Tethered particle motion in close proximity to a polystyrene surface

Tese de mestrado, Engenharia Física, Universidade de Lisboa, Faculdade de Ciências, 2010Tethered particle motion is a powerful technique to study the dynamics and mechanics of macromolecules, e.g. Deoxyribonucleic acid (DNA). It involves the analysis of Brownian motion of a particle (bead) tethered to a surface by a DNA molecule. In this work we describe the method to construct the system of a magnetic bead tethered to polystyrene surface by a DNA linker molecule. As tether we used a double helix DNA chain which is cut from a DNA plasmid form (circular shape) and provided with two different specific groups to connect the beads and the polystyrene surface respectively. Using magnetic tweezers we submit 500 nm diameter superparamagnetic beads to different magnetic forces and record the Brownian motion of the beads. Using an in-house made Matlab™ script we were able to track the 2 dimensional bead’s movement in order to study the influence of the magnetic force on the Brownian motion, with emphasis to the maximum mean radius of the bead’s excursion. We determined the mean radial displacement by calculating the 2-dimensional mean square displacement and fitting it with a modified relation from the work of Kusumi et al [1]. describing restricted diffusion. These results were verified by comparing them to the probability distribution function of the bead displacement plots. The observed results shown that the radial distance of the project center of the bead has a dependency of 1∕√Fmag with the magnetic force and the DNA spring constant depends linearly of the applied force. This results allows us to draw conclusions about the quality of the different methods used for the sample preparation and data analysis