| Popis: |
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. The 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. The 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. With 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. |
