DNA precipitation revisited: A quantitative analysis

Abstract

AbstractWe have quantitatively investigated the precipitation of DNA in different valence cations and in various ethanol concentration solution. In free ethanol condition, monovalent ion cannot induce DNA condensation and in turn to precipitation. Divalent ion can cause DNA precipitation as the concentrations of both Mg2+ (MgCl2) and Ca2+ (CaCl2) go up to a critical value of about 50 mM. After adding ethanol, monovalent ion and divalent ion can promote DNA precipitation significantly, but it has no effect on the process by trivalent ions. In monovalent ion solution of 100 mM, the critical volume ratio of ethanol is about 52% for DNA precipitation. However, the value decreases significantly to 4% in 25 mM divalent ion solution. Based on the precipitation curves, we quantitatively calculated the binding energies between the cations and phosphate group of DNA. For monovalent ions and in aqueous solution, the Na+‐DNA binding energy is about −5.52 KJ·Mol−1 (−2.63 kBT, the precipitation temperature is 253 Kelvin) and the K+‐DNA binding energy is about −4.77 KJ·Mol−1 (−2.27 kBT). For divalent ions and in the same condition, the Ca2+‐DNA binding energy is about −8.25 KJ·Mol−1 (−3.93 kBT) and the Mg2+‐DNA binding energy is about −8.67 KJ·Mol−1 (−4.13 kBT). The scenario of DNA precipitation modulation by ethanol and zwitterionic ions is demonstrated by atomic force microscopy (AFM) imaging intuitively.