Biomedical Engineering Reference
In-Depth Information
Fig. 6.17
Force curve of
dsDNA. B-S transition
occurred at 65 pN, followed
by a force-induced melting at
150 pN (Reprinted with
permission from Ref. [
41
].
Copyright 2007 American
Chemical Society)
force is
65 pN (i.e., the B-S transition is not completed yet), the transition is
reversible since the stretching and retraction curve can be superimposed well. After
the B-S transition, the stretching and retraction curve cannot be superimposed well,
unless enough time is allowed for the chains to restore to the B-DNA form when the
force is below 65 pN.
6.4.2
Force-Induced Melting of dsDNA
In 1999, Gaub et al. studied the single-molecule mechanics of dsDNA by the AFM-
based SMFS [
31
]. By higher force stretching, they found new mechanical properties
of dsDNA (see Fig.
6.17
).
When dsDNA is stretched further after the B-S transition, the dsDNA will
undergo a force-induced melting at
150 pN, that is, the double-stranded structure
is compelled to turn to two single strands by high force stretching. Interestingly,
the needed force for the melting process is dependent on the stretching velocity: the
higher stretching velocity, the larger melting force [
42
]. This result indicates that
the melting process is of nonequilibrium. Before the melting process is completed,
the stretching and retraction curve can be superimposed well, which implies that
DNA is annealed during retraction and the double-stranded structure is recovered.
As the melting process is completed, the higher regime in the force curve is
attributed to the ssDNA stretching, and the stretching and retraction curve cannot
be superimposed well.
In the recent 20 years, people have proposed several models to explain the force-
induced structural transition of dsDNA. However, this problem is still pending,
since no irrefutable experimental results, such as crystal structure or CD spectra,
are obtained. Some experimental results support that the B-S transition corresponds
to the melting process of dsDNA and the dsDNA is turned into ssDNA at the end of
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