Recognition of Amino Acid Salts by Temperature-Dependent Allosteric Binding with Stereodynamic Urea Receptors

Positive homotropic artificial allosteric systems are important for the regulation of cooperativity, selectivity and nonlinear amplification. Stereodynamic homotropic allosteric receptors can transmit and amplify induced chirality by the first ligand binding to axial chirality between two chromophores. We herein report stereodynamic allosteric urea receptors consisting of a rotational shaft as the axial chirality unit, terphenyl units as structural transmission sites and four urea units as binding sites. NMR titration experiments revealed that the receptor can bind two carboxylate guests in a positive homotropic allosteric manner attributed to the inactivation by intramolecular hydrogen-bonding between urea units within the receptor. In addition, the VT-CD spectra observed upon binding of the urea receptor with l- or D-amino acid salts in MeCN showed interesting temperature-dependent Cotton effects, based on the differences of the receptor shaft unit and the guest structure. The successful discrimination of hydrocarbon-based side chains of amino acid salts indicated that the input of chiral and steric information for the guest was amplified as outputs of the Cotton effect and the temperature-dependence of VT-CD spectra through cooperativity of positive allosteric binding.

Tritopic Bis-Urea Receptors for Anion and Ion-Pair Recognition

This work reports on the synthesis and characterization of three tritopic receptors and their binding properties toward various anions, as their tetrabutylammonium salts, and three alkali metal-acetate salts by UV-vis, fluorescence, ¹H, ⁷Li, ²³Na, and ³⁹K NMR in MeCN/dimethyl sulfoxide (DMSO) 9:1 (v/v). Molecular recognition studies showed that the receptors have good affinity for oxyanions. Furthermore, these compounds are capable of ion-pair recognition of the alkali metal-acetate salts studied through a cooperative mechanism. Additionally, molecular modeling at the density functional theory (DFT) level of some lithium and sodium acetate complexes illustrates the ion-pair binding capacity of receptors. The anion is recognized through strong hydrogen bonds of the NH- groups from the two urea sites, while the cation interacts with the oxygen atoms of the polyether spacer. This work demonstrates that these compounds are good receptors for anions and ion pairs.

Allosteric binding of sodium deoxycholate by a bis(β-cyclodextrin)-2,2′-bipyridine receptor

The allosteric effect of a new bis(β-cyclodextrin) receptor amounts to a more than 18-fold increase of its binding affinity towards sodium deoxycholate upon addition of a zinc(ii) phenanthroline complex as an effector.

A Tetra-Porphyrin Host Exhibiting Interannular Cooperativity

Recently identified as another form of cooperativity, interannular cooperativity is rarely observed in supramolecular chemistry. A tetra-porphyrin molecular tweezer with two bis-porphyrin binding sites is reported that exhibits archetypal interannular cooperativity when complexing 1,4-diazabicyclo[2.2.2]octane (DABCO). The UV/Vis titration data best supported a 1:2 plus 2:2 plus 1:4 complexation model (host:guest), giving K₁₂ =6.32×10¹³  m^-2 , K₂₂ =3.04×10²⁰  m^-3 , and K₁₄ =1.92×10¹⁶  m^-4 in CHCl₃ . The NMR titration data supported the formation of two sandwich species, including tetra-porphyrin⋅(DABCO)₂ as the major species, although there are speciation differences between UV/Vis and NMR concentrations. Using statistical analysis, interannular cooperativity (γ) for tetra-porphyrin⋅(DABCO)₂ was determined to be negative (γ=2.41×10^-3 ), which may be explained by DABCO being too small to be optimally bound simultaneously at both bis-porphyrin binding sites.

Strain effects determine the performance of artificial allosteric systems: calixarenes as models

It is shown that the performance of allosteric systems regarding the efficiency and the speed of response depends critically on the strain energy of the equilibrating conformers and of the corresponding interconversion transition state. The affinity of a substrate A can be large enough to overcome in the absence of an effector E by induced fit the strain involved in the formation of an optimal conformation for binding A. The efficiency as given by the ratio KAE/KA of binding constants in the presence or absence of an effector is, for many published synthetic allosteric systems, relatively low; in practice this means that these only function within rather limited concentration ranges. A small KAE/KA ratio means that the binding strength of A or the corresponding signal will increase only little by adding an effector, and may need higher concentration of E. Implementation of steric distortion in the minor conformer can lead to reduced binding of A in the absence of the effector E. Destabilization of conformers can also result from the inclusion of high energy water molecules within a cavity. Furthermore, until now it has been overlooked that strain in the transition state can lead to reaction times of up to days, and thus to the neglect of experimental observation. The role of conformational changes within an allosteric molecule is characterized with a variety of calixarenes and other compound classes, offering a clue for the design of more efficient synthetic systems with high cooperativity.

Response speed control of helicity inversion based on a "regulatory enzyme"-like strategy

In biological systems, there are many signal transduction cascades in which a chemical signal is transferred as a series of chemical events. Such successive reaction systems are advantageous because the efficiency of the functions can be finely controlled by regulatory enzymes at an earlier stage. However, most of artificial responsive molecules developed so far rely on single-step conversion, whose response speeds have been difficult to be controlled by external stimuli. In this context, developing artificial conversion systems that have a regulation step similar to the regulatory enzymes has been anticipated. Here we report a novel artificial two-step structural conversion system in which the response speed can be controlled based on a regulatory enzyme-like strategy. In this system, addition of fluoride ion caused desilylation of the siloxycarboxylate ion attached to a helical complex, resulting in the subsequent helicity inversion. The response speeds of the helicity inversion depended on the reactivity of the siloxycarboxylate ions; when a less-reactive siloxycarboxylate ion was used, the helicity inversion rate was governed by the desilylation rate. This is the first artificial responsive molecule in which the overall response speed can be controlled at the regulation step separated from the function step.

Spatial mismatch, non-additive binding energies and selectivity in supramolecular complexes

A geometric mismatch in supramolecular complexes often leads to deviations from the additivity of binding energies with the consequence of large changes in selectivity and binding mode.