Biomedical Engineering Reference
In-Depth Information
references nor a value can be measured easily by traditional ensemble methods.
In the previous section, Cui et al. found that an energy of 1.4 kJ/molbase is
needed for the water rearrangement (also a partial dehydration) upon elongation
of single ssDNA chain [ 33 ]. It is expected that this value is very close to
G 10 ,thatis,
G 10 1.4 kJ/molbase. Then by Eq. 6.12 , we can estimate that
G 11 7.1 kJ/molbase. This finding indicates that Eq. 6.11 is a spontaneous
reaction but with a small equilibrium constant.
For other water soluble polymers, the water rearrangement also occurred dur-
ing chain elongation. For instance, the energy for the water rearrangement is
13.0 kJ/molunit for poly(N-vinyl-2-pyrrolidone) (PVPr) [ 38 ] and 7.2 kJ/molunit
for poly(ethylene-glycol) (PEG) [ 10 ]. It can be observed that ssDNA consumes the
minimum energy for the water rearrangement upon elongation. If
G 3 was a much
larger value like the PVPr or PEG [ 10 ],
G 9 would be larger than zero, and the
self-organization from ssDNA to dsDNA will not be a favorable process. One may
conclude that it is the weak disturbance of water molecules on ssDNA that makes
the self-organization possible at the physiological condition [ 33 , 39 ].
6.4
Transition from dsDNA to ssDNA and the Corresponding
Force Spectroscopy
6.4.1
Transition from dsDNA to ssDNA
Bustamante et al. utilized optical tweezers to study the stretching behavior of
dsDNA [ 19 ]. They found that when the external force was increased to 65 pN, the
dsDNA chain yielded. It is believed that the dsDNA chain underwent a structural
transition, in which the chain length extends 70%, while the force needed for the
transition was a constant.
In this transition, B-DNA is elongated to the overstretched (S) state. Therefore,
the transition is called B-S transition. After the transition, the external force will
increase upon further elongation (see Fig. 6.16 ). They found that when the external
160
140
120
strand separation
100
WLC
80
60
Fig. 6.16 Force-induced B-S
transition of dsDNA
(Reprinted with the
permission from Ref. [ 40 ].
Copyright 2002 American
Chemical Society)
40
B-S transition
20
0
0.2
0.3
0.4
0.5
0.6
0.7
DNA extension per base pair (nm)
 
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