Non-adsorbing polymers induce attractive interactions between colloidal. For example, two microtubules (MTs) in a solution of poly-ethyleneglycol (PEG) spontaneously form a bundle that maximizes its overlap within seconds. We combined single filament imaging with optical trapping in order to measure the free energy of this interaction. This approach also makes it possible to measure forces acting on microtubules, which are responsible for the overlap maximization.
This experimental technique can be used to measure pairwise interactions between various biological or synthetic filaments and complements information extracted from other experiments (for example, measurements based on bulk osmotic stress).
In order for this type of measurement to work, the bundle must be freely sliding. If the two MTs are linked to each in some way (for example, through friction or by microtubule-associated proteins), the depletion force might be insufficient to drive the overlap maximization. The case of strong frictional coupling between bio-polymers is explored here.
This research has been published in Physical Review Letters