Biomedical Engineering Reference
In-Depth Information
[126]. It is therefore of obvious interest to determine dissociation rates. As
illustrated by the many studies on leukocyte-endothelium interaction, the
force dependence of interaction plays a dominant role in some situations.
•
Second, as described above and in other chapters, some techniques allow
experimental determination of
k
off
, that is, the rupture frequency of a
given bond in the presence of a disruptive force
F.
This may be achieved
with a flow chamber that yields direct determination of
k
off
(
F
)
[157] or with
AFM or BFP since theoretical models allow us to relate constant-force bind-
ing frequencies and unbinding forces measured at constant pulling speed
[56] [75]. An important point is that the force-free dissociation rate and
mechanical resistance may behave as different parameters: Thus, when the
ligand CD34 of L-selectin was subjected to mild periodate oxidation, the
force-free dissociation rate
k
off
(
F
)
was not substantially altered, in contrast
with dissociation rates measured in presence of disruptive forces, as evi-
denced with a flow chamber [162]. This example supports the use of con-
sidering the force dependence of dissociation rates.
(
0
)
•
Third, an important question is to know whether
k
off
may be viewed
as an intrinsic property of a given ligand-receptor complex
AB.
While a
positive answer might have appeared obvious a few years ago, two recent
papers [124] [159] reflected the feeling that bond lifetime and dissociation
rates were not intrinsic parameters since they depended on the
history
of
studied complexes. This apparent paradox is indeed a consequence of a
clear approximation in our language. It is only an approximation to refer
to a complex
AB
since it is well known that AB may span a number of
states that appear as local minima in a multidimensional energy landscape
or even in a 1D reaction path [129] [146] [204]. Therefore, if the amount of
time required to reach equilibrium is higher than the period of time between
complex formation and dissociation rate determination, measured parame-
ters will depend on the initial state of the molecular complex and on the
time allowed for equilibration between different substates before beginning
measurements. An additional point is that the dissociation probability of a
molecular complex subjected to a time-dependent disruptive force is depen-
dent on the history of force application [124] [193] and possibly, as sug-
gested by molecular dynamics simulation, on the precise location of atoms
at the moment of force application [193].
(
F
)
. Firstly,
dissociation may depend not only on the intensity of a disruptive force but also on
its direction [8] [202]. This may be important if free rotation is not allowed between
binding molecules and surfaces. Secondly, dissociation is not only dependent on the
properties of binding sites but also on linker molecules connecting these sites to
surfaces [63] [193].
Fourth, several authors developed theoretical models to relate dissociation fre-
quencies under constant load or loading rates to the location and depth of energy
landscapes. The next step would be to relate these geometric and energy parameters
There are other properties that hamper the universality of function
k
off
(
F
)
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