Force Measurements of a Single DNA Molecule in the Collapsing Phase Transition.

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  • Murayama Yoshihiro
    Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577
  • Sano Masaki
    Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577 Department of Physics, The Graduate School of Science, The University of Tokyo, Tokyo 113-0033

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The elastic force for a single DNA molecule (15.7 kilobasepairs) during a transition between an elongated coil and a collapsed globule state was measured by dual-trap optical tweezers. Under highly diluted conditions, the DNA concentration was 2 nM in nucleotides, the force change in a collapsing process and the elastic response in the collapsed state were observed at the single molecule level. To induce the transition, a trivalent cation, spermidine, was used. Two types of force changes in different timescales occurred in the intramolecular collapsing process; a fast force change from about 0.2 to 0.9 pN within 20 s, and a slow force change from 0.9 to 1.5 pN for 100 s. After the force change, a force plateau at about 1.8 pN was observed in a stretching process; however, after the process, the elastic response was the same as the one in a coil state. This hysteresis indicates the existence of a metastable state such as a supercooling liquid state.

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