Yassien Temerk

Electrochemical behaviour of mono- and oligonucleotides: Part VII. Comparative studies on the adsorption of cytidine at the mercury-aqueous solution interface by pulse polarography and ac polarography

Abstract Cytidine has been investigated in order to explore the potentialities of normal pulse polarography and phase-sensitive ac polarography (out-of-phase mode) in connection with the DME for the study of the adsorption and interfacial behaviour of nucleotides. The results are compared with those of a previous study [8] by single-sweep and phase-sensitive ac voltammetry at the HMDE. While the joint application of these latter approaches yields more comprehensive and also more reliable information on the adsorption parameters, normal pulse polarography is also able to provide, with good approximation, the values of certain adsorption parameters for the dilute layer (adsorption coefficient b, free adsorption enthalpy −G0, interaction coefficient a) provided the span of the drop life before the application of the pulse is adjusted to a period sufficiently long to attain adsorption equilibrium. Normal pulse polarography is particularly valuable for estimation of the kinetic parameters of the electrode process of non-adsorbed cytidine provided that the electrode surface is incompletely covered. Phase-sensitive ac polarography at the DME yields — for the adsorption parameters b, −G0 and a — data of equivalent reliability to the more laborious approach by phase-sensitive ac voltammetry at the HMDE provided the drop time of the DME has been adjusted to a period sufficiently long to attain adsorption equilibrium.

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.

Electrochemical behaviour of mono-and oligonucleotides and the corresponding nucleosides: Part VI. Comparative voltammetric study on the adsorption of 1-β-D-arabinofuranosylcytosine and 1-β-D-ribofuranosylcytosine at charged interfaces

Abstract Cytostatic actions of 1-β-D-arabinofuranosylcytosine have been reported. Therefore a systematic comparative study was undertaken on the adsorption and association of 1-β-D-arabinofuranosylcytosine (ara-C) and 1-β-ribofuranosylcytosine (r-C) in acid and neutral solution at the mercury-solution interface constituting a model for charged biological interfaces in the living cell with respect to the biophysicochemical behaviour of the studied nucleosides. The adsorption was followed by out-of-phase a.c. voltammetry and by single sweep voltammetry at the hanging mercury drop electrode (HMDE). Characteristic properties and adsorption parameters of the dilute and compact adsorption layers of ara-C and r-C were evaluated from the obtained double step Frumkin isotherm and from the potential dependence of adsorption. While the properties in the dilute layer are rather similar for both, ara-C and r-C, the compact layer of ara-C is more dense indicating a substantial influence of the different steric arrangement in the sugar moiety of both otherwise similar nucleosides. The importance of the obtained results for the biochemical behaviour at natural charged surfaces is discussed.

Electrochemical behaviour of mono- and oligonucleotides: Part IX. Voltammetric studies on the adsorption and association of the oligonucleotide cytidylyl-(3 → 5)-cytidine at the mercury—solution interface

Abstract The adsorption and the interfacial behaviour of the dinucleotide cytidylyl-( 3 → 5 )-cytidine (CpC) has been studied at the hanging mercury drop electrode by phase-sensitive ac voltammetry and single-sweep voltammetry. The adsorption equilibrium and its attainment has been investigated as a function of various parameters such as pH, adsorption potential Es, adsorption time ts, temperature and the bulk concentration of CpC. Furthermore, the role of the anion of the supporting electrolyte on the formation of the condensed film of the adsorbed dinucleotide has been elucidated. The adsorbed layer is transformed into a condensed film at all bulk concentrations of CpC due to significant stacking forces acting between the adjacent rings of the cytosine residues. From the limiting value of the surface concentration an average surface area per adsorbed mononucleotide unit of about 0.4 nm2 was found which is comparable to the value resulting under the same conditions for cytidine and poly-C. It was concluded that adsorption occurs via the cytosine residues of the short dinucleotide chain. The adsorption parameters of CpC have been evaluated at various pH and compared with those of cytidine. This study emphasizes again the usefulness of the mercury—aqueous electrolyte interface as a model of charged biological interfaces in investigations of the physicochemical aspects of the interfacial behaviour of biomolecules.

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.

Binding mode and thermodynamic studies on the interaction of the anticancer drug dacarbazine and dacarbazine-Cu(II) complex with single and double stranded DNA.

The binding mode and thermodynamic characteristics of the anticancer drug dacarbazine (Dac) with double and single stranded DNA were investigated in the absence and presence of Cu(II) using cyclic voltammetry, square wave voltammetry and fluorescence spectroscopy. The interaction of Dac and Dac-Cu(II) complex with dsDNA indicated their intercalation into the base stacking domain of dsDNA double helix and the strength of interaction is independent on the ionic strength. The interaction of Dac with dsDNA in the presence of Cu(II) leads to a much stronger intercalation. The interaction mode of Dac molecules with ssDNA is electrostatic attraction via negative phosphate on the exterior of the ssDNA with Dac. The binding constants, stoichiometric coefficients and thermodynamic parameters of Dac and Dac-Cu(II) complex with dsDNA and ssDNA were evaluated. Comparison of the mode interaction of Dac with dsDNA and ssDNA was discussed. The decrease of peak current of Dac was proportional to DNA concentration, which was applied for determination of dsDNA and ssDNA concentration.

Voltammetric studies on the absorption and association of cytidine dinucleotides in solutions of varying pH at charged interfaces

The adsorption and association in the adsorbed state of cytidylyl-(3−′ → 5−′)-adenosine (CpA), cytidylyl-(3−′ → 5−′)-guanosine (CpG) and cytidylyl-(3−′ → 5−′)-uridine (CpU) has been studied at the hanging mercury drop electrode by phase-sensitive ac voltammetry and single-sweep voltammetry. The association of cytidine dinucleotides in the adsorbed state has been investigated as a function of pH, adsorption time, adsorption potential and the bulk concentration of the cytidine dinucleotide. At all bulk concentrations, the adsorbed layer is transformed into a condensed film due to significant stacking forces acting between adjacent rings of the base residues. The adsorption parameters for the condensed film were evluated at various pH values. The limiting value of the surface concentration calculated from the voltammetric measurements indicates that the average surface area per adsorbed mononucleotide unit is 0.43 nm2, 0.50 nm2 and 0.53 nm2 for CpA, CpG and CpU, respectively. These values are comparable to the values resulting under the same conditions for CpC and poly-C. It is concluded that in all cases adsorption occurs via the base residues attached to the nucleotide chain.

Square wave adsorptive stripping voltammetric determination of anticancer drug nilutamide in biological fluids using cationic surfactant cetyltrimethylammonium bromide

A sensitive square wave adsorptive stripping voltammetric method was described for the determination of anticancer drug nilutamide (NLM) in biological fluids based on the enhancement effect of the cationic surfactant: cetyltrimethylammonium bromide (CTAB). With pH 6.0 Britton–Robinson (BR) buffer as the supporting electrolyte and in the presence of 1.45 × 10−4 mol L−1 CTAB, NLM yields a well-defined and sensitive reduction peak at the ZnO nanoparticle modified carbon paste electrode (ZnONPs/CPE). Various chemical and instrumental parameters affecting the monitored electroanalytical response were investigated and optimized for NLM determination. The electrochemical parameters such as the surface concentration (Γ), electron transfer coefficient (α) and standard rate constant (ks) of NLM at the modified electrode were calculated. The achieved limits of detection and quantification were 3.21 × 10−9 mol L−1 and 1.07 × 10−8 mol L−1 by square wave adsorptive stripping voltammetry (SWAdSV), respectively. The proposed method was successfully applied for the detection of NLM in blood and urine samples with good accuracy and precision.

Kinetics of hydrolysis of some N′-(4-substituted benzylidene) salicylohydrazides

The rates of hydrolysis of a series of substituted benzylidenesalicylohydrazide derivatives in 40%(v/v) ethanol–buffer mixture have been investigated by differential pulse polarography. The hydrolysis is catalysed by H+ and its rate follows strictly first-order kinetics. The rate constant decreases with increasing pH up to around pH 3.9, beyond which no measurable reaction was observed; here the protonation of imine begins to be significantly incomplete. The attack of water on the protonated substrate is considered to be the rate-determining step. The effects of pH, molecular structure, and temperature on the reaction rate and the activation energy are reported. Thermodynamic parameters (ΔG‡, ΔH‡, and ΔS‡) for the hydrolysis are discussed.