Abiotic molecular recognition of dicarboxylic anions in methanol

Recognition of chiral carboxylic anions by artificial receptors

Many carboxylic molecules, ranging from drugs to flavors and fragrances, contain chiral centers. As a consequence, research has been carried out in order to design and synthesize artificial receptors for carboxylic anions. Many problems have to be solved for binding anions. The results obtained in the binding of carboxylic anions by guanidine, secondary ammonium and metal-center have been selected. The last part of this review focuses on chiral recognition of carboxylic anions by organic and metal-based chiral receptors.

Synthesis of Functionalized Guanidino Amino Acids

We report the synthesis of guanidino amino acids (GuAA), which are structurally related to Arg and resemble a dipeptide consisting of alpha- and gamma-amino acid with a guanidinium group in the main chain. The compounds are available with different protecting groups in gram amounts and are intended as synthetic building blocks for the construction of synthetic oxoanion or peptide receptors. Tyr, Trp or dansyl-functionalized Lys can be introduced as the alpha-amino acid part, which leads to luminescent GuAAs. The compounds signal carboxylate binding in MeOH, DMSO and buffered water by change of the emission intensity. The property may find use in the construction of chemosensors.

Rational Design, Synthesis, and Application of a New Receptor for the Molecular Recognition of Tricarboxylate Salts in Aqueous Media

A rational design of a tripodal receptor for the molecular recognition of tricarboxylate salts in aqueous media, based on squaramide, has been performed using high-level DFT calculations (RI-BP86/SVP level of theory) in solution using the COSMO treatment, including some preliminary ab initio calculations at the higher RI-MP2/TZVP level of theory, comparing the ability of squaramide to bind carboxylate salts with two widely used guanidinium salts. The tripodal receptor has been synthesized using a new methodology that has been recently reported by some of us, and its capability of recognizing several mono-, di-, and tricarboxylate salts has been studied experimentally by means of microcalorimetry experiments in a very high competitive media, H(2)O:EtOH 1:3. These experiments give enthalpic and entropic data, which are unfortunately scarce in the literature of molecular recognition of anions. Finally, a fluorimetric ensemble of the receptor with fluorescein has been found to be useful for the fluorimetric determination of zinc citrate in a commercial toothpaste using competition assays.

Ion-pairing molecular recognition in water: aggregation at low concentrations that is entropy-driven

Investigations into the thermodynamic parameters that characterize the binding of citrate to tris-guanidinium host 1 in water are reported. The parameters K(a), DeltaH degrees, DeltaS degrees, and DeltaG degrees for the binding event were quantified using isothermal titration calorimetry (ITC) techniques. The 1:1 binding stoichiometry was verified by a Job plot derived from NMR data, and the microcalorimetry data was collected for solutions of 1 and citrate ranging from 1 to 100 mM using phosphate buffer concentrations of 5 and 103 mM. At low buffer concentrations (low ionic strength) complexes with greater than 1:1 stoichiometries were observed by ITC, and K(1) was determined to range from 2.0 x 10(3) to 3.0 x 10(3) M(-1). At higher buffer concentrations (high ionic strength) the higher-order complexes were not detected, and K(1) was determined to be 409 M(-1). The 1:1 association of host 1 and citrate is characterized by a large favorable entropy component and negative enthalpy. However, the complexes with higher-order stoichiometry arise from desolvation processes that result from the association of polyions in aqueous media and is entirely entropy driven. This leads to an unusual observation: the dilution of one component of the host/guest complex leads to the formation of the higher-order complexes. The reason for this observation is discussed.

Hydrogen Bonding in Anion Recognition: A Family of Versatile, Nonpreorganized Neutral and Acyclic Receptors

The diamides and disulfonamides m-C(6)H(4)(CONHAr)(2) (Ar = Ph, 1; p-n-BuC(6)H(4), 2, 2,4,6-Me(3)C(6)H(2), 3), m-C(6)H(4)(SO(2)NHPh)(2), 4, and 2,6-C(6)H(3)N(CONHPh)(2), 5, readily synthesized on a multigram scale, bind strongly to halides and acetate in organic solvents with K(a)'s as high as 6.1 x 10(4) (NMR spectroscopy). The binding stoichiometry is 1:1 in solution for all cases except for the 4.F(-) and 4.OAc(-) complexes, where both 1:1 and 1:2 binding stoichiometries were found. The association constants in CD(2)Cl(2) ((1)H NMR) follow the trend Cl(-) > Br(-) > I(-) for all the receptors. F(-) and OAc(-) binding may be stronger or weaker than Cl(-) depending on the nature of the receptor. The presence of the pyridine nitrogen in 5 and of the more rigid amide in 1-3 and 5 vs the less rigid sulfonamide structure in 4 increases selectivity for smaller anions. The enthalpy and entropy of formation for 2.Cl(-) were DeltaH = -31 kJ/mol; DeltaS = -23 J/(mol.K) (VT-NMR). The X-ray structure of [PPh(4)](2)[1.Br][Br].CH(2)Cl(2), shows 1:1 complexation of Br(-) via two N-H.Br(-) hydrogen bonds and a syn-syn nonplanar binding conformation for 1. Solution hydrogen bonding was confirmed by FT-IR and NMR spectroscopy. The receptor conformation changes on complexation. Trends in structure/binding relationships show receptor flexibility is an important factor in anion recognition.

Design, synthesis and evaluation of synthetic receptors for the recognition of aspartate pairs in an alpha-helical conformation

The specific targeting of protein surface functional groups remains a largely unexplored aspect in molecular recognition. In this study, a series of zwitterionic, 16-mer peptides serve as models for the recognition of carboxylate pairs in proteins. A receptor is described that contains two guanidinium groups separated by 4-5 A by a rigid bicyclo[3.3.0]octane spacer. Modeling studies indicate that such a receptor would be suitable for binding with two aspartate carboxylates when the amino acids are separated by two (i + 3) or three (i + 4) other amino acids in an alpha-helical peptide. Studies employing circular dichroism spectroscopy demonstrated that the addition of the receptor to the i + 3 peptide substrate caused a 23% enhancement of helical structure in 15% water/methanol at 25 degrees C. Other substrate peptides [(i + 1), (i + 4), (i + 7), (i + 10)] showed lower helical induction. Similar, but weaker binding and helical induction were observed under buffered conditions (10 mM Tris-Mes, pH 7.0). These results, along with studies employing a series of related di-cationic receptors, suggest a 1:1 binding model composed of specific hydrogen interactions between each receptor guanidinium with each substrate carboxylate when the peptide adopts a helical conformation.

Neutral Ferrocenoyl Receptors for the Selective Recognition and Sensing of Anionic Guests

A range of neutral, hydrogen-bonding ferrocenoyl anion receptors and redox sensors operable in nonaqueous solvents are reported and a series of anion-binding and -sensing experiments presented. Thioamide-based receptor L2 binds halide anions more effectively than its carboxamide analogue L1, with the thioamide (N-H) group proving to be a better NMR antenna for detecting the recognition event. The binding of this class of neutral hydrogen-bonding receptor has favorable DeltaH degrees and unfavorable DeltaS degrees. Multidentate amide receptor L5 binds halide guests more strongly, with the effect of solvent on this binding process being studied. The introduction of a primary amine functionality (L4) causes remarkably strong HSO(4)(-) binding, the first reasoned report of selectivity for this acidic anionic guest. Analogously to many biological anion recognition processes, different binding modes operate dependent on guest acidity. In this way, the chemical properties of the substrate are addressed, yielding novel anion selectivities. All the receptors investigated exhibit electrochemical anion recognition. Typically, an EC mechanistic response is observed as ferrocene oxidation "switches-on" electrostatic interactions with the bound guest. Remarkable cathodic shifts of the ferrocene oxidation wave are also induced (up to 220 mV with HSO(4)(-) and 240 mV with H(2)PO(4)(-)) as the proximate bound negative charge stabilizes positively charged ferrocenium. Difunctional receptor L8 shows a large, novel UV-visible spectroscopic enhancement with H(2)PO(4)(-).