Low field electric birefringence study of monovalent cation-DNA interactions

The effects of monovalent cations on DNA have been studied using static and dynamic electric birefringence. Kerr's law is obeyed in a limited E range (<30 Vcm_?1) and the steady state birefringence values are close for the different cations. The birefringence kinetics have been analysed in terms of three relaxation times. On a semilogarithmice plot of Δn(t), the tail of the curve is linear over a wide range of time for Na⁺, K⁺, NH₄⁺ and Li⁺. Only for Cs⁺ solution is no linear part found and a much longer relaxation time is determined. This only contributes a small part of the total birefringence. With Cs⁺ this contribution is more field-dependent than for the other cations and we observe a larger molecular flexibility. On the other hand, with Li⁺ a greater stiffness of the DNA molecule appears. The electrical polarizabilities anisotropies decrease in the order: Cs⁺ >NH⁺₄ >K⁺ >Na⁺ >Li⁺. There are no significant differences in the optical anisotropy factors.