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. 2007 Jun 8;93(7):2400–2409. doi: 10.1529/biophysj.107.106112

FIGURE 4.

FIGURE 4

Measurements with 30-basepair-long double-stranded DNA (DNA30s). (a) Example of a force-extension curve of DNA30s possessing a B-S transition. The force-extension curve shows a rupture event of a hybridized 30 basepair DNA duplex, recorded at a retract velocity of 895 nm/s. At a force of 64 pN, the force-extension curve shows a short plateau with a length of ∼6 nm (see inset; it spans an area of 40 pN and 18 nm), which corresponds to the B-S transition. During this transition the double-stranded DNA dissociates. The black curve shows a two-state FJC-fit to the force-extension curve, which describes the elastic behavior of the PEG-DNA complex. The FJC-fit follows the force-extension curve up to the point where the DNA elongates due to the B-S transition. This region of constant force diverges from the FJC-fit by a value of up to 6 nm (shortly before rupture). (b) Example of a force-extension curve of DNA30s possessing a B-S transition followed by an additional force increase. At a force of 64 pN, the force-extension curve, which has been recorded at a retract velocity of 895 nm/s, shows a short plateau with a length of ∼6 nm. Again, this plateau corresponds to the B-S transition. After this transition the force increases to a value of 78 pN, where the DNA duplex finally dissociates. The elastic behavior of the PEG-DNA complex is described with a two-state FJC-fit (black curve), which follows the force-extension curve up to the plateau of the B-S transition. This example curve is representative for ∼10% of the measurements.