Single Molecule Analysis Of DNA Charge Inversion Induced By The Counterions

Interaction between multivalent counter-ions and bio-macromolecule(DNA) had already been an important research topic in the physical biology. DNA molecule was important genetic material for the living body, it had to study the electrical properties for the mixture(DNA-counterion) by using the single molecule biological technology of Single molecule fluorescence electrophoresis(SMFE), dynamic light scattering(DLS) and atomic force microscopy(AFM). The main research content was as follows;1. It could measure the electrophoresis mobility(EM) of interaction between the DNA and the mixing counter-ions by with the SMFE and DLS, The mixing counter-ions covers spermine and monovalent sodium, divalent magnesium, trivalent spermidine or hexammine cobalt, chitosan. The experimental results showed that, DNA electrophoresis mobility increases with the increase of concentration of spermine. When the monovalent sodium or divalent magnesium poured into spermine solution, as the concentration of extra counter-ions, the EM of the mixtures become smaller, then it suppressed the DNA charge inversion. At the case of only spermine buffer(c=0.7mM), the value of DNA’s EM become positive. Then adding the Mon- or div- counterions into the buffer of spermine, the value of DNA’s EM changed the electrical property. And at the case of same spermine concentration, the monovalent ions have stronger suppression than divalent ions. However, the scenario changes dramatically when we add trivalent spermidine into the solution of DNA and spermine. In contrast, the charge neutralization and inversion of DNA was promoted by mixing with trivalent ions. As the concentration of extra counter-ions, the DNA electrophoresis mobility gradually increased at the same concentration of spermine buffer, and the hexammine cobalt ions could have stronger promotion than the spermidine ions. While the chitosan added the buffer, the promoting effects could be strongest than other counter-ions in my paper, the DNA’s electrophoresis mobility increase with the concentration of the spermine or extra counter-ions. So it was the same conclusion by the two single molecule technologies.2. In order to explain the observed experimental phenomena, it acquired the configuration of the mixture of DNA-counterions at the same concentration of spermine(10 mm) and different extra concentration of the counter-ions by atomic force microscopy. The experimental results showed that, while the buffer covered only spermine ions, the configuration of DNA-spermine’s mixture was a flower and the dispersibility of complexes was small and particle size of complexes was uniformity. When extra Mon- or div- counterions added in the buffer containing the spermine, the flower patterns condensed loosely a little and the size larger than it in 10 μM spermine solution only. While the trivalent counter-ions added into the spermine solution, mixtures of the flowers patterns condensed compactly and the size of large than before. So we explain the observation by the counter-ions complexation which is related with DNA condensation.