Single-Molecule Mechanics of DNA - DNA Nanotechnology: From Structure to Function - page 94

Fig. 6.11 0, ( b ) supercoiled DNA in

10 mM PBS (Reprinted from Ref. [ 28 ]. Copyright 1998, with permission from Elsevier)

Force curves for negatively, < 0, ( a ) and positively, >

of dsDNA is 0. At a stretching force of 0.2 pN, the
dsDNA was shortened significantly when

At the initial state, the

is increased
or decreased. At a stretching force of 8 pN, it was found that the end-to-end distance
( R ) of dsDNA is independent of

is changed, no matter how

. At a stretching force of 1 pN, two situations can

be observed: when < 0.02, R is independent of ; while > 0.02, R decreases

rapidly as increases. At zero external force, dsDNA may exist as supercoil. The

force needed to unwind the supercoil is 0.6 pN.
Strick et al. also found that the larger j j , the larger force is needed in stretching.
However, when the force is larger than a critical value, the stretching elasticity will
become independent of j j (see Fig. 6.11 [ 28 ]). This is the result of conversion from
Wr to Tw. The telephone wire is a similar example in daily life.

6.2.2.3 Twist Elasticity of dsDNA

In 2003, an elegant system was designed by Bustamante et al., which was used to
detect the twist elasticity of dsDNA directly [ 29 ]. To ensure that there is no Wr
conformation in the helix, the dsDNA was stretched by optical tweezers. With the
influence of fluid flow, the rotor is fixed in the position. The micropipette is rotated
to increase Tw.
The fluid flow was stopped when the dsDNA is twisted into the P-DNA
conformation [ 30 ]. Then, the dsDNA is free and starts to relax the Tw added before,
and the conformation transits to B-DNA gradually. The torque of dsDNA can be
calculated by the formula D ! D 14r 3 !

in which ” is the rotational

drag, is the viscosity of the solution. The torsional

modulus ( C ) of dsDNA can be calculated to be C D 410 ˙ 30 pN nm 2 by the formula

! is the angular velocity, and ˜