Z.A. Ahmed

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.

The effect of pH on the adsorption and the association of cytidine, cytidine-5′-monophosphate and cytidine-5′-diphosphate at the charged interface☆

Abstract The effect of pH on the adsorption and association of cytidine, cytidine-5′-monophosphate and cytidine-5′-diphosphate was studied by phase-sensitive a.c. polarography. At pH ≤ 5.2 and at lower concentrations of N(3)-protonated cytidine or cytidine phosphate, a single-step Frumkin isotherm was obtained, whereas at elevated bulk concentration the formation of a compact film reflects a double-step Frumkin isotherm. In the neutral and slightly alkaline range (pHs 7.0, 8.0), the alternating stacking model, due to hydrophobic interactions of the bases of the substances investigated, decreases and consequently pit formation does not occur. The effect of pH on the adsorption parameters of cytidine and cytidine phosphate is discussed. The influence of the phosphate group on the extent of adsorption of cytidine is also considered.

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.

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.

Adsorptive stripping analysis of thiocytosine at the static mercury electrode surface

SummaryThe cathodic adsorptive stripping method has been applied for the trace determination of thiocytosine (Tcyt) using differential pulse voltammetry at the HMDE. The strong interaction of Tcyt with the mercury electrode surface leads to the formation of an adsorbed film of Hg(II)-thiolate complex at the electrode surface. The formation of this film is followed by cyclic DC- and differential pulse voltammetry. It has been found that formation of such film depends on various parameters, e.g. pH, concentration, adsorption time and adsorption potential. The influence of these parameters on the shape and the peak height of the cathodic stripping peak during the negative scan of potential has been determined. The influences of anions (SO42−, Cl−, ClO4−, NO3−) and cations [Cu(II), Ni(II), Pb(II) and Cd(II)] on the method of analysis are considered. The calibration curve for the determination of Tcyt is linear and the detection limit is 1.49×10−8 mol/l, whereas in the presence of Cu(II) the method is developed and the detection limit for Tcyt determination is lowered to 4.99×10−9 mol/l. A statistical analysis of the calibration curve data is included.

The interfacial behaviour of guanosine mono-, di- and triphosphate in solutions of varying pH at charged interfaces

Abstract A systematic study of the adsorption and interfacial behaviour of guanosine mono-, di- and triphosphate (5′-GMP, 5′-GDP and 5′-GTP) at the h.m.d.e., has been carried out in different buffer solutions by phase-sensitive a.c. voltammetry. At low bulk concentrations, molecules of guanosine phosphate adsorbed at the maximum adsorption potential in a dilute adsorption layer are oriented planar to the electrode surface, where the interaction of π electrons with the interface favours adsorption. At bulk concentrations above a threshold value, the stacking interactions between vertically oriented molecules lead to association and formation of a compact layer. The adsorption can be described quantitatively for both types of adsorption layers by single- and double-step Frumkin isotherms, respectively. The resulting adsorption parameters are evaluated and conclusions on the respective interfacial behaviour, orientations and interactions of these substances are discussed. The influence of the phosphate group on the surface reorientation of guanosine at the charged interface was also considered.

Adsorption and association of 6-thiopurine and 6-thiopurine riboside at charged interfaces

Abstract A systematic study on the adsorption and association of 6-thiopurine (6-TP) and 6-thiopurine riboside (6-TPR) has been carried out at various pH values and the adsorption parameters were determined quantitatively. The adsorption was followed by out-of-phase alternating current voltammetry and cyclic voltammetry at a hanging mercury drop electrode. A comparative study was undertaken on the adsorption and association of the investigated thiopurines and the similar type of nucleic acid components containing purine bases. The base-containing thio group enhances stacking interaction and facilitates formation of the perpendicularly stacked layer on the electrode surface.

Voltammetry of adsorbed cancerostatic actinomycins

Abstract A systematic study of the adsorption and association of the cancerostatic drug actinomycin-C 1 (ACT) at a hanging mercury drop electrode (HMDE) has been conducted using phase-sensitive a.c. voltammetry and cyclic voltammetry (CV). At all bulk concentrations, the adsorbed layer is transformed into a condensed film by the significant stacking forces acting between adjacent rings of the phenoxazone residues. The nucleation and growth mechanism is confirmed and the data are analysed using the Avrami equation. The adsorption parameters for the condensed film were evaluated at various pH values. In addition, the preparative electrochemical reduction of ACT was performed using the large-scale electrolysis and differential pulse polarography. The consequences for DNA interaction and membrane adsorption are discussed.

Comparative studies of the interfacial behaviour of some nucleic acid bases with and without thio group

Abstract A comparative study on the adsorption and association of nucleic acid bases with and without thio group such as guanine (Gua), guanosine (Guo), 6-thioguanine (6SGua), 6-thioguanosine (6SGuo) and 8-thioguanosine (8SGuo) was conducted using phase sensitive a.c. voltammetry. The reduction of the adsorbed species at the charged interface was studied using cyclic voltammetry and controlled potential coulometry. The influence of the thio group on the stacking interaction between vertically oriented molecules is discussed. The limiting value of the surface concentration Γm and the average surface area Sm per adsorbed molecule indicates the rather more compact interfacial structure for thioguanine and thioguasine than for guanine and guanosine. The extent of intermolecular and intramolecular association is most pronounced for the nucleic acid bases containing thio group.

VOLTAMMETRIC STUDIES OF THE ADSORPTION AND CONFORMATION OF POLYRIBOCYTIDYLIC ACID AT THE MERCURY ELECTRODE SURFACE

Abstract The adsorption and interfacial behaviour of polyribocytidylic acid (poly-C) was studied by phase sensitive ac voltammetry and single-sweep voltammetry. The two helical conformations of poly-C were investigated as a function of various parameters such as pH, adsorption potential, temperature and the nature of the anions of supporting electrolyte. The adsorbed acid form of poly-C undergoes an opening of the double helix because the electric field at the charged interface renders some of cytosine moieties accessible for reduction. The limiting value of the surface concentration of poly-C indicates that the adsorption occurs via the base residues attached to the nucleotide chain. The results are compared with the similar adsorption behaviour of polyriboadenylic acid (poly-A) and DNA.