Biomedical Engineering Reference
In-Depth Information
to determine the kinetic properties of the system in the framework of the Bell-Evans
model.
A collection of unbinding force as a function of the logarithm of the loading rate
from different works on the strepta(avidin)-biotin systems is shown in Figure 6.5.
This figure shows that at the same loading rate, different values for the unbinding
force have been found; this being in agreement with the variability observed in the
previously mentioned experiments. For some experiments (e.g., Rico and Moy, 2007;
Guo et al., 2008), the unbinding force data cluster on two distinct linear trends point-
ing out the existence of two distinct barriers in the energy landscape. In general,
the values for
k
off
and
x
β
extracted by a fit through Equation 6.2, and reported in
Table 6.1, exhibit a wide range of values. The dissociation rate values, ranging from
about 10
−
7
to 100 s
−
1
, suggest the existence of both a slow and a fast processes. It is
interesting to note that the lifetime of the slower process is substantially lower than
the value in bulk (10
7
s). Some variability, even if less marked, has been found for
the energy barrier width
x
β
. This also has stimulated wide discussions and deeper
investigations as witnessed by the abundant literature (see, e.g., Pincet and Husson,
2005; Rico and Moy, 2007; Teulon et al., 2011). A variety of possible explanations
have been suggested, without, however, reaching a general consensus. For example,
it has been hypothesized that the observed discrepancies could arise from differences
in the used setup, for example, the kind of substrates (silicon, glass, agarose, gold,
mica), the cantilever stiffness, the immobilization strategies, and so on (Patel et al.,
2004; Walton et al., 2008). In other cases, more than one biomolecular pair has been
postulated to be involved in the unbinding process, leading to significant errors in the
De Paris et al., 2000
Yuan et al., 2000
William et al., 2000
Patel et al., 2004
Rico and Moy, 2007
Guo et al., 2008
300
200
100
0
0.01
0.1
1
10
100
1000
Loading rate (nN/s)
FIGURE 6.5
An overlay of the unbinding force as a function of the logarithm of the loading
rate from a collection of DFS experimental data for the avidin-biotin complex. Dashed lines
are obtained by a fit through Equation 6.2.
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