A Molecular Tweezer for Lysine and Arginine

Molecular recognition of organic ammonium ions in solution using synthetic receptors

Ammonium ions are ubiquitous in chemistry and molecular biology. Considerable efforts have been undertaken to develop synthetic receptors for their selective molecular recognition. The type of host compounds for organic ammonium ion binding span a wide range from crown ethers to calixarenes to metal complexes. Typical intermolecular interactions are hydrogen bonds, electrostatic and cation-π interactions, hydrophobic interactions or reversible covalent bond formation. In this review we discuss the different classes of synthetic receptors for organic ammonium ion recognition and illustrate the scope and limitations of each class with selected examples from the recent literature. The molecular recognition of ammonium ions in amino acids is included and the enantioselective binding of chiral ammonium ions by synthetic receptors is also covered. In our conclusion we compare the strengths and weaknesses of the different types of ammonium ion receptors which may help to select the best approach for specific applications.

Differential effects of Mg(ii) and N(alpha)-4-tosyl-l-arginine methyl ester hydrochloride on the recognition and catalysis in ATP hydrolysis

The supramolecular interactions of Mg(ii) and N(alpha)-4-tosyl-l-arginine methyl ester hydrochloride (TAME) with ATP have been investigated using (1)H and (31)P NMR spectra. Furthermore, the hydrolysis of ATP catalyzed by Mg(ii) and TAME has been studied at 60 degrees C and pH 7 using (31)P NMR spectra. In the Mg(ii)-ATP-TAME ternary system, the binding interaction of Mg(2+) with ATP involves not only N1 and N7 in the adenine ring but also beta- and gamma-phosphate of ATP. The binding forces are mainly electrostatic interaction and cation (Mg(2+))-pi interaction. The guanidinium group and the aromatic ring of TAME interacts with ATP by beta and gamma phosphate and the adenine ring of ATP. The binding forces are mainly electrostatic interactions and pi-pi stacking. A significant difference between the binary and the ternary system indicates that TAME is essential to the stablization of the intermediate. Kinetic studies show that the hydrolysis rate constant of ATP is 2.16 x 10(-2) h(-1) at pH 7 in the Mg(ii)-TAME-ATP ternary system. The Mg(ii) ion and TAME can accelerate the ATP hydrolysis process. A possible mechanism has been proposed that the hydrolysis occurs through an addition-elimination, in which the phosphoramidate intermediate was observed at 3.21 ppm in the (31)P NMR of the ternary system. These results provide further information concerning the effect of the key amino acid residue and metal ions as cofactors of ATPase on ATP synthesis/hydrolysis at the molecular level.

Molecular tweezer and clip in aqueous solution: unexpected self-assembly, powerful host-guest complex formation, quantum chemical 1H NMR shift calculation

The newly prepared water-soluble naphthalene tweezer 2a and anthracene clip 4a (substituted both with lithium methanephosphonate groups in the central spacer unit) undergo an unexpected self-assembly in aqueous solution. The highly ordered intertwined structures of the self-assembled dimers [2a]2 and [4a]2 were elucidated by quantum chemical 1H NMR shift calculations. 2a and 4a form extremely stable host-guest complexes with N-methylnicotinamide 8 in methanol and water as well. According to the thermodynamic parameters determined by 1H NMR titration experiments at various temperatures the self-assembly of 2a and 4a and their strong binding to NMNA 8 observed in aqueous solution are enthalpy driven (DeltaH << 0); the enthalpic driving force is partially compensated by an unfavorable entropy (TDeltaS < 0). Self-assembly and the host-guest binding are therefore beautiful examples of the nonclassical hydrophobic effect.

Supramolecular Chemistry in Water

Supramolecular chemistry in water is a constantly growing research area because noncovalent interactions in aqueous media are important for obtaining a better understanding and control of the major processes in nature. This Review offers an overview of recent advances in the area of water-soluble synthetic receptors as well as self-assembly and molecular recognition in water, through consideration of the functionalities that are used to increase the water solubility, as well as the supramolecular interactions and approaches used for effective recognition of a guest and self-assembly in water. The special features and applications of supramolecular entities in aqueous media are also described.

Inherently Chiral Molecular Clips: Synthesis, Chiroptical Properties, and Application to Chiral Discrimination

Inherently chiral molecular clips (MCs), pseudoenantiomeric anti-1 and anti-2, as well as mesoid syn-3, were synthesized by diastereodifferentiating repetitive Diels-Alder reactions of the achiral bisdienophile 6 with chiral diene 5 generated in situ from (-)-menthyl 3,4-bis(dibromomethyl)benzoate 4. These MCs were successfully separated by chiral HPLC to give optically active anti-1 and anti-2 and almost optically inactive syn-3. The structures of anti-1, anti-2, and syn-3 were assigned by high-resolution NMR and the absolute configurations of anti-1 and anti-2 were determined by the exciton-chirality method. Optically active anti-2 can serve as a chiral host. It binds the HCl adduct of D-tryptophan methyl ester (D-TrpOMeHCl) 3.5 times stronger than the L-enantiomer (KD/KL=3.5).

Oxacalixarenes and oxacyclophanes containing 1,8-naphthyridines: a new class of molecular tweezers with concave-surface functionality

The first examples of oxacalix[4]arenes and [1(4)]oxacyclophanes bearing 1,8-naphthyridine units are reported, and these systems function as molecular tweezers containing inner-cavity hydrogen bond acceptors.

Molecular recognition in molecular tweezers systems: quantum-chemical calculation of NMR chemical shifts

Quantum-chemical calculations for molecular tweezers systems are presented, where the focus is not only on the recognition process in the host-guest systems, but on the self aggregation of the tweezers host as well. Such intermolecular interactions influence the corresponding NMR spectra strongly by up to 6 ppm for proton chemical shifts, since ring-current effects are particularly important. The quantum-chemical results allow one to reliably assign the spectra and to gain information both on the structure and on the importance of intra- and intermolecular interactions. In addition, we study the accuracy of a variety of density functionals for describing the present host-guest systems, where we observe a considerable underestimation of ring-current effects on (1)H NMR chemical shifts at the density functional theory (DFT) level using smaller basis sets such as 6-31G**, so that larger bases like TZP are required. This stands in contrast to the behavior of the Hartree-Fock scheme, where small basis sets, such as 6-31G**, provide reliable (1)H NMR shieldings for molecular tweezers systems.