The Experimental Research Of Divalent Manganese Ion-Induced DNA Conformation Change

In this article,the direct evidence of the Mn²⁺ ion-induced DNA condensation was provided by single molecule measurements and atomic force microscope characterization.This single molecule observation,ruling out the interaction between the DNA molecules,undoubtedly indicates the Mn²⁺-induced DNA condensation at room temperature.To see whether this condensation occurs in ensemble experiments,AFM was used to characterize the morphology of the Mn²⁺ induced DNA conformation in the same buffer condition.At [Mn²⁺]=1mM,DNA remains to be the typical coil conformation.Above 3mM,typical aggregated structures were observed,suggesting the DNA condensation behavior.The finding suggests the failure of the Manning theory to explain the Mn²⁺-DNA interaction and alters the view that divalent metal ion cannot cause DNA condensation.For divalent metal ions,the theory predicts that about ⁸8% charges of DNA chain are screened,and the electrostatic resistance resulting from the remaining charges still prevents the further DNA condensation.However,the theory fails to explain the present finding.The failure can be traced back to the premise of diffusive binding with which the theory sustains.Diffusive binding allows the mobility of the counterions in the bound region.The binding of the alkyl metal ions on DNA is through the electrostatic attraction between the ions and the charge of the phosphate part of DNA chain,which fits the characteristic of diffusive binding.In contrast,previous studies have concluded that the binding of Mn²⁺ on DNA prefers the N7?G?atom in the groove region of DNA and could even alter the local DNA structure,which suggests the strong localized binding mode;hence the freedom of the bound ions is limited.In order to evaluate the effect of the elevated temperature,the MT measurement was conducted at a temperature of 55°C.Out of the expectation,the increased temperature has no significant effect on the suspending DNA with the length approximately constant.Increasing the incubation temperature up to 55°C in the AFM can facilitate the process and form more compact structures showing higher profiles,in contrast with the cases in room temperature.It is shown that elevated temperature is not a requirement to induce DNA condensation,but helps to promote the process at the ensemble level.