Binding in subunit systems

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.

Contortions of encapsulated alkyl groups

Rotors are recalled as early molecular devices that transmit information through changes in conformation. Specific cases involve bipyridyls and biphenyls in which the biaryl bond acts as a fulcrum to relay applied stresses from one site to another. New types of molecular stress encountered by encapsulated molecules are identified--including bending, straightening, squeezing, grinding and compression. For flexible molecules in reversibly formed capsules a fluid model of recognition is proposed that is neither lock-and-key nor induced fit. Instead, the guest assumes the shape that best fills the available space, even if contortions to higher energy conformations are required. For encapsulated alkanes, a delicate balance of attraction and repulsion exists when the size of a guest molecule approaches the space available to it. The complexes are analyzed by both NMR and computational methods and detailed maps of the host-guest interfaces in solution are provided. The reversible transition of an encapsulated alkane between a compressed, coiled conformation and a relaxed, extended one is described. The system is a spring-loaded molecular device under the control of acids and bases that offers an alternative to the rotors of current molecular machinery.

Bisthioxanthylidene biscrown ethers as potential stereodivergent chiral ligands

The concept of bisthioxanthylidene biscrown ethers as potential stereodivergent chiral ligands in asymmetric synthesis is introduced. Substituted bisthioxanthylidenes may be chiral and can exist as stable enantiomers due to their folded structure. As a result, both a right-handed helix (P) and left-handed helix (M) are present in this type of molecule. This offers the unique possibility to construct two crown ether moieties, attached to the same molecule, of which one exhibits (P)-helicity and the other (M)-helicity. When the crown ether moieties differ in size they can be complexed selectively with a base containing a cation of appropriate diameter. In this manner the (P)-helix and the (M)-helix can be activated selectively to serve as a chiral environment for base catalyzed asymmetric synthesis. Thus, we envisioned the new concept of a single chiral ligand to separately synthesize two enantiomers of a chiral product just by varying the added base. For this purpose, four new bisthioxanthylidene monocrown ethers and two new bisthioxanthylidene biscrown ethers were synthesized. Two biscrowns and two monocrowns were separated into their respective enantiomers (HPLC) and optical data (UV and CD) were collected to ensure stability of enantiomers at ambient temperatures. Ion complexation of one mono- and two biscrown ethers with potassium and sodium cations was investigated.