Electrochemical recognition of ammonium and alkali metal cations with calix[4]arenediquinone

Direct Detection and Discrimination of Nucleotide Polymorphisms Using Anthraquinone Labeled DNA Probes

A novel electrochemical detection approach using DNA probes labeled with Anthraquinone (AQ) as a reporter moiety has been successfully exploited as a method for the direct detection of DNA targets. This assay uses simple voltammetry techniques (Differential Pulse Voltammetry) to exploit the unique responsiveness of AQ to its chemical environments within oxygenated aqueous buffers, providing a specific detection mechanism as a result of DNA hybridization. This measurement is based on a cathodic shift of the reduction potential of the AQ tag and the concurrent reduction in peak current upon DNA binding. The further utility of this approach for discrimination of closely related DNA targets is demonstrated using DNA strands specific to B. anthracis and closely related bacillus species. DNA targets were designed to the rpoB gene incorporating nucleotide polymorphisms associated with different bacillus species. This assay was used to demonstrate that the shift in reduction potential is directly related to the homology of the target DNA. The discriminatory mechanism is dependent on the presence of oxygen in the measurement buffer and is strongly linked to the position of the nucleotide polymorphisms; with homology at the terminus carrying the AQ functionalised nucleotide critical to achieving accurate discrimination. This understanding of assay design was used to demonstrate an optimized assay capable of discriminating between Yersinia pestis (the causative agent of plague) and closely related species based on the groEL gene. This method is attractive as it can not only detect DNA binding, but can also discriminate between multiple Single Nucleotide Polymorphisms (SNPs) within that DNA without the need for any additional reagents, reporters, or processes such as melting of DNA strands. This indicates that this approach may have great potential to be exploited within novel biosensors for detection and diagnosis of infectious disease in future Point of Care (PoC) devices.

Bistriazole-p-benzoquinone and its alkali salts: electrochemical behaviour in aqueous alkaline solutions

Lithium, sodium and potassium salts of bistriazole-p-benzoquinone were synthetized and studied by single crystal X-ray, VT-XRPD and thermogravimetric analysis, and CV measurements.

Electrochemical Reduction of Quinones in Different Media: A Review

The electron transfer reactions involving quinones, hydroquinones, and catechols are very important in many areas of chemistry, especially in biological systems. The therapeutic efficiency as well as toxicity of anthracycline anticancer drugs, a class of anthraquinones, is governed by their electrochemical properties. Other quinones serve as important functional moiety in various biological systems like electron-proton carriers in the respiratory chain and their involvement in photosynthetic electron flow systems. The present paper summarizes literatures on the reduction of quinones in different solvents under various conditions using different electrochemical methods. The influence of different reaction conditions including pH of the media, nature of supporting electrolytes, nature of other additives, intramolecular or intermolecular hydrogen bonding, ion pair formation, polarity of the solvents, stabilization of the semiquinone and quinone dianion, catalytic property, and adsorption at the electrode surface, are discussed and relationships between reaction conditions and products formed have been presented.

Synthesis and binding characteristics of novel calix[4]arene(amidocrown) diquinones

A series of new calix[4]arene(amidocrown) diquinones (3a–3d) have been synthesized, characterized, and evaluated for cation recognition. It has been observed that 3b interacts with alkali metal ions (Li+, Na+, and K+) and ammonium ions to induce an unprecedented downfield shift in the NH proton resonance, which can be attributed to polarization of the amidocrown ring of the calix[4]arene diquinone receptor. The observation has been confirmed by a significant anodic shift (Li+ > NH4+ > Na+> K+) of the corresponding amidocrown-diquinone redox couple in cyclic and square wave voltammetric experiments. Both NMR and electrochemical studies of the binding characteristics of 3b with alkali metal cations and ammonium ions revealed a 1:1 binding stoichiometry for all and exhibited the highest association constant for lithium ions. This indicated that the receptor 3b selectively binds the lithium ion in preference to other alkali metal cations and ammonium ions.

Structure-selective recognition by voltammetry: enantiomeric determination of amines using azophenolic crowns in aprotic solvent

The enantiomeric recognition of amines by voltammetry using electroactive macrocyclic molecules, nitroazophenolic crown ethers, is reported. The oxidation potential of the nitroazophenol moiety in nitroazophenols with 18-crown-6 sensitively depends on the structure of alkyl amines. Based on this phenomenon, enantiomeric amines and even the quantitative assay of the R/S ratio in enantiomeric mixtures can be selectively recognized by using 18-crown-6 azophenol (3-H) with chiral centers. In the case of phenylglycinol, the association constants (K) of 3-H for the R and S forms have an R/S value of 3.5. The peak potential of the R form in square-wave voltammograms reproducibly differs from that of the S form by 32 mV, within which the peak potential linearly varies with the enantiomeric ratio. Free energy perturbation and molecular dynamics simulation provide deeper understanding of the enantiomeric recognition in this system. The theoretical analysis indicates that the free energy difference between diastereomeric complexes agrees well with the experimental results, and the pi-pi or charge-charge interaction plays a key role in enantiomeric recognition.

Bis(calix[4]diquinone) receptors: cesium- and rubidium-selective redox-active ionophores

A new class of redox-active ionophore comprised of two calix[4]diquinone moieties connected through either alkylene or pyridylene linkages has been developed. Spectroscopic and electrochemical investigations, X-ray crystal structure analyses, and molecular modeling studies show butylene- and propylene-linked members of this family of redox-active receptors exhibit remarkable selectivity preferences and substantial electrochemical recognition effects toward cesium and rubidium cations.

Self-assembled monolayer of a redox-active calix[4]arene: voltammetric recognition of the Ba2+ ion in aqueous media

The Redox-active monolayer of a novel calix[4]arene recognizing redox-inactive ionic species by voltammetry is reported. Calix[4]arene-disulfide-diquinone, which is not only redox-active but is also a highly selective ionophore for the Ba2+ ion, spontaneously forms a stable and dense monolayer film on gold. The redox-active calixarene monolayer selectively recognizes Ba2+ ion in aqueous media, and the voltammetric signals are proportional to the ionic concentration. A new voltammetric peak can be detected by square-wave voltammetry upon adding a dilute solution containing Ba2+ ion having a concentration as low as 1.0 x 10(-6) M. The Langmuir plot (1/ip vs 1/[Ba2+]) shows a linear slope in the range from 1.0 x 10(-6) M to 1.0 x 10(-4) M. This modified electrode does not show any significant interference from alkali and alkaline earth metal ions except for Sr2+ and Ca2+. Only 100- and 500-fold concentrations of Sr2+ and Ca2+ ions, respectively, can lead to voltammetric responses comparable to that of Ba2+.