DNA gelation in concentrated solutions

Sonicated calf-thymus DNA (200 ± 30 base pairs) spontaneously forms viscoelastic gels over a wide range of concentration, temperature, and buffer conditions. Quasielastic light scattering (QLS) can be used to monitor this process, because the ratio of dynamic-to-static scattering intensity decreases dramatically as gelation occurs. Using QLS, we have explored the effects of DNA concentration and mono- and divalent cations on the thermal stability of DNA gels. We found that the gel–sol transition temperature (T_gel) varies linearly with [DNA] from 7.5 to 17 mg/mL. Both Na⁺ and Mg²⁺ strongly stabilize the gel state. The sharpness of the transition increases with increasing ionic and DNA concentrations. Analysis of the Na⁺-dependent gelation indicates that the process requires the association of one Na⁺ per 118 base pairs. Mg²⁺ effectively stabilizes the gel at concentrations 10-fold below those required for Na⁺. The unexpectedly large effect of Mg²⁺ suggests that ion-specific interactions may play an important role in determining gel stability.