Deepened chiral cavitands

Preparative scale and convenient synthesis of a water-soluble, deep cavitand

Cavitands are established tools of supramolecular chemistry and molecular recognition, and they are finding increasing application in sensing and sequestration of physiologically relevant molecules in aqueous solution. The synthesis of a water-soluble, deep cavitand is described. The route comprises six (linear) steps from commercially available precursors, and it relies on the fourfold oligomeric cyclization reaction of resorcinol with 2,3-dihydrofuran that leads to the formation of a shallow resorcinarene framework; condensation with aromatic panels, which deepens the hydrophobic binding cavity; construction of rigid urea functionalities on the upper rim; and the introduction of the water-solubilizing methylimidazolium groups on the lower rim. Late intermediates of the synthesis can be used in the preparation of congener cavitands with different properties and applications, and a sample of such a synthetic procedure is included in this protocol. Emphasis is placed on scaled-up reactions and on purification procedures that afford materials in high yield and avoid chromatographic purification. This protocol provides improvements over previously described procedures, and it enables the preparation of sizable amounts of deep cavitands: 7 g of a water-soluble cavitand can be prepared from resorcinol in 13 working days.

Bis-resorcin[4]arene–bridged porphyrin conjugates: synthesis, fluorescence and binding studies

The synthesis of bis-resorcin[4]arene–bridged porphyrin conjugates and effect of the structure type on binding, and fluorescence quenching behaviors are presented.

Bulky metallocavitands with a chiral cavity constructed by aluminum and magnesium atrane-likes: enantioselective recognition and separation of racemic alcohols

Novel tetranuclear metallocavitands with a chiral cavity were synthesized via self-assembly of aluminum/magnesium atrane-likes and were successfully applied to the chiral resolution of 2-butanol.

Peptide-biphenyl hybrid-capped AuNPs: stability and biocompatibility under cell culture conditions

In this study, we explored the biocompatibility of Au nanoparticles (NPs) capped with peptide-biphenyl hybrid (PBH) ligands containing glycine (Gly), cysteine (Cys), tyrosine (Tyr), tryptophan (Trp) and methionine (Met) amino acids in the human hepatocellular carcinoma cell line Hep G2. Five AuNPs, Au[(Gly-Tyr-Met)2B], Au[(Gly-Trp-Met)2B], Au[(Met)2B], Au[(Gly-Tyr-TrCys)2B] and Au[(TrCys)2B], were synthesised. Physico-chemical and cytotoxic properties were thoroughly studied. Transmission electron micrographs showed isolated near-spherical nanoparticles with diameters of 1.5, 1.6, 2.3, 1.8 and 2.3 nm, respectively. Dynamic light scattering evidenced the high stability of suspensions in Milli-Q water and culture medium, particularly when supplemented with serum, showing in all cases a tendency to form agglomerates with diameters approximately 200 nm. In the cytotoxicity studies, interference caused by AuNPs with some typical cytotoxicity assays was demonstrated; thus, only data obtained from the resazurin based assay were used. After 48-h incubation, only concentrations ≥50 μg/ml exhibited cytotoxicity. Such doses were also responsible for an increase in reactive oxygen species (ROS). Some differences were observed among the studied NPs. Of particular importance is the AuNPs capped with the PBH ligand (Gly-Tyr-TrCys)2B showing remarkable stability in culture medium, even in the absence of serum. Moreover, these AuNPs have unique biological effects on Hep G2 cells while showing low toxicity. The production of ROS along with supporting optical microscopy images suggests cellular interaction/uptake of these particular AuNPs. Future research efforts should further test this hypothesis, as such interaction/uptake is highly relevant in drug delivery systems.

Guest recognition in a partially bridged deep cavitand

The synthesis of a new deep cavitand partially bridged via disulfide bonds is described. Its thermodynamic and kinetic complexation parameters are compared with those of an unbridged analogue. The disulfide bridges cause deviations in ΔH and ΔS but result in only small differences in ΔG of complexation. The bridges increase the activation barrier for guest dissociation and lead to complexes with enhanced kinetic stability.

Electronic and steric effects in binding of deep cavitands

A deep, self-folding cavitand responds to minor electronic differences between suitably sized adamantane guests. Binding constants range from <0.5 to 4000 M(-1) for guests as similar as 1-bromoadamantane and 1-cyanoadamantane. The barriers to guest exchange also vary up to 3 kcal mol(-1).

Self-folding cavitands: structural characterization of the induced-fit model

Deletion of structural elements in cavitand receptors is used to evaluate the thermodynamic and kinetic parameters of binding in these hosts. The use of a bulky guest showcases the importance of host-guest shape adaptations in the molecular recognition process.

Chiral encapsulation by directional interactions

The complexation of chiral guests in the cavity of dimeric self-assembled chiral capsule 1(2) was studied by using NMR spectroscopy and X-ray crystallography. Capsule 1(2) has walls composed of amino acid backbones forming numerous directional binding sites that are arranged in a chiral manner. The polar character of the interior dictates the encapsulation preferences towards hydrophilic guests and the ability of the capsule to extract guests from water into an organic phase. Chiral discrimination towards hydroxy acids was evaluated by using association constants and competition experiments, and moderate de values were observed (up to 59 %). Complexes with one or two guest molecules in the cavity were formed. For 1:1 complexes, solvent molecules are coencapsulated; this influences guest dynamics and makes the chiral recognition solvent dependent. Reversal of the preferences can be induced by coencapsulation of a nonchiral solvent in the chiral internal environment. For complexes with two guests, filling of the capsule's internal space can be very effective and packing coefficients of up to 70 % can be reached. The X-ray crystal structure of complex 1(2) superset((S)-6)(2) with well-resolved guest molecules reveals a recognition motif that is based on an extensive system of hydrogen bonds. The optimal arrangement of interactions with the alternating positively and negatively charged groups of the capsule's walls is fulfilled by the guest carboxylic groups acting simultaneously as hydrogen-bond donors and acceptors. An additional guest molecule interacting externally with the capsule reveals a possible entrance mechanism involving a polar gate. In solution, the structural features and dynamic behavior of the D(4)-symmetric homochiral capsule were analyzed by variable-temperature NMR spectroscopy and the results were compared with those for the S(8)-symmetric heterochiral capsule.

Recognition and organocatalysis with a synthetic cavitand receptor

The cyclization reaction of an epoxyalcohol is catalyzed by a synthetic cavitand receptor with an inwardly directed carboxylic acid function. The receptor features a hydrophobic pocket in which the substrate is bound and positioned to react in a regioselective manner. The nature of this substrate-catalyst complex and its dynamic properties were investigated by NMR methods and with the aid of a model compound lacking the epoxide function. The kinetic parameters of the cyclization reaction were also studied. A catalytic cycle is proposed and diverse inhibition mechanisms are identified that parallel those encounterd in enzymology.

Chemistry and catalysis in functional cavitands

Biological macromolecules use binding forces to access unfavorable chemical equilibria and stabilize reactive intermediates by temporarily isolating them from the surrounding medium. Certain synthetic receptors, functional cavitands, share these abilities and allow for the direct observation of labile intermediates by conventional spectroscopy. The cavitands feature inwardly directed functional groups that form reversible, covalent bonds with small molecules held inside. Tetrahedral intermediates of carbonyl addition reactions--hemiaminals, hemiacetals, and hemiketals--show amplified concentrations and lifetimes of minutes under ambient conditions. Labile intermediates in addition reactions of carboxylic acids to isonitriles are also stabilized by isolation in the space of the cavitands. The restricted environments channel the reactions of intermediates in cavitands along a specific path and strengthen the parallels between functional synthetic cavitands and enzymes.

Pseudo-capsule assemblies characterized by 19F NMR techniques

The formation of capsule-like dynamic assemblies which do not rely on attractions between host subunits was achieved via specific interactions with fluorinated guests. Characterization of these three component assemblies was accomplished by applications of (19)F NMR spectroscopy.