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Free solution capillary electrophoresis (CE) accurately measures small DNA diffusion but overestimates larger DNA due to electrostatic coupling. Extrapolation to zero electric field corrects these anomalous diffusion coefficients.

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Area of Science:

  • Analytical Chemistry
  • Physical Chemistry
  • Biophysics

Background:

  • Free solution capillary electrophoresis (CE) is a rapid method for determining translational diffusion coefficients of charged molecules.
  • Reversing electric field polarity enhances measurement speed by moving analytes back and forth past a detector.
  • Accurate diffusion coefficient measurement is crucial for understanding molecular behavior and interactions.

Purpose of the Study:

  • To validate the accuracy of diffusion coefficient measurements using free solution CE with double-stranded DNA (dsDNA) as a model system.
  • To investigate the cause of discrepancies observed for larger DNA molecules compared to literature values.
  • To develop a method for obtaining accurate diffusion coefficients for larger dsDNA using CE.

Main Methods:

  • Utilized free solution capillary electrophoresis (CE) with oscillating electric fields to measure translational diffusion coefficients.
  • Employed double-stranded DNA (dsDNA) molecules ranging from 20 to 960 base pairs as model analytes.
  • Systematically varied electric field strength to analyze its effect on measured diffusion coefficients.

Main Results:

  • CE measurements accurately determined diffusion coefficients for small dsDNA (≤30 base pairs) consistent with literature values.
  • Diffusion coefficients for larger dsDNA molecules (>30 base pairs) were anomalously high and increased with DNA size.
  • Anomalous results were attributed to electrostatic coupling between DNA and counterions, influenced by counterion mobility, cation concentration, and electric field strength.

Conclusions:

  • Free solution CE can accurately measure diffusion coefficients for small DNA, but requires corrections for larger DNA due to electrokinetic effects.
  • Electrostatic coupling between DNA and counterions leads to overestimated diffusion coefficients for larger molecules.
  • Extrapolating apparent diffusion coefficients measured at various electric field strengths to zero field strength provides accurate diffusion coefficients for dsDNA across a wide size range.