Selective voltammetric and flow-injection determination of guanine and adenine on a glassy carbon electrode modified by a ruthenium hexachloroplatinate film

An inorganic film of ruthenium hexachloroplatinate (RuPtCl₆), deposited on the surface of a glassy carbon electrode, exhibits electrocatalytic activity in the oxidation of purine bases, such as adenine and guanine. Appropriate working conditions are found for fabricating a polymer film on the surface of glassy carbon and for recording the maximum electrocatalytic effect for the modified electrode. A method is developed for the selective voltammetric determination of guanine and adenine in their simultaneous presence on an electrode modified by a RuPtCl₆ film. A procedure is proposed for the amperometric detection of purine bases with this modified electrode under the conditions of flow-injection analysis. The dependence of the analytical signal on the concentration of guanine and adenine is linear up to 5 × 10^?6 and 5 × 10^?7 M in the stationary mode and to 5 × 10^?7 and 5 × 10^?8 M under flow conditions, respectively. The proposed method was tested in the analysis of calf thymus DNA for the concentrations of guanine and adenine.     

Reversed-phase high-performance liquid chromatographic analysis of endogenous purine ribonucleotide pools in BHK monolayer cultures

A rapid, sensitive, and reproducible separation of purine bases, nucleosides and nucleotides by reversed-phase high-performance liquid chromatography is described. A multi-step gradient of acetonitrile in 0.1 _M potassium phosphate was used to detect picomole amounts of each compound within 22 min. The high sensitivity of the method made it suitable for the analysis of purine nucleotide pools extracted from 1--2 × 10⁶ hamster fibroblasts grown as monolayer (BHK line) and for the detection of purine bases and nucleosides leaked form the cells into the incubation media. The chromatographic procedure was applied to study the effects of hexavalent chromium (potassium dichromate) on purine metabolism. Several steps are affected by this treatment, causing a marked unbalance of both the guanylate and adenylate pool, which was seen as a strong decrease in the level of triphosphates and accumulation of their precursors.     

Flow-injection determination of meptazinol with electrochemical detection

A flow-injection analysis system incorporating a glassy carbon voltammetric detector cell is described. Meptazinol (0.01–10 μg ml^-1) can be determined by electro-chemical oxidation in a carrier stream of 0.05 M sodium acetate—0.1 M acetic acid in 98% ethanol at sampling rates up to 80 samples per hour.     

Voltammetric determination of N-alkylated anilines by adsorption/extraction at a carbon-paste electrode

The voltammetric behaviour of aniline and some of its N-alkylated derivatives at a carbon-paste (Nujol/graphite) electrode is examined. The N-alkylated anilines, in contrast to aniline, are accumulated at the electrode by a combined adsorption/extraction process. The organic bases are determined in the rane 10^?7?2×10^?6 M after a 2-min preconcentration period by differential pulse voltammetry; N,N-dimethylaniline can be determined in the presence of excess of aniline (10^?5 M) by using medium-exchange. The voltammetric method is used for evaluation of the distribution ratios of the organic bases in the Nujol/aqueous buffer system. The enhancement of the voltammetric signal of each base is correlated with the respective distribution ratio.     

Selective Voltammetric and Flow-Injection Determination of Guanosine and Adenosine at a Glassy Carbon Electrode Modified with a Ruthenium Hexachlororuthenate Film

An inorganic film of ruthenium hexachlororuthenate (RuRuCl₆), deposited on the surface of a glassy carbon electrode, exhibits electrocatalytic activity in the oxidation of purine nucleosides, such as guanosine and adenosine. Appropriate operating conditions are found for fabricating a polymer film on the surface of glassy carbon and for recording the maximum electrocatalytic current for the modified electrode. A method for the selective voltammetric determination of guanosine and adenosine in their simultaneous presence at an electrode modified by a RuRuCl₆ film is developed. A procedure is proposed for the amperometric detection of purine nucleosides with this modified electrode under the conditions of flow-injection analysis. The linear dependence of the analytical signal on the concentration of guanosine and adenosine is observed up to 5 × 10^–6 M in the stationary mode and up to 5 × 10^–7 M in the flow system. The proposed method for the selective determination of guanosine and adenosine was tested in the analysis of human urine.     

Determination of iron(II) and iron(III) by flow injection and amperometric detection with a glassy carbon electrode

Flow injection analysis can be used for the determination of both iron(II) and iron(III) with an amperometric detector. The flow-through cell contains a glassy carbon electrode. Selection of the appropriate voltammetric technique, choice of the indication potentials, sample size, composition of the carrier stream, etc., are discussed. The limit of determination is about 10^-6 M; the calibration curves are linear in the concentration ranges 10^-3–10^-5 M for iron(III) and 5 × 10^-4–10^-5 M for iron(II). To illustrate the potentialities of the proposed method, standard rocks have been analysed.     

Characterization of multiple carbon fibre microelectrodes in a flow-through cell and amperometric detection with dilute electrolyte for liquid chromatography

A simple, convenient and sensitive flow-through cell incorporating multiple carbon fibres in a polyvinyl chloride tube was constructed for high performance liquid chromatography. The voltammetric behaviour, electrode treatment, stability and hydrodynamic voltammograms of such electrodes are discussed. An in-situ electrochemical pretreatment is proposed for activation and recovery of the activity of the carbon fibre electrode in flow-stream detection. The desirable detector was found to be nearly flow-rate independent. Catecholamines can be detected at concentrations as low as 2 × 10⁻⁹ M. The cell could be used in the mobile phase with little electrolyte. It was found that the solution resistance in the flow pathway was the major source of distortion of the shape of hydrodynamic voltammograms and of high noise at dilute electrolyte. Parallel detection using dual working electrodes is demonstrated for reversed-phase chromatographic separation of catecholamines.     

Simultaneous detection of purine and pyrimidine at highly boron-doped diamond electrodes by using liquid chromatography

Highly boron-doped diamond (BDD) electrode, have been examined for simultaneous detection of purine and pyrimidine bases in mild acidic media by using HPLC with amperometric detection. Cyclic voltammetry at as-deposited (AD) and anodically oxidized (AO) BDD were used to study the electrochemistry and to optimize the condition for HPLC applications. At AO BDD electrode, due to its higher overpotential of oxygen evolution reaction, well-defined anodic peaks were observed for the oxidation of purine and pyrimidine bases in acid medium, whereas at AD BDD the oxidation peak of thymine was overlapped with the anodic current of oxygen evolution. The chromatograms of adenine, guanine, cytosine, thymine and 5-methylcytosine mixture were well resolved by using a silica-based column and a solution of 5% acetonitrile in 100 mM ammonium acetate buffer (pH 4.25) as the mobile phase. The detection was carried out at AO BDD electrode at an applied potential of 1.6 V versus Ag/AgCl. Linear calibration curves were obtained within the concentration range from 0.1 to 10 μM with the limits of detection (S/N = 3) ranging from 26.3 to 162.1 nM, resulting in an order of magnitude higher sensitivities than those at conventional electrodes. HPLC analysis with diamond amperometric detector was successfully applied for determination of 5-methylcytosine in real DNA samples with high reproducibility. No deactivation of the electrode was found during cyclic voltammetric and HPLC measurements, indicating the high stability for analysis of biological samples.     

Simultaneous determination of uric acid,xanthine, hypoxanthine and caffeine in human blood serum and urine samples using electrochemically reduced graphene oxide modified electrode

This paper describes the fabrication of graphene on glassy carbon electrode (GCE) by electrochemical reduction of graphene oxide (GO) attached through 1,6-hexadiamine on GCE and the simultaneous determination of structurally similar four purine derivatives using the resultant electrochemically reduced GO (ERGO) modified electrode. The electrocatalytic activity of ERGO was investigated toward the oxidation of four important purine derivatives, uric acid (UA), xanthine (XN), hypoxanthine (HXN) and caffeine (CAF) at physiological pH. The modified electrode not only enhanced the oxidation currents of the four purine derivatives but also shifted their oxidation potentials toward less positive potentials in contrast to bare GCE. Further, it successfully separates the voltammetric signals of the four purine derivatives in a mixture and hence used for the simultaneous determination of them. Selective determination of one purine derivative in the presence of low concentrations other three purine derivatives was also realized at the present modified electrode. Using differential pulse voltammetry, detection limits of 8.8 × 10⁻⁸ M, 1.1 × 10⁻⁷ M, 3.2 × 10⁻⁷ M and 4.3 × 10⁻⁷ M were obtained for UA, XN, HXN and CAF, respectively. The practical application of the modified electrode was demonstrated by simultaneously determining the concentrations of UA, XN, HXN and CAF in human blood plasma and urine samples.     

Microanalysis of DNA by stripping transfer voltammetry

A cathodic stripping transfer voltammetric procedure for trace determination of DNA and its components is described. The method is based on the DNA acid hydrolysis with subsequent electrochemical determination of released purine bases. In the first step, DNA is hydrolyzed for 30 min in 0.5 M perchloric acid at 75 degrees C. The electrochemical step involves generation of Cu(I)-purine base complex on a mercury electrode surface, transfer of electrode with accumulated complex into supporting electrolyte where voltammetric measurement is performed. Analysis is carried out in 14-microl drop volume (two-electrode connection) or in 30-microl drop (three-electrode connection) on a platinum plate, which is used as a counter electrode. Blank electrolyte contains 0.05 M borate buffer, pH 9.2 with 6.3 microM Cu(II). We could observe voltammetric signal at hydrolyzed nucleosides, nucleotides, ODN, and DNA containing purine bases. We are able to accumulate under the controlled potential and determine subnanomolar concentration of DNA corresponding to the amount of 200 pg of DNA.     

Determination of traces of purine by cathodic stripping voltammetry at the hanging copper amalgam drop electrode

In the presence of purine, the copper(II)/copper(Hg) couple splits into copper(II)/copper(I) and copper(I)/copper(Hg) couples, which form two well-separated systems of peaks under voltammetric conditions. The copper(I)/purine complex adsorbs on the electrode surfacer and can be deposited on the electrode surface by electroreduction of copper(II) ions at the HMDE or by electro-oxidation of the hanging copper amalgam drop electrode (HCADE). The deposit can be stripped either cathodically or anodically over the pH range 2–9. The cathodic stripping variant at the HCADE, in solution with pH 2, offers the best results, with linear response for the range 5 × 10^?9–1.5 × 10^?7 mol dm^?3 purine after an accumulation time of 3 min. The detection limit found with the HMDE in the presence of copper(II) ions is higher.     

Determination Of Nucleic Acid Bases At Nanomolar Concentrations By Means Of Cathodic Stripping Voltammetry

Abstract

It is shown that nucleic acid bases and some of their derivatives can be determined by means of cathodic stripping voltammetry (CSV). The limit of detection of adenine is about 2 × 10^?9 M. Presence of an excess of DNA does not interfere with the determination of free bases. CSV may be used also for the determination of nucleosides and nucleotides derived from purine bases.     

Separation of six purine bases by capillary electrophoresis with electrochemical detection

Capillary zone electrophoresis with electrochemical detection (ED) has been employed for the separation and determination of adenine (A), guanine (G), theophylline (Thp), hypoxanthine (HX), xanthine (Xan) and uric acid (UA). Effects of several important factors such as the acidity and concentration of running buffer, separation voltage, injection time and detection potential were investigated to acquire the optimum conditions. The detection electrode was a 300 μm carbon disc electrode at a working potential of +0.95 V (versus saturated calomel electrode (SCE)). The six purine bases can be well separated within 14 min in a 40 cm length fused-silica capillary at a separation voltage of 10 kV in a 100 mmol/l borate buffer (BB, pH 10.0). The current response was linear over about three orders of magnitude with detection limits (S/N=3) ranging from 0.157×10⁻⁶ to 0.767×10⁻⁶ mol/l for all compounds. The proposed method was successfully applied to determine Thp in tea and aminophylline tablets, UA in human urine, and two purine bases in DNA.     

Electrochemical investigation and determination of ceftazidime in pharmaceutical dosage forms and human urine

A voltammetric study of the oxidation of Ceftazidime (CEFT) has been carried out at the glassy carbon electrode by cyclic, differential pulse (DPV) and square wave (SWV) voltammetry. The oxidation of CEFT was irreversible and exhibited diffusion controlled process depending on pH. The oxidation mechanism was proposed and discussed. According to the linear relationship between the peak current and concentration, DPV and SWV voltammetric methods for CEFT assay in pharmaceutical dosage forms and human urine were developed. For analytical purposes, a well resolved diffusion controlled voltammetric peak was obtained in 0.1 M H₂SO₄ at 1.00 and 1.02 V for differential pulse and square wave voltammetric techniques, respectively. The linear response was obtained within the range of 4 × 10^?6?8 × 10^?5 M with a detection limit of 6 × 10^?7 M for differential pulse and 4 × 10^?6–2 × 10^?4 M with a detection limit of 1 × 10^?6 M for square wave voltammetric technique. The determination of CEFT in 0.1 M H₂SO₄ was possible over the 2 × 10^?6–1 × 10^?4 M range in urine sample for both techniques. The standard addition method was used for the recovery studies.     

Voltammetric Determination of Purine Bases Using a Carbon Electrode Modified With Hybrid Silica Film

Hybrid silica films containing cation‐exchange polyelectrolytes were designed and used to modify a glassy carbon electrode (GCE) for voltammetric determination of purine bases. Hybrid silica‐polyelectrolyte films synthesised in the presence of adenine as structure directed component have demonstrated enhanced sorption capacity to purine base. The anodic peaks of adenine and guanine at a hybrid film‐modified GCE were observed at +1.55 and +1.1 V, respectively, in phosphate buffer solution at pH 3.5. Oxidation currents of adenine and guanine were proportional to their concentration in the range of 0.02–0.50 mM, with a detection limit of 0.015 mM. The developed method was used to determine adenine in adenosine triphosphate and purine bases in hydrolyzed solutions of DNA and demonstrated good metrological characteristics.     

A helium ionization detector with a thermionic electron emitter for gas chromatography

The principal circuit of a helium ionization detector with a thermionic electron emitter (He-IDTEE) is described. The detector works at atmospheric pressure. The characteristics of the detector were studied. The difference between the voltammetric characteristics of He-IDTEE and that of a helium ionization detector with a radioactive source was shown. The dependences of the analytical signal, background signal, and noise on an accelerating voltage were analyzed. The dependences of the signal-to-noise ratio on the accelerating voltage were studied at different cathode temperatures. The optimal working conditions of the detector were selected. The detection threshold and linear dynamic range were measured. For isobutane, they were 3 × 10^?9 vol % and 2–4 orders of magnitude, respectively.     

PHOTOSENSITIZED REDUCTION OF PYRIMIDINE BASES BY TRYPTOPHAN*

Abstract— Thymine and uracil were chemically altered when irradiated with UV light in aqueous solution containing tryptophan as a photosensitizer. The reaction is inhibited by oxygen and is therefore not an example of photodynamic action. Unlike the pyrimidine bases, purine bases were not altered under similar reaction conditions. Two major photoproducts were identified. One of the products was identified as the reduced base, dihydrouracil or dihydrothymine. The quantum yield for formation of dihydrouracil was 0.24 times 10^--⁴ to 13.6 times 10^--⁴, depending upon the concentrations of uracil and the reaction temperature. Radical scavengers such as cysteine and nitrous oxide decreased the rate of dihydrouracil formation. Other indole compounds also sensitized the photoreduction of uracil, their quantum yields ranging from <1 × 10^--⁵ for tryptamine to 1.3 times 10^--³ for indole-3-acetic acid. A reaction mechanism is presented whereby the pyrimidines are reduced by electron transfer from a metastable charge transfer complex originating from the first excited singlet state of tryptophan.     

Voltammetric Detector for Liquid Chromatography: Determination of Triclosan in Rabbit Urine and Serum

A flow-electrolytic cell containing a strand of carbon fibers has been designed and characterized for use in a voltammetric detector for high-performance liquid chromatography. The detector was used for determination of triclosan (2,4,4-trichloro-2-hydroxydiphenyl ether) in rabbit serum and urine. Analysis of rabbit serum and urine 1 day and 1 to 5 days, respectively, after ingestion of oral triclosan revealed that the concentration of triclosan was higher than for control serum and urine. The concentration reached maximum levels after 6 h and 34 h or 44 h in serum and urine, respectively. When triclosan was determined in rabbit samples with the method proposed the results obtained were comparable with those obtained by high-performance liquid chromatography with ultraviolet detection.     

Computational design and characterization of new thieno-expanded tricyclic purine analogs

Unnatural nucleobases are under intense research due to their widespread applications in nucleic acids research. In this work, four new thieno-expanded purine analogs comprising ^ttzA, ^tthA, ^ttzG, and ^tthG were computationally designed based on the isomorphic tz- and th-bases. These base analogs can also be seen as modified derivatives of the previously reported tricyclic purine analogs (^ttA and ^ttG). The structural, electronic, and photophysical properties are studied by means of DFT and TDDFT calculations. We find out that these new bases can form stable Watson-Crick base pairs with natural counterparts, thus potentially mimicking natural nucleobases in DNA/RNA duplexes. Calculations reveal that these bases have smaller AIPs and HOMO-LUMO gaps than natural ones, suggesting that they are candidates for applications in nanowire technology. Particularly, the photophysical properties were explored, and the results are compared with those for tz-, th-, and tt-bases. The nature of the low-lying excited states is discussed, and analyses reveal that the thiophene-homologation would not change excitation maxima of ^thA and ^tzA, while it will result in large red-shifts of those of ^thG and ^tzG. Meanwhile, thiophene insertion has relatively larger influences on the emissions ^thA and ^tzG, for which the fluorescence was 37 nm blue-shifted and 19 nm red-shifted, respectively. Taking these new bases as derivatives of ^ttA and ^ttG, it was found that the modifications would result in large red-shifts of both the excitation maxima and the fluorescence. The effects of water solution and base paring on the phtotophysical properties were also considered.     

Determination of nanomolar levels of guanine by differential-pulse adsorptive cathodic stripping voltammetry of its copper(II) complex

Guanine is determined at the 5.0×10^–10 –2.0×10^–7 mol/l level by differential-pulse adsorptive stripping voltammetry at a hanging mercury drop electrode using the reduction peak of its copper (II) complex at –0.21 V vs. Ag-AgCl electrode. The optimum analytical conditions were found to be Britton-Robinson buffer solution (pH 4.8), an accumulation potential of 0.0 V and an accumulation time of 3 min. Under these conditions, the detection limit is 5.0×10^–10 mol/l and the relative standard deviation 2.6% for 1.0×10^–7 mol/l guanine. The method is compared with the previous voltammetric methods. The presence of some purine derivatives does not interfere.