In particular it will be shown that the nature of the links between the calixarene units plays a determinant role in the product properties and that singly-linked calixarenes can be exploited in varied applications, including those as efficient receptors of large molecules, as electrochemical and luminescent sensors in ion detection, or as new materials allowing capsule formation suitable for the storage of small guests (82 references).

Counterion-Dependent Proton-Driven Self-Assembly of Linear Supramolecular Oligomers Based on Amino-Calix[5]arene Building Blocks

Self-assembly of a calix[5]arene bearing a 12-aminododecyl pendant group on the lower rim into supramolecular oligomers through intermolecular iterative inclusion events is readily triggered by contact with acid solutions and is reversed to the amino monomer precursor by treatment with a base. 1H NMR data are consistent with the formation of head-to-tail assemblies derived from endo-cavity inclusion of the alkylammonium moiety. Diffusion NMR and light-scattering studies provide evidence for the presence of oligomers in solution and show that different counterions and concentrations result in different oligomer sizes, whereas ESI-MS and SEM investigations, respectively, indicate that self-assembly also takes place in the gas phase and in the solid state. The growth of these supramolecular oligomers is concentration-dependent; however, as a consequence of the saline nature of the monomer, it also shows a distinct counterion-dependence owing to ion-pairing/solvation effects.

A designed receptor for pH-switchable ion binding in water

Molecules with conditional (switchable) properties are of considerable fundamental interest and are potentially useful for a broad range of applications, including chemical sensing. We have prepared a novel receptor, derived from the peracylation of cyclohexane 1,3,5-trimethanol with tyrosine, that suggests that the phenol-amine hydrogen bond may be an effective structural tool in the preparation of molecules with pH-switchable conformations. This conclusion is based on several key observations. First, the proton longitudinal relaxation rates for this molecule change in a pH-dependent fashion, while those for closely related structures do not. Second, NOESY spectra for the receptor change markedly depending on pH, with the spectrum acquired at pH 9.5 displaying NOEs consistent with the calculated "closed" conformation. Third, this molecule serves as a receptor for anions and cations in aqueous solution at high pH, but not at low pH, as demonstrated by UV-vis titrations and isothermal titration calorimetry (ITC).

Controlling a recognition-mediated reaction using a pH switch

The selective recognition-mediated reaction between a nitrone bearing a urea recognition site and a maleimide bearing a proton switchable recognition site can be turned 'on' and 'off' by the addition of base and acid respectively.

Carbon dioxide and supramolecular chemistry

Concepts and techniques of supramolecular chemistry are applied to the century-old chemistry between CO2 and amines to design novel sensing systems and nanoscale, self-assembling polymeric materials and networks.

CO2 in Supramolecular Chemistry: Preparation of Switchable Supramolecular Polymers

CO2 gas was used to construct novel types of supramolecular polymers. Self-assembling nanostructures 11 and 13 were prepared, which employ both hydrogen bonding and dynamic, thermally reversible carbamate bonds. As precursors, calixarene ureas 1 and 2 were synthesized, which strongly aggregate/dimerize (K(D)>/=10(6) M(-1) per capsule) in apolar solution with the formation of self-assembling capsules 7 and linear polymeric chains 8, respectively, and also possess "CO2-philic" primary amino groups on the periphery. CO2 effectively reacts with molecules 7 and 8 in apolar solvents and cross-links them with the formation of multiple carbamate salt bridges. Oligomeric aggregate 11 and three-dimensional polymeric network 13 were prepared and characterized by 1H and 13C NMR spectroscopy. The morphology of supramolecular gel 13 was studied by scanning electron microscopy. Addition of a competitive solvent destroyed the hydrogen bonding in assembling structures 11 and 13, but did not influence the carbamate linkers; carbamate salts 12 and 14, respectively, were obtained. On the other hand, thermal release of CO2 from 11 and 13 was easily accomplished (1 h, 100 degrees C) while retaining the hydrogen-bonding capsules. Thus, three-dimensional polymeric network 13 was transformed back to linear polymeric chain 8 without breaking up. Encapsulation and storage of solvent molecules by 11 and 13 was demonstrated. This opens the way for switchable materials, which reversibly trap, store, and then release guest molecules. A two-parameter switch and control over hydrogen bonding and CO2-amine adducts was established.