Bond-Force measurements

This chapter presents a new technique to measure the bond-force between two biomolecules. The manipulation method presented in chapter 3, together with a special customised design, is used to pull at a bond between two biomolecules until it breaks. One biomolecule is attached to the magnetic bead and the other is connected to the sample surface.

Figure 4.1: Sketch for the bond-force measurements

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The setup for these experiments is shown in figure 4.1. Two conducting lines (a wide and a narrow one) are structured and covered by SiO$_2$ (see section 3.1 for a comprehensive explanation). The sample surface is then covered in a special process (see section 4.3) by one kind of biomolecules (red in figure 4.1). Commercially available magnetic markers that are covered with a matching biomolecule (green in figure 4.1) are put in solution on the sample surface. The beads are collected on top of the thin conducting line with a small current, so that the biomolecules at the markers can bind to the biomolecules on the surface. After a few minutes, the small current is turned off and a current through the wide conducting line is turned on. This current, and with it the magnetic force on the marker, is increased very slowly, while the beads on the sample are recorded by a CCD-camera (confer section 2.7 for the complete description of this setup). All beads that did not bind to the surface are directly drawn off the thin conducting line. The other beads stay until the applied force $\vec{F}_{\rm bond}$ is so high that the bond between the biomolecules breaks. Because the current and all other needed variables (confer equation 1.10) during the rupture events are known, the magnetic force $\vec{F}_{\rm mag}$ can be calculated ^4.1. And because the angle $\nu \leq 5$° in our experiments (confer figure 4.1), we can state with an error of less than 1% that $F_{\rm bond} \approx F_{\rm mag}$. Hence, the rupture force between the magnetic marker and the surface can be calculated directly.