Circularization, photomechanical switching, and a supercoiling transition of actin filaments.

We confine actin filaments onto a 2D surface using depletion interactions and show that this significantly increases the probability of intramolecular circularization. Quantitative analysis reveals that the resulting semiflexible rings fluctuate significantly less then their linear counterparts with equal stiffness-an effect induced by the constraint of circular geometry. When exposed to fluorescence excitation light, rhodamine-phalloidin-labeled filaments undergo a change in their natural twist. This photomechanical transition induces a localized small-wavelength supercoiling transition of absorbed actin rings. Upon completion of the photoinduced reaction, the twist of neighboring monomers in an actin filament changes by approximately 0.26 degrees .