Role of counterions in the gigahertz relaxation of wet DNA

We have measured the dielectric properties of concentrated solutions and gels (≤30 mg/ml) of random-sequenced DNA from E. coli in the 400-MHz–26-GHz range. Two Debye-type relaxations are evident, one with a relaxation time near 9 ps and attributable to the classical Debye relaxation of water. More noteworthy is a second relaxation process with a characteristic time in the 20–200-ps range, i.e., a relaxation frequency in the 0.8–8-GHz range, depending upon the species of the counterions and the temperature. The slower relaxation process has an enthalpy of 3.3 kcal/mol and is accounted for by a counterion-based relaxation process. These experimental results are considered in terms of two models from polyelectrolyte theory, one by Oosawa [Biopolymers 9, 677 (1970)] and Wyllie [in Dielectric and Related Molecular Processes, edited by M. Davies (American Chemical Society, Washington, D.C., 1972), Vol. 1], and the other by Manning [Q. Rev. Biophys. 11, 179 (1978); Acc. Chem. Res. 12, 443 (1979)], and we propose that different ion-based relaxation mechanisms dominate in different hydration regimes.