Towards synthetic adrenaline receptors--shape-selective adrenaline recognition in water

Selective Binding of Dopamine and Epinephrine in Water by Molecularly Imprinted Fluorescent Receptors

Catecholamines play important roles in biology but their structural similarity makes it challenging to construct synthetic receptors with selective binding. A combination of covalent and noncovalent binding groups in the hydrophobic core of water-soluble nanoparticles enabled them to recognize dopamine and epinephrine with an association constant (Ka ) of 3-4×104  M-1 in water, an order of magnitude higher than those of previously reported synthetic hosts. In addition, minute structural changes among analogues were detected including the addition or removal of a single hydroxyl or methyl group.

Chemical sensing of neurotransmitters

This review focuses on the chemosensors for neurotransmitters published for the last 12 years, covering biogenic amines (dopamine, epinephrine, norepinephrine, serotonin, histamine and acetylcholine), amino acids (glutamate, aspartate, GABA, glycine and tyrosine), and adenosine.

Induction of intracellular calcium concentration by environmental benzo(a)pyrene involves a β2-adrenergic receptor/adenylyl cyclase/Epac-1/inositol 1,4,5-trisphosphate pathway in endothelial cells

Polycyclic aromatic hydrocarbons (PAHs) such as benzo(a)pyrene (B(a)P) are widely distributed environmental contaminants, known as potent ligands of the aryl hydrocarbon receptor (AhR). These chemicals trigger an early and transient increase of intracellular calcium concentration ([Ca(2+)](i)), required for AhR-related effects of PAHs. The mechanisms involved in this calcium mobilization were investigated in the present study. We demonstrated that B(a)P-mediated [Ca(2+)](i) induction was prevented in endothelial HMEC-1 cells by counteracting β2-adrenoreceptor (β2ADR) activity using pharmacological antagonists, anti-β2ADR antibodies, or siRNA-mediated knockdown of β2ADR expression; by contrast, it was strongly potentiated by β2ADR overexpression in human kidney HEK293 cells. B(a)P was shown, moreover, to directly bind to β2ADR, as assessed by in vitro binding assays and molecular modeling. Pharmacological inhibition and/or siRNA-mediated silencing of various signaling actors acting downstream of β2ADR in a sequential manner, such as G protein, adenylyl cyclase, Epac-1 protein, and inositol 1,4,5-trisphosphate (IP(3))/IP(3) receptor, were next demonstrated to prevent B(a)P-induced calcium signal. Inhibition or knockdown of these signaling elements, as well as the use of chemical β-blockers, were finally shown to counteract B(a)P-mediated induction of cytochrome P-450 1B1, a prototypical AhR target gene. Taken together, our results show that B(a)P binds directly to β2ADR and consequently utilizes β2ADR machinery to mobilize [Ca(2+)](i), through activation of a G protein/adenylyl cyclase/cAMP/Epac-1/IP(3) pathway. This β2ADR-dependent signaling pathway activated by PAHs may likely be crucial for PAH-mediated up-regulation of AhR target genes, thus suggesting a contribution of β2ADR to the health-threatening effects of these environmental pollutants.

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.

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.

Entirely artificial signal transduction with adrenaline

Multifunctional transmembrane-building blocks with recognition sites for adrenaline on one end and the reaction partners for an SN2 reaction on the opposite end have been embedded in DPPC-liposomes. These doped vesicles can be quantitatively reduced at their disulfide head groups by externally added reducing agents; their composition and chemical processes taking place within can be monitored by NMR spectroscopy and--with limitations--by UV/Vis spectroscopy. Attempted release of thiopyridine as a second messenger into the interior of the liposome on external adrenaline addition could not be proven unambiguously because the detection system does not fulfill the necessary rigorous specificity and sensitivity requirements.

The Sodium Salt of Diethyl 1H-pyrazole-3,5-dicarboxylate as an Efficient Amphiphilic Receptor for Dopamine and Amphetamines. Crystal Structure and Solution Studies

The sodium salt of the diethyl 1H-pyrazole-3,5-dicarboxylate (2) of amphiphilic character is able to interact with (+)-amphetamine, (+)-methamphetamine, and dopamine, yielding stable complexes. Crystal structure of 2 with (+)-amphetamine leads to a very nice double helical supramolecular structure.

Selective complexation of N-alkylpyridinium salts: binding of NAD+ in water

A new class of receptor molecules is presented that is highly selective for N-alkylpyridinium ions and electron-poor aromatics. Its key feature is the combination of a well-preorganized molecular clip with an electron-rich inner cavity and strategically placed, flanking bis-phosphonate monoester anions. This shape and arrangement of binding sites attracts predominantly flat electron-poor aromatics in water, binds them mainly by pi-cation, pi-pi, CH-pi, and hydrophobic interactions, and leads to their highly efficient desolvation. NAD(+) and NADP, the important cofactors of many redox enzymes, are recognized by the new receptor molecule, which embraces the catalytically active nicotinamide site and the adenine unit. Even nucleosides such as adenosine are likewise drawn into the clip's cavity. Complex formation and structures were examined by one- and two-dimensional NMR spectroscopy, Job plot analyses, and isothermal titration microcalorimetric (ITC) measurements, as well as quantum chemical calculations of (1)H NMR shifts. The new receptor molecule is a promising tool for controlling enzymatic oxidation processes and for DNA chemistry.

Artificial Receptors That Provides a Preorganized Hydrophobic Environment: A Biomimetic Approach to Dopamine Recognition in Water

[structure: see text] The recognition of dopamine in water has been achieved with tripodal oxazoline-based artificial receptors, capable of providing a preorganized hydrophobic environment by rational design, which mimics a hydrophobic pocket predicted for a human D2 receptor. The receptors show an amphiphilic nature owing to the presence of hydrophilic sulfonate groups at the periphery of the tripodal oxazoline ligands, which seems to contribute in forming the preorganized hydrophobic environment. The artificial receptors recognized dopamine hydrochloride in water with reasonable selectivity among various organoammonium guests examined. The observed binding behavior of the receptors was explained by evoking guest inclusion in the preorganized hydrophobic pocket-like environment and not by simple ion-pairing interactions. The rationally predicted 1:1 inclusion binding mode was supported by binding studies such as with a reference receptor that cannot provide a similar binding pocket, Job and VT-NMR experiments, electrospray ionization mass analysis, and guest selectivity data. This study implies that an effective hydrophobic environment can be generated even from an acyclic, small molecular artificial receptor. Such a preorganized hydrophobic environment, as being utilized in biological systems, can be effectively used as a complementary binding force for the recognition of organoammonium guests such as dopamine hydrochloride in water.

Selective Detection of Catecholamines by Synthetic Receptors Embedded in Chromatic Polydiacetylene Vesicles

A new detection scheme for catecholamines was constructed through embedding synthetic receptors within vesicles comprising phospholipids and polydiacetylene. Fluorescence emission of the polydiacetylene was induced through specific interactions between the soluble ligands and the vesicle-incorporated hosts. The system demonstrated remarkable selectivity among structurally similar ligands and achieved much lower detection thresholds compared to that of other reported catecholamine sensors. The chromatic assembly provides a generic route for high sensitivity detection of ligand-receptor interactions.

Understanding the Template Preorganization Step of an Artificial Arginine Receptor§

A biomimetic complex which mimics the arginine-phosphonate diester interaction of the arginine fork is investigated with respect to structure and energetics of stable configurations. Within this work, we provide knowledge on local minima of the isolated system obtained from first-principles calculations. Non-negligible solvation effects are studied in a microsolvation approach. The interactions which govern the structural patterns of molecular recognition in this tweezer-guest complex can be significantly modulated by the action of hydrogen bond accepting and donating solvent molecules, such as dimethyl sulfoxide or water, which were present in experimental investigations on this system. Different tweezer-guest structures are evaluated with respect to their temperature-dependent thermodynamical properties as products of the first association reaction step of the bisphosphonate tweezer template and the guanidinium moiety.

Multipoint Recognition of Catecholamines by Hydrindacene-Based Receptors Accompanied by the Complexation-Induced Conformational Switching

The molecular recognition of catecholamines by hydrindacene-based receptors 1 and 2, as well as the durene-based receptor 3, and the guest-induced conformational changes are reported. These receptors selectively bind adrenaline and dopamine salts through the guests' ammonium group and 3-hydroxyl group on the aromatic ring. In the case of adrenaline, an additional hydrogen bond with a benzylic hydroxyl group is formed. In 2 % CD3CN/CDCl3, the association constants are of the order of 10(4) M(-1), which is much larger than with guests without the 3-hydroxyl groups (10(3) M(-1)). The two amide groups of receptor 1 can rotate freely around the C(aromatic)--C(amide) bond, whereas the tert-amide in 2 changes between two stable conformations at a slow enough rate to allow detection by (1)H NMR spectroscopy. In the absence of a guest molecule, the syn-conformer is less stable than the anti-conformer. On complex formation with adrenaline, the syn-conformer becomes dominant due to an intramolecular dipole-reversal effect in addition to multipoint hydrogen bonding.

Selective Amine Recognition: Development of a Chemosensor for Dopamine and Norepinephrine

[reaction: see text] A boronic acid-containing coumarin aldehyde was designed and synthesized. The sensor binds to catecholamines such as dopamine and norepinephrine by forming an iminium ion with the amine as well as a boronate ester with the catechol. An internal hydrogen bond produces a colorimetric response to these analytes with good selectivity for catecholamines over simple amines. The fluorescence of the sensor is quenched by the catechol.

Chromogenic Charge Transfer Cleft-Type Tetrahydrobenzoxanthene Enantioselective Receptors for Dinitrobenzoylamino Acids

A combination of a benzoxanthene cleft-type receptor with an electron-rich aromatic ring capable of establishing charge-tranfer interactions provides enantioselective receptors for dinitrobenzoylamino acids. Racemic mixtures of the receptor can be resolved with TLCs impregnated with the guest. The structure of the complexes has been established in an X-ray study. Enantiomeric amino acids provide complexes with different colors.

Complexation of phenolic guests by endo- and exo-hydrogen-bonded receptors

This article describes the complexation of phenol derivatives by hydrogen-bonded receptors. These phenol receptors are formed by self-assembly of calix[4]arene dimelamine or tetramelamine derivatives with 5,5-diethylbarbiturate (DEB) or cyanurate derivatives (CYA). The double rosette assemblies 3(3).(DEB)6/(CYA)6 have their phenol-binding functionalities (ureido groups) at the top and at the bottom of the double rosette (exo-receptors). The tetrarosette assemblies 4(3).(DEB)12/(CYA)12 form a cavity with binding sites between the two double rosettes for guest encapsulation (endo-receptors). An intrinsic binding constant Ka of 202 M-1 and 286 M-1 for the binding of 4-nitrophenol to the ureido functionalized exo- and endo-receptors, respectively, was observed. For the exo-receptor a 1:6 stoichiometry was observed while for the endo-receptor 1:4 binding stoichiometry was determined by Job plot and MALDI-TOF MS. The important role that the hydroxy group's acidity plays in the complexation of 4-nitrophenol is clarified by binding studies with different phenol derivatives. The hydrogen-bonded receptors showed a much smaller response towards less acidic phenol derivatives.