DISSERTATION

______________________________________________________________________ Introduction
Figure 1.7. a) Potential distribution in relation to distance from the electrode for different
ionic strengths. Φ0 of 100 mV was used for the calculation. Figure adapted with
permission from ref. 42 . Copyright (2010) American Chemical Society. b) Potential profile
calculated for different Φ0 values and an ionic strength of 10 mM. Figure adapted from
ref. 43 .
In solutions of intermediate ionic strength, where the Debye length spans over few DNA base
pairs the influence of the applied potential is more significant, scaling with the distance from
the electrode surface (Figure 1.8, b). In this case, it is possible to manipulate the conformation
of dsDNA by applying positive (invoking a lying conformation) or negative potentials
(invoking up-right conformation). Still, control of dsDNA is achieved with less difficulty than
of ssDNA, due to their persistence lengths, while ssDNA manipulation is only partial depending
on the Debye length. While at a positively polarized electrode ssDNA will remain in the lying
conformation, negative potentials evoke at least partially an up-right orientation. The upper part
of the ssDNA is not exposed to the electric field and it exhibits a randomized conformation.
In solutions of low ionic strength charge screening is weak resulting in high charge repulsion.
Therefore, dsDNA is not stable under these conditions. Furthermore, applied electric potentials
are very long ranged, which leads to an efficient repulsion of the ssDNA from the surface
(Figure 1.8, c). Since the persistence length depends on the ionic strength, in solutions of low
ionic strength ssDNA exhibits a rigid conformation. Thus, combining these two effects, ssDNA
1.2 DNA 14