Vladimír Vetterl

Alternating current polarography of nucleosides

Summary A.c. polarograms of nucleosides currently occurring in nucleic acids exhibit a minimum at a potential of about −0.4 V, caused by the adsorption of nucleosides on the electrode surface. At higher concentrations of deoxycytidine, adenosine, guanosine, and deoxyguanosine, association of the adsorbed molecules takes place in the neighbourhood of this potential. With deoxyadenosine, association of the molecules occurs at about a potential of −1.2 V. With adenosine, association occurs at both −0.4 V and −1.2 V, approximately. As with bases, the transition from the non-associated to the associated state occurs over a close concentration interval in which the adsorption isotherm has an inflection point. With uridine, thymidine, and cytidine, no association of the adsorbed molecules takes place under the given experimental conditions, even at concentrations approaching saturation value. At pH=7.0, most of the nucleosides tested are polarographically non-reducible and the maxima observed on the a.c. polarograms are of a capacitive character. Only the peak for cytidine and deoxycytidine at a potential of −1.6 V is caused by reduction of cytosine.

Electric field effect in the adsorption of adenosine and cytosine

Abstract The influence of pH and structure-breaking anions ClO4− on the adsorption of cytosine was studied and compared with the results obtained for adenosine. The a.c. polarograms of acid solutions of cytosine exhibit at higher cytosine concentrations two pits, indicating the region of potentials at which the cytosine molecules are oriented perpendicularly to the electrode surface and associate. At low pH values the reorientation of cytosine molecules from the planar to the perpendicular position can be explained by the interaction of the electric field of the electrode with the protonated N1 of cytosine. With neutral molecules of cytosine and adenosine the reorientation to the perpendicular position is obviously supported by the interaction of the electric field with the permanent and induced dipole moments of the molecules. The mechanism of adsorption and association of adenosine and cytosine on the negatively charged surface is discussed. ClO4− anions disturb the stacking interactions between adsorbed molecules of cytosine and prevent the association.

On the stability of condensed adsorption films

Abstract The conditions under which condensed adsorption films form are investigated theoretically. On the basis of the Ising lattice gas model, we determine on which parameters the critical temperature and the condensation temperature of the condensed adsorption film depend. In particular, it follows that, contrary to the condensation temperature, the critical temperature is independent of the concentration of the adsorbate in the solution. This statement of the theory has been corroborated by way of the adsorption film formation of adenine at the mercury/electrolyte interface.

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

Abstract The pit, on alternating current polarograms of bases, indicating the region of potentials where the association of the adsorbed molecules takes place and a compact surface film is formed usually appears near the potential of the electrocapillary maximum. An exception in this respect is cytosine, which forms the pit at more negative potentials. The negative pit corresponds to the surface film formed by molecules of cytosine adsorbed electro- statically at the negatively charged mercury surface via their positive charge or the electropositive site. In the presence of Br− or I− ions the association of cytosine occurs also near the potential of the electrocapillary maximum. The I− ions allow association at the electrically neutral electrode surface to occur more easily than do the Br− ions. The surface film at the neutral electrode surface is formed by molecules of a complex of Br-cytosine, and/or I-cytosine. This complex has a higher surface activity than cytosine alone, due to the high polarizability of the bromine or iodine atoms, and the maximum adsorption is thus observed near the potential of the electrocapillary maximum, in a similar way as with 5-Br-cytosine or 5-I-cytosine.

Adsorption of nucleosides on mercury surface

SummaryAll nucleosides currently occurring in nucleic acids are, under the given experimental conditions, surface-active substances. If the concentration is sufficiently high, the most of nucleosides in the electrode surface associates and creates a surface film just as the corresponding bases do. Only with uridine, thymidine, and cytidine, unlike the corresponding bases, no associations in the electrode surface occur.

Adsorption of xanthosine, inosine and 6-azauridine on the dropping mercury electrode☆

Abstract The capacitance curves of inosine, xanthosine and 6-azauridine at pH 7 were determined by means of B reyer s alternating current polarography. The anomalous concentration dependence of the capacitance curves of inosine and 6-azauridine observed at higher concentrations (c > 10 mM) is explained by self-association of nucleoside molecules in solution. In contrast to inosine and 6-azauridine, xanthosine does not associate at the electrode surface. The coefficients of F rumkin s adsorption isotherm were determined for xanthosine and 6-azauridine.

342 - Reorientation of the Cytosine Derivatives at the Electrode surface

The alternating current polarograms of cytosine, cytidine and deoxycytidine at pH 5 and cytidine at pH 9 were measured. With increasing concentration of the cytosine derivatives a sort of a pit appears on the a.c. polarograms indicating the region of potentials at which the bases and/or nucleosides are reoriented at the electrode surface from the planar position to the perpendicular one, associate and form a compact surface film. Cytidine at pH 5 begins to form a pit starting at a concentration of about 15 mM. At pH 9 cytidine begins associate already at a concentration of 7 mM. Deoxycytidine at pH 5 associate on the electrode surface at still lower concentrations than cytidine. n nThe potential Upit of the centre of the pit of the investigated derivatives shifts to more negative values in the order deoxycytidine pH 5 (−0.45) - cytidine pH 5 (−0.70) - cytidine pH 9 (−0.85) cytosine pH 5 (−1.20 V). This order seems to correlate with the increasing value of the electrostatic attraction of the cytosine derivatives on the negatively charged electrode via their positive charge on N-3 and/or dipole moment. According to this conception the greatest electrostatic adsorption on the negatively charged electrode should be exhibited by cytosine at pH 5, which is attracted to the electrode via the protonated N-3, the next in the series being cytidine at pH 9, which is attracted to the electrode via the dipole moment. The smallest electrostatic adsorption exhibits deoxycytidine at pH 5, by which the effective value of the dipole moment is reduced due to the positive charge on N-3. n nThe more negative is the potential Upit of the pit of nucleosides, the lower is the capacitance of the minimum of the pit indicating the changes in the orientation of the adsorbed nucleosides and/or changes of the compactness of the surface film. The possible orientations of the cytosine derivatives in the compact layer on the electrode surface are presented. At more negative electrode potentials the positive charge (cytosine at pH 5) or positive end of dipole moment (cytidine at pH 9) points to the electrode surface. n nWith cytidine at pH 9 the pit disappears at concentrations higher than about 100 mM, probably due to the self-association of the neutral cytidine molecules in the bulk of solution. At pH 5 the pit is observed with all of the investigated derivatives up to the highest concentrations. The self-association is at pH 5 possibly prevented due to the repulsion of charges at N-3.

Two-dimensional condensation of benzalkonium chloride at the mercury electrode and its relation to DNA imaging using scanning force microscopy

Abstract We show two important properties of the cationic detergent benzalkonium chloride (BAC): (a) formation of condensed layers of BAC at the mercury electrode, and (b) ability of BAC to facilitate anchoring of DNA at the mica surface which makes it possible to visualize individual DNA molecules by scanning force microscopy (SFM). The adsorption behaviour of BAC (in the absence of DNA) was studied by ac voltammetry using a hanging mercury dropping electrode (HMDE) and a dropping mercury electrode (DME) at various temperatures and BAC concentrations. Two pits were observed on the capacitance-potential curve, one located at the potential of zero charge (pzc) and the other in a narrow potential range around −1.7 V. The pit at the pzc corresponds to a condensed layer formed by the BAC molecules which may be adsorbed via the hydrophobic alkyl chain pointing towards the electrode surface. The pit at the negatively charged electrode indicates a condensed layer in which the BAC molecules may be adsorbed in a different way, probably via the positively charged nitrogen. In both orientations the BAC molecules may have their benzene rings perpendicular to the electrode surface with stabilization of the condensed phase by the stacking interactions between the benzene rings. The condensation of BAC molecules on the electrode surface is stimulated by sulphate anions and hindered by bromide anions. Our results suggest that the adsorption behaviour of BAC at the liquid | liquid interface at negative charge densities can be used as a model for DNA + BAC spreading on mica as used in SFM.

Association of methyl, amino and hydroxy derivatives of adenine at a mercury electrode at various sodium chloride concentrations

Abstract The effect of methyl, hydroxy and amino substituents on the adsorption and association of adenine on the surface of a mercury electrode at various NaCl concentrations was studied. All these substituents prevent the association and surface film formation of adenine on the negatively-charged electrode surface, where protonated unsubstituted adenine associates at low temperatures. A methyl substituent in position 3 or 6 prevents the association of adsorbed adenine derivatives not only on the negatively charged mercury surface, but also on the neutral mercury surface. 1-Methyladenine associates on the neutral mercury electrode (near −0.6 V) only at higher NaCI concentrations; 2-methyladenine associates on the neutral mercury surface well at low and high ionic strengths but less at medium ionic strengths, similarly to adenine. 2-Aminoadenine associates at low ionic strengths on the positively-charged mercury surface (near −0.2 V) and at higher NaCl concentrations on the neutral surface (near −0.7 V). 8-Hydroxyadenine did not associate on the mercury electrode. The double pit or more complicated course of the capacitance pit observed with 1-methyladenine, 2-methyladenine and 2-aminoadenine at higher NaCl concentrations may be caused by a change in the structure of the surface film.

Sensing of parameters behind attachment of human plasma fibrinogens on carbon doped titanium surfaces: an optical study

Polished titanium surface and four differently carbon doped titanium surfaces are investigated to characterize adsorption and desorption of human plasma fibrinogen (HPF) molecules. The surface tension and surface energy of carbon doped titanium and other comparative titanium surfaces used in the experiments were observed by measuring optically the contact angle of water droplet on the treated surfaces. The dielectric constant of each bulk surface was measured utilizing ellipsometry in dry environment. Whereas the temporal adsorption or desorption of HPF molecules on test surfaces in background electrolyte with and without HPF molecules were measured using an optical correlator, which utilizes a diffractive optical element (DOE) in non-contact domain. The optical correlator operates in coherent and in non-coherent mode, which allows sensing of optical path differences providing information on the optical roughness (Ropt), contrary to the mechanical roughness obtained from atomic force microscope (AFM) profilometer, and reflectance of the surfaces immersed into a liquid. The knowledge of the parameters helps us to understand mechanisms behind attachment of HPF molecules on biomaterial surfaces in hard tissue replacement.