Synthetic pores with sticky π-clamps

Bioinspired multi-block molecules

Elaborately designed synthetic multiblock molecules and copolymers are able to undergo folding like biological macromolecules and form controlled and compartmentalized self-assemblies that exert characteristic functions in solution, the crystalline state, and membranous media.

Flower-like supramolecular self-assembly of phosphonic acid appended naphthalene diimide and melamine

Diverse supramolecular assemblies ranging from nanometres to micrometers of small aromatic π-conjugated functional molecules have attracted enormous research interest in light of their applications in optoelectronics, chemosensors, nanotechnology, biotechnology and biomedicines. Here we study the mechanism of the formation of a flower-shaped supramolecular structure of phosphonic acid appended naphthalene diimide with melamine. The flower-shaped assembly formation was visualised by scanning electron microscope (SEM) and transmission electron microscopy (TEM) imaging, furthermore, XRD and DLS used to determined mode of aggregation. Characteristically, phosphonic acid-substituted at imide position of NDIs possess two important properties resulting in the formation of controlled flower-like nanostructures: (i) the aromatic core of the NDI which is designed to optimize the dispersive interactions (π-π stacking and van der Waals interactions) between the cores within a construct and (ii) phosphonic acid of NDI interact with malamine through molecular recognition i.e. strong hydrogen-bonding (H-bonding). We believe such arrangements prevent crystallization and favour the directional growth of flower-like nanostructure in 3D fashion. These works demonstrate that complex self-assembly can indeed be attained through hierarchical non-covalent interactions of two components. Furthermore, flower-like structures built from molecular recognition by these molecules indicate their potential in other fields if combined with other chemical entities.

Influence of perylenediimide-pyrene supramolecular interactions on the stability of DNA-based hybrids: Importance of electrostatic complementarity

Aromatic π-π stacking interactions are ubiquitous in nature, medicinal chemistry and materials sciences. They play a crucial role in the stacking of nucleobases, thus stabilising the DNA double helix. The following paper describes a series of chimeric DNA-polycyclic aromatic hydrocarbon (PAH) hybrids. The PAH building blocks are electron-rich pyrene and electron-poor perylenediimide (PDI), and were incorporated into complementary DNA strands. The hybrids contain different numbers of pyrene-PDI interactions that were found to directly influence duplex stability. As the pyrene-PDI ratio approaches 1:1, the stability of the duplexes increases with an average value of 7.5 °C per pyrene-PDI supramolecular interaction indicating the importance of electrostatic complementarity for aromatic π-π stacking interactions.

Amino acid derivatized arylenediimides: a versatile modular approach for functional molecular materials

Nature's elegant molecular designs and their assemblies with specific structure-property correlations have inspired researchers to design and develop bio-mimics for advanced functional applications. To realize such advanced molecular materials, naturally evolved amino acids are arguably the ideal auxiliaries due to their remarkable molecular/chiral recognition and distinctive sequence specific self-assembling properties. Over the years, this modular approach of derivatizing naphthalenediimides (NDIs) and perylenediimides (PDIs) with amino acids and peptides have resulted in several hitherto unknown molecular assemblies with phenomenal impact on their performance. Derivatization with versatile arylenediimides is especially interesting due to their wide spread applications in fields ranging from biomedicine to electronics. Herein some of these seminal reports of this rapidly emerging field and the design principles embraced are discussed.

Recent synthetic transport systems

This critical review covers progress with synthetic transport systems, particularly ion channels and pores, between January 2006 and December 2009 in a comprehensive manner. This is the third part of a series launched in the year 2000, covering a rich collection of structural and functional motifs that should appeal to a broad audience of non-specialists, including to organic, biological, supramolecular and polymer chemists. Impressive breakthroughs have been achieved over the past four years in part because of a fruitful expansion toward new types of interactions, including metal-organic, π-π, aromatic electron donor-acceptor, anion-π or anion-macrodipole interactions as well as dynamic covalent bonds (169 references).

Artificial beta-barrels

In biology, beta-barrels, cylindrically rolled-up forms of beta-sheets, are ubiquitous structural motifs within various binding proteins, pores, and enzymes. This biological multifunctionality suggested that synthetic artificial beta-barrels would provide access to many different functions beyond the limitations of peptide chemistry. Unlike the relative ease of formation of synthetic (de novo) alpha-helix bundles, the synthesis of artificial beta-barrels remains a challenge. To bypass the folding problems involved, we have employed "unfoldable" rigid-rod scaffolds as privileged staves (staves are the wood strips that form the sides of macroscopic barrels); the resulting barrel-stave supramolecules exhibit their expected multifunctionality. Several "rigid rod" beta-barrels that act as receptors, ion channels, pores, catalysts, and sensors have been prepared and studied. The most recent topic of interest concerns the use of artificial beta-barrels as multicomponent sensors ("artificial tongues") in complex analyte matrices. For multicomponent sensing, we have designed artificial beta-barrels to form pores that can open and close in response to chemical stimulation within lipid bilayers. With use of fluorogenic vesicles, changes in pore activity are readily detectable with either the naked eye or multiwell screening formats. The varying responsiveness to substrates and products makes synthetic pores versatile detectors of chemical reactions, of the activity of the enzymes that catalyze these reactions, and of their inhibitors. In sensing applications, the "perfect" selectivity of enzymes is exploited to generate analyte-specific signals. Reactive signal amplifiers are then covalently linked to the products of enzymatic signal generation to enhance their pore blockage potency. With the help of signal generators and amplifiers, we have employed artificial beta-barrel pores to sense sweet (sucrose, lactose), sour (acetate, lactate, citrate), and umami ("deliciousness", glutamate) components in various food samples. This breakthrough naturally led us to design and synthesize refined pores for advanced sensing applications. We have developed methods to build guest-binding sites not only at internal and external barrel surfaces but also near the core or near the periphery of the pore. Further refinements include the introduction of asymmetric staves for voltage gating and anchoring of the pore at the membrane-water interface.

Controllable biomolecule release from self-assembled organic nanotubes with asymmetric surfaces: pH and temperature dependence

The release behavior of fluorescent dyes, oligo DNAs and spherical proteins from self-assembled organic nanotubes having 7-9 nm inner diameters has been studied in terms of novel nanocontainers with high-axial ratios. Both much smaller inner diameters and asymmetric inner and outer surfaces are characteristic of the nanotubes. The acid-dissociation constant (pKa) of the amino groups located at the inner surface and the thermal phase transition temperature (Tg-l) of the nanotube were evaluated based on the pH titration and variable-temperature circular dichroism (CD) spectroscopic experiments, respectively. Each guest was slowly released from both open ends of the nanotube under weak alkaline conditions (pH 8.5), as a result of the decrease in electrostatic attraction between the inner surface and the guests. Elevated temperatures above the obtained Tg-l converted the monolayer membrane of the nanotube from a solid state to a fluid one, promoting the remarkably fast release of the guests. The unique release properties of the nanotube as a nanocontainer with two terminal open ends were compared with those of liposomes that posses a closed hollow space covered with fluid bilayer membranes.

Artificial tongues and leaves

The objective with synthetic multifunctional nanoarchitecture is to create large suprastructures with interesting functions. For this purpose, lipid bilayer membranes or conducting surfaces have been used as platforms and rigid-rod molecules as shape-persistent scaffolds. Examples for functions obtained by this approach include pores that can act as multicomponent sensors in complex matrices or rigid-rod π-stack architecture for artificial photosynthesis and photovoltaics.

Synthetic pores with reactive signal amplifiers as artificial tongues

The sensation of taste is mediated by activation or deactivation of transmembrane pores. Artificial stimulus-responsive pores are enormously appealing as sensor components because changes in their activity are readily detectable in many different ways. However, the detection of multiple components in complex matrices (such as foods) with one pore sensor has so far remained elusive because the specificity necessary for sensing a target compound in complex mixtures is incompatible with the broad applicability needed for the detection of multiple components. Here, we present synthetic pores that, like our tongues, can sense flavours in food and in addition make them visibly detectable. Differential sensing and pattern recognition are solutions based on empirical and biomimetic approaches. They have been explored with synthetic receptor arrays and electronic tongues. In contrast, our approach is non-empirical as it exploits reactive amplifiers that covalently capture elusive analytes after enzymatic signal generation and drag them into synthetic pores for blockage. Reactive amplification proved to be highly sensitive and adaptable to various analytes and pores. Moreover, it can be combined with reactive filtration for minimizing interference. The system was tested on real food samples for detection of sucrose, lactose, lactate, acetate, citrate and glutamate to demonstrate the feasibility of these synthetic pores as universal sensors.