Molecular Recognition of Zwitterions: Enhanced Binding and Selective Recognition of Miltefosine by a Squaramide-Based Host

Molecular Recognition of Zwitterions with Artificial Receptors

Many biomolecules exist as internal ion pairs or zwitterions within a biologically relevant pH range. Despite their importance, the molecular recognition of this type of systems is specially challenging due to their strong solvation in aqueous media, and their trend to form folded or self-assembled structures by pairing of charges of different sign. In this Minireview, we will discuss the molecular recognition of zwitterions using non-natural, synthetic receptors. This contribution does not intend to make a full in-depth revision of the existing research in the field, but a personal overview with selected representative examples from the recent literature.

Self-assembly of amphiphilic aryl-squaramides in water driven by dipolar π–π interactions

Amphiphilic aryl-squaramides self-assemble via unprecedented dipolar π–π interactions forming well-defined supramolecular aggregates and self-consistent hydrogels in water

Surface Modification of Pseudoboehmite-Coated Aluminum Plates with Squaramic Acid Amphiphiles

The functionalization of interfaces has become very important for the protection or modification of metal (metal oxides) surfaces. The functionalization of aluminum is particularly interesting because of its relevance in fabricating components for electronic devices. In this work, the utilization of squaramic acids for the functionalization of aluminum substrates is reported for the first time. The physicochemical properties of the interfaces rendered by n-alkyl squaramic acids on aluminum metal substrates coated with pseudoboehmite [Al(O)x(OH)y] layers are characterized by contact angle, grazing-angle Fourier-transform infrared spectroscopy, atomic force microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and matrix-assisted laser desorption ionization time-of-flight. Moreover, we could confirm the squaramic functionalization of the substrates by diffuse reflectance UV-vis spectroscopy, which cannot be used for the characterization of UV-vis-inactive substrates such as carboxylates and phosphonates, commonly used for coating metallic surfaces. Remarkably, the results of sorption experiments indicate that long-chain alkyl squaramic acid desorbs from activated-aluminum substrates at a reduced rate compared to palmitic acid, a carboxylic acid frequently used for the functionalization of metal oxide surfaces. Theoretical calculations indicate that the improved anchoring properties of squaramic acids over carboxylates are probably due to the formation of additional hydrogen bonding interactions on the interface. Accordingly, we propose N-alkyl squaramic acids as new moieties for efficient functionalization of metal oxides.

Synthesis and supramolecular self-assembly of glutamic acid-based squaramides

We describe the preparation and characterization of two new unsymmetrical squaramide-based organogelators. The synthesis of the compounds was carried out by subsequent amine condensations starting from dimethyl squarate. The design of the gelators involved a squaramide core connected on one side to a long aliphatic chain and on the other side to a glutamic acid residue. The gelator bearing the free carboxylic groups showed a lower gelation capacity than its precursor diester derivative. Some selected gels were further studied by infrared spectroscopy, rheology and electron microscopy. Critical gelation concentrations and gel-to-sol transition temperatures were also determined for each case. In addition, the superior squaramide diester gelator was compared with an analogue triazole-based gelator in terms of critical gelation concentration, gelation kinetics and thermal phase transition.

Selective recognition of quaternary ammonium ions and zwitterions by using a biomimetic bis-calix[6]arene-based receptor

A biomimetic bis-calix[6]arene binds cationic and zwitterionic species with a high selectivity for carbamylcholine.

Self-assembled fibrillar networks of a multifaceted chiral squaramide: supramolecular multistimuli-responsive alcogels

Chiral N,N'-disubstituted squaramide has been found to undergo self-assembly in a variety of alcoholic solvents at low concentrations leading to the formation of novel nanostructured supramolecular alcogels. The gels responded to thermal, mechanical, optical and chemical stimuli. Solubility studies, gelation ability tests and computer modeling of a series of structurally related squaramides proved the existence of a unique combination of non-covalent molecular interactions and favorable hydrophobic/hydrophilic balance in that drive the anisotropic growth of alcogel networks. The results have also revealed a remarkable effect of ultrasound on both the gelation kinetics and the properties of the alcogels.

Synthesis of luminescent squaramide monoesters: cytotoxicity and cell imaging studies in HeLa cells

Luminescent squaramide monoesters functionalised with fluorophore groups have been explored as cytotoxic and imaging agents. The biodistribution behaviour differs depending on the fluorescent moiety; lysosomal and nuclear localisation have been observed.

Cell uptake and localization studies of squaramide based fluorescent probes

Cell internalization is a major issue in drug design. Although squaramide-based compounds are receiving much attention because of their interesting bioactivity, cell uptake and trafficking within cells of this type of compounds are still unknown. In order to monitor the cell internalization process of cyclosquaramide compounds we have prepared two fluorescent probes by covalently linking a fluorescent dye (BODIPY derivative or fluorescein) to a noncytotoxic cyclosquaramide framework. These two probes (C2-BDP and C2-FITC) rapidly internalize across live cell membranes through endocytic receptor-mediated mechanisms. Due to its higher fluorescence and photochemical stability, C2-BDP is a superior dye than C2-FITC. C2-BDP remains sequestered in late endosomes allowing their fast and selective imaging in various live cell types. Cyclosquaramide-cell membrane interactions facilitate cell uptake and have been investigated by binding studies in solution as well as in live cells. Cyclosquaramide 1 (C2-BDP) can be used as a highly fluorescent probe for the rapid and selective imaging of late endosomes in live cells.