Biomolecular Recognition: The Current Challenge - Dynamic Force Spectroscopy and Biomolecular Recognition - page 11

Ea

150

E'a

100

50

0

0 10 20 30

Distance

FIGURE 1.4 Bell's law. Bell interpreted bond rupture as the exit of an energy well on a
unidimensional energy landscape. Assuming that the frequency of particle attempts at crossing
the barrier was a constant, the probability of success was estimated at exp ,whereEa

is the activation energy. The effect of a force is to lower the energy curve in proportion to the
distance (broken line) [12].

( Ea / k B T )

between red cells bound by a minimal amount of antibodies [60]. He estimated at a
few tens of piconewtons the rupture force and ascribed it to the uprooting of mem-
brane molecules, a possibility already suggested by Bell [12]. Soon thereafter, lam-
inar flow chambers (Figure 1.5) were used to monitor the formation and rupture of
attachments between moving particles and surfaces coated with receptor and ligand
molecules.
This approach proved a highly sensitive way of observing single bond formation
and dissociation, since a cell size sphere subjected to a wall shear rate on the order of
afews − 1 displays a translational velocity of a few

m/s and is subjected to a distrac-
tive force on the order of a piconewton, which is sufficiently low to permit a single
weak bond to maintain a particle at rest during a detectable amount of time. The
lifetime of single bonds formed between E-selectin molecules and ligands borne by
flowing neutrophils was estimated at about 2.4 s [106]. During the following years,
flow chambers were used to estimate Bell's F 0 coefficient for the force dependence of
dissociation rates, yielding about 90 pN for P-selectin/PSGL-1 couple [5]. However,
it was soon reported that single bond rupture was more complex than that predicted
with Bell's law, since ligand-receptor association behaved as a multiphasic reaction
[146], [147]. Another problem that was later emphasized was the difficulty of ensur-
ing that single bonds were indeed observed [201]. This difficulty may provide an
explanation for the discrepancy found between different estimates of parameter F 0
[5], [65].

µ