Effect Of Divalent Ions On DNA Coagulation At Different PH Values

DNA condensation and charge reversal play an important role in life sciences,polymer physics,and medical applications.In solution,the condensation of DNA depends on the valence of the counterion in the solution,and cations having a valence of three or more are usually required to cause DNA condensation.However,the most common small counterions in cells,such as Na⁺,K⁺,Ca²⁺,and Mg²⁺,are less than three metal cations that do not normally condense DNA.We have found that by changing the pH of the solution,divalent metal cations can cause DNAto coagulate.Through single-molecule Magnetic Tweezers?MT?experiments,we have clearly demonstrated that by changing the pH of the solution,Mg²⁺and Ca²⁺can cause DNA condensation.When the pH of the solution is between 5 and 8,the DNA after the action of the divalent ions is in a free stretch state.When the pH of the solution is about4,the DNA is in an unstable transition state,that is,a transition from free stretching to condensed state.When the pH of the solution is about 3,the DNA is in a stable coacervation state,and the coagulation process is reversible,which can be accomplished by adjusting the pH of the solution.At the same time,we also found that at low pH,as the concentration of divalent ions increases,the cohesive force?Fc?of DNA increases first and then decreases,and deagglomeration occurs.We also confirmed the dependence of the condensed form of DNA on pH by atomic force microscopy?AFM?.Furthermore,we found that the electrophoretic mobility of DNA at the critical aggregation of DNA caused by divalent counter ions is about-1.0×10^-4cm²V^-1s^-11 by dynamic light scattering experiment?DLS?,which corresponds to about neutralizing DNA89%of the charge.The bivalent counterion of the Bimanin theory is about 1%more than the 88%charge of the neutralized DNA,which is also effective for DNA condensation by the bivalent counterion in a low pH solution.Finally,we used the coarse granulation model of Langevin dynamics to simulate the effect of pH on DNA condensation,and found that the simulation results are consistent with the experimental results.