Dopamine-selective response in membrane potential by homooxacalix[3]arene triether host incorporated in PVC liquid membrane

Specific Binding of Primary Ammonium Ions and Lysine-Containing Peptides in Protic Solvents by Hexahomotrioxacalix[3]arenes

The binding of ammonium ions by two homooxacalix[3]arene-based receptors was studied using NMR spectroscopy and in silico methods. Both receptors are shown to endocomplex, even in a protic environment, a large variety of primary ammonium ions, including biomolecules. The binding mode is similar for all guests with the ammonium ion deeply inserted into the polyaromatic cavity and its NH₃⁺ head nearly in the plane defined by the three oxygen atoms of the 18-crown-3 moiety, thus enabling it to establish three H-bonds with the ethereal macrocycle. The remarkable electronic, size, and shape complementarity between primary ammonium ions and the two cavity-based receptors leads to an unprecedented specificity for primary ammonium ions over secondary, tertiary, and quaternary ones. These binding properties were exploited for the selective liquid-liquid extraction of primary ammonium salts from water and for the selective recognition of lysine-containing peptides, opening new perspectives in the field of peptide sensing.

Fifty years of oxacalix[3]arenes: A review

Hexahomotrioxacalix[3]arenes, commonly called oxacalix[3]arenes, were first reported in 1962. Since then, their chemistry has been expanded to include numerous derivatives and complexes. This review describes the syntheses of the parent compounds, their derivatives, and their complexation behaviour towards cations. Extraction data are presented, as are crystal structures of the macrocycles and their complexes with guest species. Applications in fields as diverse as ion selective electrode modifiers, fluorescence sensors, fullerene separations and biomimetic chemistry are described.

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.

Dopamine-selective potentiometric responses by new ditopic sensory elements based on a hexahomotrioxacalix[3]arene

New ditopic sensory elements 2 and 3 for catecholamines based on a hexahomotrioxacalix[3]arene, with a boronic acid substituent appended, were designed and synthesized. As an interesting mode of molecular recognition at membrane surfaces, the host, when incorporated into poly(vinyl chloride) (PVC) liquid membranes, displayed excellent potentiometric selectivity for dopamine over other catecholamines (noradrenaline and adrenaline) and inorganic cations (Na+, K+, and NH4+).

The Effects of O-Substituents of Hexahomotrioxacalix[3]arene on Potentiometric Discrimination between Dopamine and Biological Organic/Inorganic Cations

As an interesting type of molecular recognition at a membrane surface, the tri-O-acetic acid ester (host 2) of hexahomotrioxacalix[3]arene, when incorporated into poly(vinyl chloride) (PVC) liquid membranes, displays a high potentiometric selectivity for dopamine over, not only other catecholamines (noradrenaline, adrenaline), but also quaternary ammonium guests (tetramethylammonium, choline, and acetylcholine) and inorganic cations (Na+, K+, NH4+). Interestingly, changes in membrane potential based on the host-guest complexation of host 2 that were observed dopamine/inorganic cation selectivity were not displayed by the related hosts 3 and 4, which contain amide substituents. This paper describes our efforts to separately estimate the two factors contributing to the dopamine selectivities, i.e., the guest lipophilicity factor and the host-guest complexation factor, in an attempt to understand the effects of the O-substituents of these hosts. The potentiometric experiments showed that, although the guests had roughly equal lipophilicity, the electromotive force (EMF) response for dopamine by host 2 was excellent. Furthermore, host 2 displayed ca. a 20-fold stronger complexation for dopamine, compared to noradrenaline, adrenaline, K+, and NH4+ cations. These results indicate that the high potentiometric selectivity of the ion-selective electrode for dopamine mainly reflect, not the guest lipophilicity factor but the host-guest complexation factor. On the other hand, host 3 displayed ca. a 3000-fold stronger binding to Na+ than dopamine, thus explaining the reasons for the lower dopamine-selectivities of host 3 compared to host 2. It is interesting to note that the high potentiometric selectivities for dopamine were displayed by not only host 2 but also host 5, regardless of the simple structure of the O-substituents.

Preparation of a Selective Receptor for Ephedrine for the Development of an Optical Spot Test for the Detection of Ephedrine in Human Urine Using Stabilized in Air Lipid Films with Incorporated Receptor

The present technique describes the preparation of a selective receptor for ephedrine and a simple sensitive spot optical test for the rapid one-shot detection of ephedrine in human urine using lipid films with an incorporated receptor that are synthesized by a chemical reaction with a methacrylate polymer on a glass fiber filter. The selective receptor was synthesized using a resorcin[4]arene receptor and by transforming all the -OH groups into methoxy groups. The lipid films without this receptor provided fluorescence under a UV lamp. The use of the receptor in these films quenched this fluorescence, and the color became similar to that of the filters without the lipid films. A drop of aqueous solution of ephedrine provided a "switching on" of the fluorescence, which allows the rapid detection of this stimulant in human urine at the levels of 10(-8) M concentrations. The effect of potent interferences (i.e., proteins, lipids, ascorbic aid, glucose, leucine, glycine, tartrate, citrate, bicarbonate, and caffeine) was examined. The results showed no interferences from these compounds in concentration levels usually found in human urine samples. Dopamine was also investigated as a potent interfering agent, and the results have shown that the transformation of the hydroxy to methoxy groups has altered the selectivity of the receptor. This species does not cause interference at concentration levels lower than 10(-6) M. A drop of urine containing ephedrine provided also a "switching on" of the fluorescence, which allows the rapid detection of this stimulant in human urine at the levels of 10(-8) M concentrations. The reproducibility of the method was checked in approximately 100 samples, and all of them were found to provide similar results. Note that the colors of the filters remain stable for periods of more than 2 months.

Interaction of dopamine and acetylcholine with an amphiphilic resorcinarene receptor in aqueous micelle system

The molecular recognition of neurotransmitters, dopamine and acetylcholine with an amphiphilic resorcinarene receptor was investigated in an aqueous sodium dodecylsulfate (SDS) micelle system by 1H NMR measurements. The interaction distances of these neurotransmitters from the hydrophilic cavity of the amphiphilic receptor were estimated based on the calculation of the ring current shift using the atomic coordinates obtained from molecular dynamics calculation.

Potentiometric determination of dopamine in pharmaceutical preparations by crown ether-PVC membrane sensors

Two simple, rapid and sensitive sensors for the assay of dopamine hydrochloride have been developed. The methods are based upon the formation of the membrane sensors 12-crown-4-phosphotungestic acid (crown ether-PTA)-dopamine and 12-crown-4-tetraphenylborate (crown ether-TPB)-dopamine as neutral carriers. The sensors were stable and showed fast potential responses of 10 s, and near-Nernstian cationic slopes of 53.3 - 56.2 mV/decade of activity between pH 2.2 - 6 for the monovalent dopamine cation over a wide range concentrations 1 x 10(-5) - 1 x 10(-1) M. The selectivity coefficients of the developed sensors indicated excellent selectivity for dopamine over a large number of organic and inorganic species and pharmaceutical excipients. The mediator o-nitrophenyloctyl ether significantly affected the lifetime of the fabricated sensors of dopamine. Satisfactory results were obtained for the determination of dopamine in dosage form by the proposed sensors with an average recovery of 99.85% for the nominal concentration.

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.

An Optical Spot Test for the Detection of Dopamine in Human Urine Using Stabilized in Air Lipid Films

The present technique describes a simple, sensitive spot test for the rapid one-shot detection of dopamine in human urine using lipid films with incorporated resorcin[4]arene receptor that are synthesized by a chemical reaction with a methacrylate polymer on a glass fiber filter. The lipid films without the receptor provided fluorescence under a UV lamp. The use of the receptor in these films quenched this fluorescence, and the color became similar to that of the filters without the lipid films. A drop of dopamine or urine containing this stimulant provided a "switching on" of the fluorescence, which allows the rapid detection of this stimulant in human urine at 10(-8) M concentrations. The novelty of the present work is that it opens new routes in the field of biosensing, i.e., development of sensitive, rapid, and simple methods for detecting species based on the fluorescence of the lipid membranes on a polymer film, and provides a spot test technique for the rapid detection of dopamine. The effect of potent interferences including a wide range of compounds usually found in human urine (i.e., ascorbic aid, glucose, leucine, glycine, tartrate, citrate, bicarbonate, and caffeine) was examined using an aqueous buffered solution that contained the potent interference and dopamine at two lower concentration levels (i.e., 3 x 10(-8)-10(-8) M). The effect of proteins and lipids was also investigated at these two lower dopamine concentration levels in aqueous buffered solution. The results showed no interferences from all these constituents at concentrations usually found in human urine samples; for example, albumin up to 3.22 g/L concentration levels did not provide any interference (i.e., no fluorescence). A drop of urine containing this stimulant provided similar results, i.e., a "switching on" of the fluorescence that allows a technique for the rapid detection of this stimulant in human urine at 10(-8) M concentrations. The technique is not based on a calibration graph but is a semiquantitative method for the detection of dopamine in real samples of urine that can be complimentary to HPLC methods. The difference in color between the samples containing dopamine at concentration levels of 10(-8)-10(-7) M can be easily distinguished by naked eye and a digital camera. An increase of dopamine concentration from 10(-8) to 10(-7) M makes the color more blue whereas the color of the filters remains purple in the blank test (i.e., addition of a urine sample without dopamine or dopamine at concentration levels of 10(-9) M to the filters that contain the lipid membranes with incorporated receptor). The reproducibility of the method was checked in approximately 100 samples, and all of them were found to provide similar results. Note that it was also found that the colors remain stable in the samples containing dopamine for periods of more than two months.

Modification of the upper rim of homooxacalix[3]arenes and complexation between a nitrohomooxacalix[3]arene derivative and n-hexylamine

Several functional groups were introduced on the upper rim of (lower rim free) homooxacalix[3]arene for the first time. The swinging nitrohomooxacalix[3]arene host 1 was fixed in the cone conformation by complexation with n-hexylamine.

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

In spite of their key role in signal transduction, the mechanism of action of adrenergic receptors is still poorly understood. We have imitated the postulated binding pattern of the large membrane protein with a small, rationally designed synthetic host molecule. Experimental evidence is presented for the simultaneous operation of electrostatic attraction, hydrogen bonds, pi stacking, and hydrophobic interactions. By virtue of this combination of weak attractive forces, adrenaline derivatives in water are bound with high shape selectivity for the slim dopamine skeleton. We think that these findings support the postulated cooperative interplay of noncovalent interactions in the natural receptors. In addition, they provide access to a new type of adrenaline sensor. This may be the first step towards an artificial signal-transduction system.

Investigation of interactions of a resorcin[4]arene receptor with bilayer lipid membranes (BLMs) for the electrochemical biosensing of mixtures of dopamine and ephedrine

The present article investigates the interactions of a resorcin[4]arene receptor with planar bilayer lipid membranes (BLMs) that can be used for the electrochemical detection of dopamine and ephedrine. BLMs were composed of egg phosphatidylcholine and 35% (w/w) dipalmitoyl phosphatidic acid in which the receptor was incorporated. These BLMs modified with the resorcin[4]arene receptor can be used as one-shot sensors for the direct electrochemical sensing of these energizing-stimulating substances. The interactions of these compounds with the lipid membranes were found to be electrochemically transduced in the form of a transient current signal with a duration of seconds, which reproducibly appeared within 8 and 20 s after exposure of the membranes to dopamine and ephedrine, respectively. The response time for BLMs without the receptor for dopamine was about 3 min, whereas no signals were obtained for ephedrine in the absence of the receptor. The mechanism of signal generation was investigated by differential scanning calorimetric studies. These studies revealed that the adsorption of the receptor is through the hydrophobic tails of the receptor, whereas hydrophilic groups of the receptor were directed towards the electrolyte solution enhancing the ion transport through the lipid membranes. The magnitude of the transient current signal was related to the concentration of the stimulating agent in bulk solution in the micromolar range. No interferences from ascorbic acid were noticed because of the use of the negatively charged lipids in membranes. The present technique can be used as one-shot sensor for the detection of these pharmaceutical substances and future research is targeted to the determination of these chemicals in human biofluids such as urine of athletes.

Towards Synthetic Adrenaline Receptors-Shape-Selective Adrenaline Recognition in Water

A new rationally designed receptor molecule binds adrenaline derivatives in water. Its binding pattern imitates the interplay of noncovalent interactions operating in the natural receptor. High shape selectivity is achieved for the slim dopamine skeleton, and leads to rejection of substrates with an α-substituent, such as amino acid derivatives.