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STATE-SOLVENT INTERACTIONS FROM IMPEDANCE MEASUREMENTS:
CONCENTRATION DEPENDENCE OF DNA
The unique nature of impedance data exhibited by palladium lipoic acid (1:1) complex (1), a chemotherapy agent
developed in our laboratory, prompted us to investigate in detail the impedance of alkali chlorides (2) as well
as the most important biological molecule DNA (3), The alkali chloride data revealed frequency and potential dependent
orientation effects of the water molecule. DNA impedance data was subtly dependent on the nature of the alkali
metal iron. This prompted us to study further the concentration dependence of DNA impedance without any added electrolyte.
The electrochemical literature date of nucleic acids (4) is at very low concentrations of DNA and at negative potentials
of the mercury electrode. The present investigation was intended to gain information on the influence of orientation
of solvent molecules, packing, and dopant ions on the impedance. Therefore concentrations of 0.1, 1.0, 5.0, and
10.0 mg/ml. Calf-Thymus double stranded DNA (Type 1, sodium salt, pH 7.5) were used at potentials positive enough
to produce dopant ions of mercury.
Typical admittance plots(Figures 1 and 2) indicate that solvent orientation and
DNA conformation effects are more dominant (with four peaks) at higher concentrations of DNA than at lower concentrations
(two peaks). The effects are also enhanced at lower frequencies. Mott-Schottky plots indicate dominant p-type behavior.
Double layer capacitance plots exhibit two peaks at positive potentials. At potentials between 0.2 and 0.3 V, the
impedance plot (Figure 3) and especially the phase angle plot (Figure
4) indicate that the double layer is altered. Unlike alkali halides, the double layer DNA concentration...
Low frequency effects are dominant during this changeover of double layer structure.
For all concentrations of DNA, the impedance date at -1.5, -1.0, -0.6, -0.3, 0.0, 0.1, 0.2, and 0.3 V could be
fitted with the equivalent electronic circuit of R (RC)(RC)(RC). This circuit suggests a transmission line model
for DNA conductance.
References
1. C.V.Krishnan, and M. Garnett, 1st Spring Meeting of ISE, Abs. P06, Spain 2003
2. 2. C.V. Krishnan, and M. Garnett, 226th ACS National Meeting, Abs. Inor. 0028, NY 2003
3. C.V. Krishnan, and M. Garnett, 204th Meeting of ECS, Abs. 1378, Orlando 2003.
4. E. Palecek, M.Fojta, F.Jelen, and V. Vetterl in Bioelectrochemistry, Vol. 9 Chapter 12, p365, Edited by George
S. Wilson, Wiley-VCH, Weinheim, 2.
Figure 1
Figure 2
Figure 3
Figure 4
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