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Abstract
DNA compaction and charge neutralization in a mixing counterion solution involves competitive and cooperative electrostatic binding, and sometimes counterion complexation. At normal ionic strength, it has been found that the charge neutralization of DNA by the multivalent counterion is suppressed when being added extra mono- and di-valent counterions. Here, we explore the effect mixing counterion on DNA compaction and charge neutralization under the condition of low ionic strength. Being quite different from normal ionic strength, the electrophoretic mobility of DNA in multivalent counterion solution (octalysine, spermine) increases the presence of mono- and di-valent cations, such as sodium and magnesium ions. It means that the charge neutralization of DNA by the multivalent counterion is promoted rather than suppressed when introducing extra mono- and di-valent counterions into solution. This conclusion is also supported by the measurement of condensing and unraveling forces of DNA condensates under the same condition by single molecular magnetic tweezers. This mixing effect can be attributed to the cooperative electrostatic binding of counterions to DNA when the concentration of counterions in solution is below a critical concentration.
Highlights
DNA is an extraordinarily important biological macromolecule storing genetic information and carrying instructions for protein synthesis
Due to the highly charged nature of nucleic acids, they are constantly surrounded by counterions to neutralize most of their charges to reduce the electrostatic repulsion between segments of DNA, so they can be condensed into compact, orderly structures [1]
When more than one counterion exists in solution, the electrostatic interaction between DNA
Summary
DNA is an extraordinarily important biological macromolecule storing genetic information and carrying instructions for protein synthesis. The understanding of the DNA compaction process is important for the study of fundamental biological processes such as chromosome compacting, but it has potential clinical applications such as the development of new vehicles for gene therapy [2,3,4]. It is a feasible tool to study the interaction between DNA and counterions [34,35].
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