Influence of temperature and ionic strength on the low‐frequency dielectric dispersion of DNA solutions

The dielectric properties of DNA solutions at low frequencies (5 Hz to 2 kHz) have been measured by means of a four‐terminal bridge method utilized to minimize electrode polarization errors. At 24°C native salt‐free DNA has a very large specific dielectric increment, Δε/c = 9.8 × 106 l/mol and a very low frequency relaxation centered at 18 Hz. Both the dielectric increment and the relaxation time are greatly decreased by partial heat denaturation at temperatures above 60°C or by addition of salt, the effects being much larger for divalent anions. These results are shown to be in qualitative agreement with theoretical treatments of counterion fluctuation polarization by McTague and Gibbs for the equilibrium case and by Mandel for relaxation. The ratio of the relaxation time for the low‐frequency process to that previously observed at much higher frequencies suggests that these relaxations result from counterion fluctuations along the longitudinal and transverse axes of the molecule, respectively.