Allosteric Supramolecular Receptors and Catalysts

Theoretical study of crown ethers with incorporated azobenzene moiety

A series of crown ethers containing the azobenzene moiety incorporated into crowns of various sizes [Cr(O(6))(,) Cr(O(7)) and Cr(O(8))] and their corresponding alkali metal cation (Li(+), Na(+), K(+), Rb(+)) complexes have been studied theoretically. The density functional theory (DFT) method was employed to elucidate the stereochemical structural natures and thermodynamic properties of all of the target molecules at the B3LYP/6-31 G(d) and LANL2DZ level for the cation Rb(+). The fully optimized geometries had real frequencies, thus indicating their minimum-energy status. In addition, the bond lengths between the metal cation and oxygen atoms, atomic torsion angles and thermodynamic energies for complexes were studied. Natural bond orbital (NBO) analysis was used to explore the origin of the internal forces and the intermolecular interactions for the metal complexes. The calculated results show that the most significant interaction is that between the lone pair electrons of electron-donating oxygens in the cis-forms of azobenzene crown ethers (cis-ACEs) and the LP (1-center valence antibond lone pair) orbitals of the alkali-metal cations (Li(+), Na(+), K(+) and Rb(+)). The electronic spectra for the cis-ACEs [cis-Cr(O(6)), cis-Cr(O(7)) and cis-Cr(O(8))] are obtained by the time-dependent density functional theory (TDDFT) at the B3LYP/6-31 G(d) level. The spectra of the cis-isomers show broad π → π (S(0) → S(2)) absorption bands at 310-340 nm but weaker n → π (S(0) → S(1)) bands at 480-490 nm. The calculated results are in good agreement with the experimental results.

Ratiometric detection of Hg2+ ions: an allosterically synchronized Hg2+/Li+ switch based on thiacalix[4]crown

A new heteroditopic chemosensor based on the 1,3-alternate conformation of thiacalix[4]crown shows a ratiometric fluorescence response towards Hg(2+) ions. Further, a negative allosteric behaviour between Hg(2+)/Li(+) ions is observed. Thus, metal ion exchange triggers a Hg(2+)/Li(+) switchable fluorescent chemosensor.

Density functional theory study of a molecular allosteric switch for 2,2'-bipyridyl-3,3'-15-crown-5

A classical model of "molecular machine," which acts as an ON-OFF switch for 2,2'-bipyridyl-3,3'-15-crown-5 (L), has been theoretically studied. It is highly important to understand the mechanism of this switch. The alkali-metal cations (Na(+) and K(+) ) and W(CO)(4) fragment are introduced to coordinate with the different active sites of L, respectively. The density functional theory (DFT) method is used for understanding the stereochemical structural natures and thermodynamic properties of all the target molecules at B3LYP/6-31G(d) and SDD (Stuttgart-Dresden) level, together with the corresponding effective core potential (ECP) for tungsten (W). The fully optimized geometries have been performed with real frequencies, which indicate the minima states. The nucleophilicity of L has been investigated by the Fukui functions. The natural bond orbital analysis is used to study the intermolecular charge-transfer interactions and explore the origin of the internal forces of the molecular switch. In addition, the binding energies, enthalpies, Gibbs free energies, and the cation exchange energies have been studied for L, W(CO)(4) L, and their corresponding complexes. The properties of the complexes displayed by in presence or absence of the W(CO)(4) fragment are also analyzed. The calculated results of allosterism displayed by L are in a good agreement with the experimental results.

Self-assembly, stability quantification, controlled molecular switching, and sensing properties of an anthracene-containing dynamic [2]rotaxane

The preparation of a novel anthracene-containing dynamic [2]rotaxane by a templating self-assembly process between a diamine and a dialdehyde to form a [24]crown-8 macrocyclic diimine, in the presence of a dumbbell containing a secondary dialkylammonium ion center as the template, which has been exploited for its sensing properties. By appealing to the ability of the anthracene ring system--one of the two stoppers associated with the dumbbell--to act as a fluorescent probe, the fluorescence and fluorescence-quenching nature of the dynamic rotaxane in an equilibrium mixture has been investigated and quantified in the presence of external stimuli such as water, acids, salts, and an amine. The stability, as expressed by the hydrolysis of the dynamic rotaxane has been monitored by following: (i) the anthracene fluorescence and (ii) the movements of the signals in the (1)H NMR spectra. The rate of hydrolysis (t(1/2) = 6.9 min) of the dynamic rotaxane in the presence of a small amount (1 equiv.) of acid was found to be very much faster than when the hydrolysis was carried out with a large amount (>100 equiv.) of water, when t(1/2) > 140 min. Furthermore, it has been established that the anthracene fluorescence of the dynamic rotaxane rises with an increasing amount of acid. Two acid sensors have been identified with different operating modes-namely, logarithmic and linear. The combination of different inputs involving water, acids, salts and an amine leads to different fluorescence outputs from the dynamic rotaxane, hence, producing a prototype for expressing molecular logic.

Size selective recognition of small esters by a negative allosteric hemicarcerand

A bis(resorcinarene) substituted 2,2′-bipyridine was found to bind weakly to small esters like ethyl acetate whereas more bulky esters were not recognized by this hemicarcerand. This size selective molecular recognition could be controlled by a negative cooperative allosteric effect: coordination of a triscarbonyl rhenium chloride fragment to the bipyridine causes a conformational rearrangement that orientates the resorcinarene moieties in different directions so that they cannot act cooperatively in the binding of the substrate.

Binding mechanisms in supramolecular complexes

Supramolecular chemistry has expanded dramatically in recent years both in terms of potential applications and in its relevance to analogous biological systems. The formation and function of supramolecular complexes occur through a multiplicity of often difficult to differentiate noncovalent forces. The aim of this Review is to describe the crucial interaction mechanisms in context, and thus classify the entire subject. In most cases, organic host-guest complexes have been selected as examples, but biologically relevant problems are also considered. An understanding and quantification of intermolecular interactions is of importance both for the rational planning of new supramolecular systems, including intelligent materials, as well as for developing new biologically active agents.

Multicomponent assembly of heterometallic isosceles triangles

The multicomponent synthesis and solution-state characterization of three supramolecular bis-heterometallic isosceles triangles are elaborated. The triangular assemblies are isosceles both geometrically and chemically; they comprise multiple ligands, metals, and binding motifs. Variation of the length of one side of the triangle by changing the number of phenyl spacers n = 0, 1, and 2 influences the redox potential of the opposing copper(I) center, allowing translation of the nanomechanical changes into electronically readable values.

Pyrene-Appended Fluorescent Tweezers Generated via the Weak-Link Approach and Their Halide Recognition Properties

Through the Weak-Link Approach, fluorescent condensed and open Cu(I) tweezer complexes were prepared and characterized. These complexes exhibit fluorescence-sensitive binding properties for halide anions. The solid-state structure of a non-fluorescent Rh(I) tweezer analogue, determined by X-ray crystallography, shows that the counter anion, Cl(-), is trapped in between the two amide groups of the tweezer arms through hydrogen bonds. Although the tweezer binds Cl(-), the open complex also binds Cl(-), showing that the main role of the metal is to increase the local concentration of the pyrenyl amide moieties so that 2:1 binding can take place.