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Ionic effects on the elasticity of single DNA molecules
C G Baumann1, S B Smith, V A Bloomfield
1Department of Biochemistry, University of Minnesota, St. Paul, MN 55108, USA.
Summary
The elasticity of lambda-bacteriophage DNA is significantly influenced by ion type and concentration. Multivalent cations dramatically reduce DNA
Area of Science:
- Biophysics
- Molecular Biology
- Polymer Physics
Background:
- Understanding the elastic properties of DNA is crucial for comprehending its biological functions.
- The worm-like polyelectrolyte model predicts DNA's behavior based on ionic strength.
- The influence of different ion valencies on DNA elasticity requires further investigation.
Purpose of the Study:
- To investigate the elastic properties of lambda-bacteriophage DNA.
- To determine the effects of ionic strength and multivalent cations on DNA elasticity.
- To explore the relationship between ion charge distribution and DNA mechanical behavior.
Main Methods:
- Utilized a force-measuring laser tweezers apparatus.
- Measured DNA elasticity as a function of ionic strength in monovalent and multivalent salt solutions.
- Analyzed persistence length (P) and elastic stretch modulus (S).
Main Results:
- Electrostatic contribution to persistence length (P) varied inversely with monovalent ionic strength, matching polyelectrolyte model predictions.
- Multivalent cations significantly reduced P values (250-300 Å) compared to monovalent salts (450-500 Å).
- Ion charge distribution affected P: centrally concentrated ions (Mg2+, Co(NH3)63+) yielded lower P than linearly distributed ions (putrescine2+, spermidine3+).
- Persistence length (P) and elastic stretch modulus (S) showed opposite trends with ionic strength, contradicting macroscopic elasticity theory.
- A retractile force was observed in the presence of multivalent cations at extensions allowing intramolecular contacts, suggesting a 'thermal ratchet' mechanism for condensation in stretched DNA.
Conclusions:
- Ionic strength is not the sole determinant of DNA elastic properties; ion type and valency play critical roles.
- Multivalent cations drastically alter DNA's mechanical behavior, leading to reduced persistence length.
- DNA can be described as a worm-like chain even at condensation-inducing trivalent cation concentrations if stretching prevents condensation, with a 'thermal ratchet' mechanism proposed for this process.