Condensation of isolated semi-flexible filaments driven by depletion interactions

Using fluorescence microscopy, we directly visualize the condensed structures of individual semi-flexible actin filaments in a poor solvent. The condensation of filaments into either ring-like or racquet-like structures is driven by non-adsorbing polymers which induce attractive interactions between filaments via the well-known depletion mechanism. A quantitative analysis of the racquet structures yields a direct measurement of the adhesion strength between a pair of filaments. We also compare our experimental data with a theoretical model, demonstrating that in the limit of weak binding, thermal fluctuations can renormalize the effective strength of the attractive depletion interactions. Our experimental methods can be applied to other filamentous structures to directly measure their attractive intermolecular potentials.