Biomedical Engineering Reference
In-Depth Information
Fig. 6.8 Force curve of
dsDNA ( solid line )andthe
m-WLC fitting curve ( dotted
line ) with l p D
43.3 nm and
K 0 D
1,240 pN (Reprinted
from Ref. [ 20 ]. Copyright
1997, with permission from
Elsevier)
6.2.1.2
Single-Molecule Enthalpic Elasticity of dsDNA
As shown in Fig. 6.6 , a significant discrepancy can be found between the experimen-
tal force curve and fitting curve when the stretching force is enhanced. This is due to
the limit of the models. FJC and WLC model both assume that the bond length and
bond angle is fixed, which may be incorrect. To solve this problem, a modified WLC
(m-WLC) model was proposed (Eq. 6.4 ), in which an elastic modulus ( K 0 )was
introduced to enable the chain deformation upon force stretching. For macroscopic
materials, the dimension of elastic modulus is force/sectional area. For a certain kind
of single polymer chain, the sectional area is a constant value. Thus, the sectional
area of the chain is neglected, and the dimension is simplified as force. At high force
regime, bond length and bond angle can be altered, and the energy (enthalpy) of the
molecule is raised. The elasticity that appeared at the higher force is called enthalpic
elasticity [ 13 ].
l p
k B T
R
L
F
K 0
1
4.1 R=L C F=K 0 / 2
1
4
F
D
C
(6.4)
It was shown by the fitting result that the elastic modulus ( K 0 )ofdsDNAis
1,240 pN (see Fig. 6.8 [ 20 ]). This value is evidently lower than that of other
polyelectrolyte. For instance, the K 0 of poly(2-acrylamido-2-methylpropanesulfonic
acid) is 2 10 5 pN [ 24 ]. One possible reason could be that the contour length
of dsDNA at zero force is much shorter than that of the corresponding ssDNA.
Therefore, it is easy for dsDNA to deform under strain. Experimental results showed
that the elastic modulus is basically not influenced by the ionic strength of the
solution.
 
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