  The dependence of DNA assemblies conductance on relative humidity is
investigated theoretically. Following earlier suggestions, we consider the
ionic conductivity through the layers of water adsorbed by DNA molecules. The
increase in humidity results in a growing water layer. The binding energy of
ions depends on the thickness of the water layer due to change in water
polarization. This dependence is very strong at smaller thicknesses of water
layers due to the low-dimensional confinement of an electric field in water. We
show that the associated change in ion concentration can explain the six orders
of magnitude increase in conductivity, with relative humidity growing from 0.05
to 0.95
