Allostery in Guest Binding of Rim-to-Rim-Connected Homoditopic Biscavitands

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.

Negative Cooperativity in Guest Binding of a Ditopic Self-Folding Biscavitand

A brand-new self-folding biscavitand was synthesized from a feet-to-feet-connected bisresorcinarene. The X-ray crystal structure of the biscaivtand showed that the two cavities are tightly connected with four butylene linkages. The conformationally coupled two cavities accommodated two cationic guests, showing a homotropic negative cooperativity in nonpolar solvents (toluene and chloroform). A polar tetrahydrofuran solvent weakened the cyclic hydrogen bonding interactions of the biscavitand, which resulted in noncooperative guest binding.

Unimolecular artificial transmembrane channels showing reversible ligand-gating behavior

A series of peptide-appended bisresorcinarenes were synthesized, which adopted tubular conformation induced by intramolecular hydrogen bonds. The derivatives formed unimolecular artificial transmembrane channels in lipid bilayers to enable selective transport of monovalent cations. Importantly, the channels exhibited reversible ligand-gating behavior in response to alkyl amine and Cu2+.

One-dimensional arrangement of NORIA in the solid-state

NORIA is a synthetic macrocycle consisting of twelve resorcinol rings. By cocrystallization of NORIA with benzene, NORIA organized one-dimensional array.

Upper-rim functionalization and supramolecular polymerization of a feet-to-feet-connected biscavitand

An octaiodobiscavitand was synthesized via an aromatic Finkelstein iodination reaction in good yield. An octa-functionalized biscavitand self-assembled to form a supramolecular polymer in the solid state.

Absorption of chemicals in amorphous trisresorcinarene

Trisresorcinarene is an interesting class of macrocyclic host. Its unique structure and insolubility allow to function as a amorphous solid absorbent capable of absorbing various aromatic and aliphatic hydrocarbons.

Conformational Characteristics of Feet-to-Feet-Connected Biscavitands

X-ray crystallography of an acetoxy-protected bisresorcinarene and biscavitands possessing phosphonate and dialkylsilyl bridges revealed that the bisresorcinarene and the biscavitands adopt helical forms in the solid state. Helical conformations were also found in solution. The helix-helix interconversions of the biscavitands occurred with high activation barriers of more than 50 kJ mol^-1. The activation parameters of the helix-helix interconversions were determined using exchange spectroscopy (EXSY). The positive activation enthalpies and the negative activation entropies suggest that the transition states of the helix-helix interconversion process are most likely more strained and symmetric than the ground states. The compensatory enthalpy-entropy correlation is found in the series of activation parameters, giving rise to a compensation temperature of 254 K.

Substituent-controlled racemization of dissymmetric coordination capsules

We report the effect of substituents (methyl, isopropyl, methoxy, and methoxyphenyl) at the 6'-position of the 2,2'-bipyridyl arms on the racemization of dissymmetric coordination capsules 1a-d. When the capsules included (R)-4,4'-diacetoxy-2,2'-benzyloxycarboxyl-biphenyl ((R)-3), the (M)-helical conformer was enriched with a diastereomeric excess (de%) of >98% for 1a, 31% for 1b, 81% for 1c and 75% for 1d. The entrapped guests in 1a, 1c and 1d can be removed by washing the solid containing the host-guest complexes with diethyl ether. The rate of racemization in THF follows the order of 1c > 1d ≫ 1a. X-ray crystal structural analysis and density functional theory calculation of model complex 4c indicate a distorted tetrahedral coordination of the Cu(i) center, and UV-vis absorption spectroscopy indicates similar coordination environments in 1c and 4c. A series of experiments demonstrates that the racemization rate depends on the dihedral angles of the bipyridyl arms, and the angles are regulated by the substituents. The methoxy and methoxyphenyl substituents in 1c and 1d enlarge the dihedral angles of the bipyridyl arms. This facilitates the access of solvent molecules to the Cu(i) centers and promotes racemization. The slower racemization of 1d can be ascribed to the steric protection of the Cu(i) centers from incoming solvent molecules by the p-methoxyphenyl group.

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.

Facile Synthesis of an Eight-Armed Star-Shaped Polymer via Coordination-Driven Self-Assembly of a Four-Armed Cavitand

The polystyrene chains were installed at the lower rim of a resorcinarene-based cavitand via reversible addition-fragmentation (RAFT) polymerization to form a four-armed star-shaped polymer. A star-shaped polystyrene-functionalized supramolecular capsule was prepared through the coordination-driven self-assembly of the four-armed start-shaped polymer with silver ions. The eight-armed start-shaped supramolecular capsule encapsulated 4,4'-diacetoxybiphenyl as did a cavitand-based self-assembled capsule. A DOSY measurement indicated that the eight-armed star-shaped polymer was twice as large as the four-armed star-shaped polymer. The solution behaviors of these compounds resulted in a difference in their zero-shear viscosities.

Hexameric assembly of 5,17-di-substituted calix[4]arene in the solid state

Chiral 5,17-difunctionalized-25,26,27,28-tetrapropyloxycalix[4]arene possessing (S)-mandelamide arms ((S,S)-1) afforded cocrystals (S,S)-1·(solvent) (solvent = MeOH, EtOH, 1-PrOH, 2-PrOH, and CH₃CN). Four of the five cocrystals contain unusual hexameric assembly of the calix[4]arene host.