The pulling force of a single DNA molecule condensed by spermidine

Rui Zhang, B. I. Shklovskii

Research output: Contribution to journalArticlepeer-review

16 Scopus citations


In a recent experiment, a single DNA double helix is stretched and relaxed in the presence of spermidine, a short positive polyelectrolyte, and the pulling force is measured as a function of DNA extension. In a certain range of spermidine concentration, a force plateau appears whose value shows maximum as a function of spermidine concentration. We present a quantitative theory of this plateau force based on the theory of reentrant condensation and derive almost parabolic behavior of the plateau force as a function of the logarithm of the spermidine concentration in the range of condensation. Our result is in good agreement with experimental data.

Original languageEnglish (US)
Pages (from-to)563-570
Number of pages8
JournalPhysica A: Statistical Mechanics and its Applications
Issue number3-4
StatePublished - Apr 15 2005

Bibliographical note

Funding Information:
The authors are grateful to A.Yu. Grosberg, Y. Murayama, Y. Rabin, I. Rouzina, and J. Zhang for useful discussions. This work is supported by NSF No. DMR-9985785 and DMI-0210844.


  • Condensation
  • DNA
  • Pulling force
  • Spermidine


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