Force measurements of a single DNA molecule in the collapsing phase transition

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.