M.S. Ibrahim

Voltammetric studies of the interaction of lumazine with cyclodextrins and DNA.

The interaction of lumazine, an antibacterial drug, with alpha-, beta-cyclodextrins and DNA in aqueous solution was studied by differential pulse stripping voltammetry and cyclic voltammetry as well as UV-vis spectroscopy. The electrochemical and absorption spectral data indicated a 1:1 complex formation of lumazine with alpha-, beta-cyclodextrins and DNA. The nature of the process, taking place at the hanging mercury drop electrode, was clarified. It was found that the complexation of lumazine molecules enhances the stacking interactions which might facilitate the formation of a perpendicularly stacked layer of lumazine-alpha-cyclodextrin complex on the electrode surface. Based on the variations in the current or absorbance, the formation constants and consequently, the Gibbs energy of these complexes were determined. The small size cavity of alpha-cyclodextrin was found to have a greater affinity for lumazine than the beta-cyclodextrin. Moreover, the interactions of lumazine-alpha-cyclodextrin or lumazine-beta-cyclodextrin inclusion complex with DNA have been investigated by means of voltammetry. The results suggest that lumazine displayed high affinity for DNA and the inclusion complex decomposed when it binds to DNA.

Voltammetric and spectroscopic studies on binding of antitumor Morin, Morin–Cu complex and Morin–β-cyclodextrin with DNA

A systematic comparative study of the binding of antitumor Morin and its complexes with DNA has been investigated in the Britton-Robison (BR) buffer solutions using voltammetric and spectroscopic methods. The results show that Morin molecule, acting as an intercalator, is inserted into the cavity of the beta-cyclodextrin (beta-CD) as well as into the base stacking domain of the DNA double helix. The interaction of Morin-Cu complex or the inclusion complex of Morin-beta-CD with ds-DNA causes hypochromism in the absorption spectra, along with pronounced changes in the electrochemical behavior of the Morin complexes. An isobestic point and a new spectrum band appeared indicating the formation of the new system of Morin-Cu-DNA at lambda(m)=391 nm and Morin-beta-CD-DNA at lambda(m)=375 nm. The intercalation of Morin-Cu and Morin-beta-CD complexes with DNA produces an electrochemically inactive supramolecular complex. The binding constants were calculated from the increase of the solubility, the strong hypochromism, and the decrease in peak current of Morin and its complexes upon the addition of the host molecules. Calculation of the thermodynamic parameters of the interaction of the inclusion complex of Morin-beta-CD with DNA, including Gibbs free energy change, Helmholz free energy and entropy change shows that the complexation is a spontaneous process of association.

The effect of pH and the nature of the anions of the supporting electrolyte on the adsorption and the association of uracil and uridine at the charged interface

Abstract The effect of pH and the nature of the anions of the supporting electrolyte on the adsorption and association of uracil and uridine was studied by ac polarography and ac voltammetry. The adsorption equilibrium and its attainment have been investigated as a function of various parameters such as pH, adsorption potential Es, adsorption time ts and the bulk concentration of uracil and uridine. Moreover, the role of the anions of the supporting electrolyte in the formation of a condensed film of the adsorbed molecules has been elucidated. The stacking interactions between adsorbed molecules are affected by the nature of the anions of the supporting electrolyte and are hindered by their specific adsorption. Characteristic properties and adsorption parameters of dilute and compact layers of uracil and uridine were evaluated from the two-step Frumkin isotherm obtained and from the potential dependence of adsorption. The resulting adsorption parameters of uracil and uridine have been computed at various pHs and for various anions in the supporting electrolyte.

Cathodic adsorptive stripping voltammetric determination of nalidixic acid in pharmaceuticals, human urine and serum.

The quinolone antibacterial agent nalidixic acid (NAL) was studied by cyclic voltammetry (CV) and cathodic adsorptive stripping voltammetry (CASV). A sensitive method is described for the determination of NAL in its pure form, dosage forms and biological fluids. Controlled adsorptive accumulation of NAL on a hanging mercury drop electrode provides the basis for the direct stripping measurement of that compound in the nanomolar concentration level. Different variables were studied and optimized. The proposed method depends upon the voltammetric activity of NAL in Britton-Robinson buffer, whereby a well-defined cathodic peak is produced at pH 5.0 in presence of NO(3)(-). The calibration graph to determine NAL was linear in the range 7.4x10(-8)-2.5x10(-5) M by CASV. CAS voltammetry has been proved to be advantageous over a liquid chromatographic (LC) technique, allowing to detection limit signal to noise ratio, (s/n=3) of 0.766 ng ml(-1) (3.3x10(-9) M) NAL to be reached. The relative standard deviation (n=5) was 5.2% at concentration level of 1.0x10(-7) M NAL. The degree of interference from coexisting metal ions on the CASV signal for NAL was evaluated. The method was applied to two different commercial pharmaceutical products (Negram tablets and suspension) with very good recoveries. It was also shown that the method was successfully applied to the determination of NAL in human urine and blood serum. Mean recoveries were 98.8+/-0.3 and 98.9+/-0.41%, respectively.

Square-wave cathodic adsorptive stripping voltammetric determination of 3-hydroxyflavone, Morin and Hesperidin in bulk form and biological fluids in absence and presence of Cu(II)

Devido a atividade farmacologica dos flavonoides, na presente investigacao, foi desenvolvido um metodo eficiente e economico para a determinacao de tracos dos mesmos. Neste contexto, o comportamento interfacial de tres flavonoides, 3-hidroxiflavona (3HF), morin e hesperidina (Hesp) foi estudado em eletrodo de gota pendente de mercurio (HMDE) por voltametria ac (fase sensivel) e voltametria ciclica (CV). Os flavonoides investigados mostraram-se fortemente absorvidos, o que e um pre-requisito para a aplicacao de voltametria adsortiva de redissolucao catodica, na determinacao de alguns flavonoides. Baseado no carater de adsorcao dos flavonoides investigados, na superficie de HMDE, foi descrito um procedimento de voltametria adsortiva de redissolucao catodica com onda quadrada (SWCASV), validado, simples, rapido e sensivel, para a quantificacao dos flavonoides sob investigacao na forma granel e em fluidos biologicos. Alem disso, a acumulacao adsortiva controlada do complexo de Cu(II) de flavonoides foi tambem obtida e usada para a determinacao indireta de 3HF, morin e Hesp via reducao do complexo formado. As condicoes operacionais e de solucao para a determinacao quantitativa ultra-traco dos compostos flavonoides investigados, foram otimizadas na ausencia e presenca de Cu(II). Os limites de deteccao de 4,4 × 10-9, 7,19 × 10-9 e 7,54 × 10-9 mol L-1 para 3HF, morin e Hesp, respectivamente, na forma granel, foram atingidos usando SWCASV.

Interactions of an anticancer drug Formestane with single and double stranded DNA at physiological conditions.

Mode of interactions of anticancer drug Formestane (FMT) with single and double stranded DNA has been investigated at different temperatures and at two physiological pH values i.e. 7.4 (human blood pH) and 4.7 (stomach pH). Fluorescence spectroscopy, UV-Vis spectroscopy, cyclic voltammetry and square wave voltammetry were employed to probe the interaction between FMT and DNA. The observed fluorescence quenching of dsDNA-ethidium bromide system by the anticancer drug FMT confirmed the intercalative mode of binding between the FMT and dsDNA. The absorption spectra and voltammetric results indicate FMT gets intercalated between dsDNA bases and the strength of interaction is independent on the ionic strength. Comparison of the mode of interaction of FMT with dsDNA and ssDNA was discussed. The calculated binding constants for FMT-dsDNA and FMT-ssDNA complexes at pH 7.4 were found to be 1.52×10(5)M(-1) and 1.24×10(6)M(-1), respectively. Stoichiometric coefficients and thermodynamic parameters of FMT-dsDNA and FMT-ssDNA complexes were evaluated. The association between the anticancer drug FMT with DNA is maximum at pH 7.4 which depicts the most stable complexes are formed at human blood pH. The decrease in peak current of FMT resulting from its interaction with DNA was employed for determination of dsDNA and ssDNA concentration at physiological conditions.

Adsorptive stripping voltammetric behaviour of hypoxanthine

The effective adsorption of hypoxanthine at a hanging mercury drop electrode (HMDE) has been studied by using ac and cyclic voltammetry (CV). It was found that the complexation of hypoxanthine molecules enhances the stacking interactions which might facilitate the formation of a perpendicularly stacked layer of Cu(II)-hypoxanthine complex on the electrode surface. A rapid and sensitive controlled adsorptive accumulation of copper complexes with hypoxanthine on the HMDE provides the basis for the direct stripping measurement of this compound on the subnanomolar concentration level. The height of the chelate peak is shown to be proportional to the hypoxanthine concentration. Experimental and instrumental parameters for the quantitative determination are optimized. Under optimum conditions the calibration curve was linear and the detection limit was 1 × 10−8M (1.36 μgl−1), where in the presence of Cu(II) the method is developed and the detection limit is lowered to 4 × 10−10M (0.05 μgl−1). The interferences of several organic compounds are described.

Chelate adsorption for trace voltammetric measurements of 2-thiouracil and 4-thiouridine

A very sensitive electrochemical stripping method for trace measurements of 2-thiouracil and 4-thiouridine in presence of Cu(II) is described. The chelate of Cu(II) with 2-thiouracil and 4-thiouridine is adsorbed on the hanging mercury drop electrode, and the reduction current of the accumulated complex is measured by cathodic stripping voltammetry. The adsorption and redox behaviour are indicated by cyclic voltammetry. Optimum experimental conditions include a preconcentration potential of 0.0 V, solution of pH 7.2, adsorption time 5 min, pulse amplitude 100 mV, and a linear scan mode. The sharp chelate peak, associated with the effective interfacial accumulation, coupled with the flat baseline, facilitates measurements at the nanomolar and submicromolar concentration levels.

Adsorption stages and interfacial orientations of cytidine sulphate at the mercury/electrolyte interface

Abstract The adsorption and interfacial orientations of cytidine sulphate were studied by out-of-phase ac voltammetry. The adsorption equilibrium and its attainment have been investigated as a function of various parameters such as pH, adsorption potential, adsorption time, temperature and the bulk concentration of cytidine sulphate. The time dependence of the electrode impedance indicates that the formation of a compact film controlled by nucleation/growth mechanism and the data were analysed according to the Avrami equation. Moreover, the role of the anions of the indifferent supporting electrolyte in the formation of the condensed film of the adsorbed molecules has been elucidated. The stacking interactions between adsorbed molecules are affected by the nature of the anions of the supporting electrolyte and are hindered by their specific adsorption. The adsorption parameters of cytidine sulphate has been computed at various pHs and for various anions in the supporting electrolyte.

Cathodic adsorptive stripping voltammetry of 6-thiopurine in absence and presence of some metal ions

SummaryCathodic stripping voltammetry of an adsorbed 6-thiopurine at HMDE was investigated in solutions of varying pH. A rapid and sensitive differential pulse voltammetric method was selected for its trace determination. A method has also been developed for the determination of 6-thiopurine in presence of Cu(II), due to the strong adsorption of the Cu-6-thiopurine complex at the surface of the HMDE and subsequent reduction of the surface-bound complex. A detection limit of 9.9×10−9 mol/l was achieved in presence of Cu(II) and the slope of the straight line was seven times the slope in absence of Cu(II). Cathodic adsorptive stripping (CAS) voltammetry of 6-thiopurine in presence of Ni(II), Pb(II), Cd(II), and Fe(III) was also investigated. The influence of several operational parameters has been considered. Statistical analysis of the calibration curve data is included.