Atomic Force Microscopy and Spectroscopy - Dynamic Force Spectroscopy and Biomolecular Recognition - page 83

lines on the left side of the arrows in Figures 2.16a and 2.16b mark this subset.
Introducing this data into Equation 2.40, we obtain the force versus distance curve
displayed in Figure 2.16c.
As it is well known from previous studies of dextran (Rief et al., 1997, 1998;
Haverkamp et al., 2007) the force curve reveals a kink in the range of about 700 pN.
It originates from a conformational transition within each dextran monomer. Due to
the external applied force, each sugar ring of the dextran monomer flips into a new
conformation (Rief et al., 1998), resulting in an additional elongation (about 10%) of
the monomer. This is a thermodynamic process that can be described with a“single-
click” model (Haverkamp et al., 2007). The solid line in Figure 2.16c shows a fitof
this model to the experimental data. The agreement is remarkable because the only fit
parameter is the number of monomers in the dextran molecule. All other parameters
were fixed and chosen as given by Rief et al. (1998) and Haverkamp et al. (2007).
This agreement was also observed for other stretching events. Figure 2.17 summa-
rizes the measured force curves of three dextran strands of different length. Again,
we could fit the “single-click” model to this data. Since this model is not time or
velocity dependent and is based on the assumption that the number of folded and
unfolded molecules are in a thermodynamic equilibrium, this outcome proves that
we indeed measured the equilibrium force of the dextran molecule even with our
dynamic approach.
As mentioned previously, it is also possible to calculate the dissipated energy
per oscillation cycle via Equation 2.39 in dynamic force spectroscopy. The resulting
dissipation curve is shown in Figure 2.16d as a function of the tip-sample distance
and shows a maximum at a position corresponding to the kink in the force curve.

-2.0

n = 381

Retraction
heory

n = 266

-1.5

n = 191

-1.0

-0.5

0.0

0 50 100 150 200

Tip position D up (nm)

FIGURE 2.17 Force versus extension curves of three different dextran strands measured
with the proposed dynamic approach. The symbols represent experimental data that is well
reproduced by the theoretical force curves, where only the number of dextran monomers were
used as a fit parameter (n = 191, 266, and 381, respectively).