The Study Of Counter-ions Condensation On DNA

Charged biological macromolecules—DNA condensation is closelyrelated to the life basic phenomena and the application of gene therapy. Inthis paper, we mainly studied counter-ions condensation on DNA and theDNA condensation induced by protein by dynamic light scattering （DLS）and single molecular magnetic tweezers （MT）. The main researchcontents include:1. DLS and single molecular fluorescence electrophoretic technologyare use to investigated the interaction between DNA and counter-ions ofdifferent valence, including sodium chloride （Na⁺）, magnesium chloride(Mg²⁺), hexammine cobalt Ⅲ ([Co（NH₃）₆]³⁺), and spermine([C₁₀N₄H₃₀]4+). It is found that the electrophoretic mobility of DNAreached as a flat plateau by increasing the concentration of counter-ions,and the ratio of electrophoretic motility of DNA in a buffer containingonly Na⁺to the mobility in a buffer containing only Mg²⁺is about2:1,when the concentration of counter-ions c≥5mM. But the ratio of DNAmotilities in a buffer containing only Na⁺to the mobility in a buffercontaining only [Co（NH₃）₆]³⁺is about4.5:1. The experimental results arein good agreement with the Manning counter-ions condensation theory（CC） when the concentration of counter-ions is above some saturationvalues and ion-valency is less then three. Below the saturation, thecondensed fractions of counter-ions on DNA grow with the increasingconcentration of counter-ions while it should be independent of theconcentration in CC theory. This phenomenon can not be explained bythe free energy theory for short DNA proposed by Manning and hiscoworkers since a long λ-DNA is used in our measurements. Specifically,the condensed fractions for counter-ions of higher valency increase muchrapidly with their concentrations than the ones with lower valency. In themean while, this phenomenon is observed directly by single molecule fluorescence experiment. However, when the valency of counter-ions isequal to three, the experimental data deviates from the expectation of thetheory significantly. For the quadrivalent counter-ions, the DNA chargeinversion occurs, and the CC theory which is based on the average fieldfails. Furthermore, through the atomic force microscopy （AFM）, it isfound that DNA molecules condensed into compact structures when thevalency of counter-ions is equal or greater than three. Thus, theconformation of polyelectrolyte in free solution and ion correlation playan important role in the migration process of polyelectrolyte.2. A certain concentration of multivalent ions and protein are usuallyused to induce DNA condensation. The interaction between aglycoprotein—avidin and DNA is investigated by MT and DLS. Theexperimental results show that, with the increase of the concentration ofavidin, the hydrodynamic radius of DNA decrease from140nm to125nm,and the electrophoretic mobility of DNA reduce and tend to zero. Fromthe results by DLS, we can confirm that DNA condensation occur inpresent of avidin in the buffer. Meanwhile, the magnetic tweezersexperiments also confirm the results. It is found that the mechanism ofavidin cause DNA condensation is similar to the mechanism of histonecause DNA condensation. A single avidin molecular can wrapped aroundmultiple DNA base pair, it makes DNA molecular closely pack, probablyin an organized structure. This DNA-binding property of avidin may thusshed light on a potentially new physiological role for the protein in itsnatural environment.