Effect of Cl−, Br− and I− ions on the adsorption and association of cytosine at a mercury electrode

Adsorption and two-dimensional condensation of 5-methylcytosine

Purine and pyrimidine derivatives occurring in nucleic acids posses an extraordinary high ability of self-association at the electrode surface and can form there by a two-dimensional (2D) condensation a monomolecular compact film (self-assembled monolayer-SAM). The effects of methyl substituent on the 2D condensation were studied using the 5-methylcytosine molecule which is involved in gene silencing and has a great biological impact. At acid pHs, 5-methylcytosine forms at the mercury electrode a physisorbed self-assembled 2D layer at potentials close to the potential of electrocapillary maximum. From the temperature dependence of the electrode double layer capacitance, the standard Gibbs energy of adsorption (Delta G(m)=-12.7 kJ mol(-1)), lateral interaction coefficient of the Frumkin adsorption isotherm (a(c)=2.05) and area occupied by one molecule (A=1.31 nm(2)) in the 2D layer were determined. Measurements performed on a single-crystal Au(111) surface show that the 2D condensation can take place on other substrates as well.

Adsorption of Cytosine on a Mercury Electrode

The electrosorption behavior of cytosine at the mercury electrode/acetic buffer of pH 4 and 5 interfaces was determined from the double-layer differential capacity measurements extrapolated to zero frequency. Solutions of cytosine were prepared to cover the range from 1 × 10-4 to 6 × 10-3 mol dm-3. Adsorption of cytosine was described by the adsorption isotherms constants derived from the surface pressure data as a function of electrode charge density and bulk concentration. The obtained values of the relative surface excesses Γ′ were higher in the acetic buffer of pH 4 than of pH 5. Maximum of cytosine adsorption in the mentioned buffers was at -581 and -551 mV, respectively. The values of the standard Gibbs energy ∆G° obtained from the Frumkin isotherm were higher in the buffer of pH 4 than of pH 5. The values of the interaction parameter A indicated weaker repulsive interaction between adsorbed molecules of cytosine in the former buffer. The adsorption parameters obtained from the virial isotherm confirmed corresponding parameters obtained from the Frumkin isotherm. The dependences of ΦM-2 on the relative surface excess at a constant charge density were analyzed in order to calculate the electrostatic parameters of the inner layer.

On the formation kinetics of two-dimensional cytidine films

The kinetics of phase transitions of cytidine adsorbed on mercury are studied by chronoamperometry and capacitance measurements. Cytidine forms highly ordered two-dimensional adlayers in a broad range of pH. In acid solvent, only one kind of condensed layer is formed. In the alkaline solution, cytidine forms two different two-dimensional (2D) adlayers. The minimum capacitance value in adlayer II at pH 5 is 7.0 microF cm(-2) and, at pH 8.3, it is 5.1 microF cm(-2); in adlayer III, the minimum capacitance is 10.6 microF cm(-2). The formation of a physisorbed film of cytidine molecules adsorbed at the mercury surface proceeds by complex mechanisms. From j-t transients, it can be seen that the phase transformations from dilute adlayer Ia to condensed physisorbed film II is accompanied by the reorientation of cytidine molecules at the mercury surface (inverted current transient). The interfacial transformations of the cytidine film yield a sigmoidal C-t transient. This experimentally measured C-t transient were analysed by Avrami theorem. The rate of the transformations from dilute adlayer Ia to condensed film II of cytidine at pH 5 depends strongly on temperature but is only slightly affected by temperature at pH 8.3. The effect of pH and ionic composition of the supporting electrolyte on the rate of transformation of cytidine films was studied as well.

Adsorption and Two-Dimensional Condensation of Thiabendazole at a Mercury/Solution Interface

The adsorption behavior of the fungicide thiabendazole [2-(4'-thiazolyl) benzimidazole] was studied with out-of-phase ac voltammetry using a hanging mercury drop electrode in 0.3 M KCl solutions at various temperatures and thiabendazole concentrations. A state of "normal" adsorption and a condensed two-dimensional layer at moderately charged and rather negatively charged surfaces, respectively, have been characterized. Copyright 1999 Academic Press.