Investigations on the spermine provoked liquid crystalline phase behavior of high molecular weight DNA in the presence of alkali and alkaline earth metal ions

Abstract

The effect of alkali and alkaline earth metal ions on the spermine induced condensates of high molecular weight DNA was studied. The spermine induced DNA precipitation was facilitated by the alkali and alkaline earth metal ions, suggesting that the metal ions instead of getting into competition with spermine in DNA binding, had acted in concert to precipitate DNA from solution. Spermine-DNA–metal ion condensates exhibited mainly two different phases and at least one of them was fluid in nature, which is the cholesteric phase and a columnar hexagonal phase with a restricted fluidity where the DNA molecules are more closely packed. In the presence of alkali metal ions spermine–DNA aggregates were mainly in the cholesteric phase but at higher spermine concentrations, in the presence of Rb⁺ and Cs⁺, they adopted a cholesteric to columnar arrangement, suggesting that the increased size of Rb⁺ and Cs⁺ facilitated the cholesteric to columnar phase transition. Among the alkaline earth metal ions, the existence of fluidic cholesteric textures in the presence of Mg²⁺ can be thought of probably as a clue to the possible synergistic role of polyamines and the Mg²⁺ in the cell nucleus, in preserving the fluidity required for the biological functions of DNA within the condensates. The evolution of the columnar phase from the cholesteric phase in the presence of Ca²⁺ also indicates the probable role of Ca²⁺ in packing DNA into hexagonal arrangement in vivo. Among all the metal ions studied, the behavior of Na⁺ was exceptional in inducing DNA resolubilization at a very low concentration of 12 mM spermine whereas with the other metal ions DNA got resolubilized at or above 400 mM spermine concentration. Small angle X-ray diffraction peaks obtained at 2θ value between 0 and 5 also indicate the formation of the columnar hexagonal phase with Rb–DNA, Cs–DNA and Ca–DNA systems.