European Biophysics Journal

, Volume 43, Issue 2–3, pp 71–79 | Cite as

Optical tweezers reveal force plateau and internal friction in PEG-induced DNA condensation

  • Heikki Ojala
  • Gabija ZiedaiteEmail author
  • Anders E. Wallin
  • Dennis H. Bamford
  • Edward Hæggström
Original Paper


The simplified artificial environments in which highly complex biological systems are studied do not represent the crowded, dense, salty, and dynamic environment inside the living cell. Consequently, it is important to investigate the effect of crowding agents on DNA. We used a dual-trap optical tweezers instrument to perform force spectroscopy experiments at pull speeds ranging from 0.3 to 270 μm/s on single dsDNA molecules in the presence of poly(ethylene glycol) (PEG) and monovalent salt. PEG of sizes 1,500 and 4,000 Da condensed DNA, and force–extension data contained a force plateau at approximately 1 pN. The level of the force plateau increased with increasing pull speed. During slow pulling the dissipated work increased linearly with pull speed. The calculated friction coefficient did not depend on amount of DNA incorporated in the condensate, indicating internal friction is independent of the condensate size. PEG300 had no effect on the dsDNA force–extension curve. The force plateau implies that condensation induced by crowding agents resembles condensation induced by multivalent cations.


Poly(ethylene glycol) Force–extension curve Single molecule Coil–globule transition Internal friction Dissipated work 



This study was supported by Academy Professor (Academy of Finland) funding grants 255342 and 256518 to D.H.B. and Academy of Finland grant (128518) to E.H. H.O. acknowledges support from the Finnish Academy of Science and Letters (Väisälä Foundation).


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Copyright information

© European Biophysical Societies' Association 2014

Authors and Affiliations

  • Heikki Ojala
    • 1
  • Gabija Ziedaite
    • 1
    • 2
    Email author
  • Anders E. Wallin
    • 3
  • Dennis H. Bamford
    • 2
    • 4
  • Edward Hæggström
    • 1
  1. 1.Department of PhysicsUniversity of HelsinkiHelsinkiFinland
  2. 2.Department of BiosciencesUniversity of HelsinkiHelsinkiFinland
  3. 3.Centre for Metrology and AccreditationEspooFinland
  4. 4.Institute of BiotechnologyUniversity of HelsinkiHelsinkiFinland

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