Abstract
We observed the diffusive motion of a micron-sized bead in an entangled-DNA solution to investigate the effect of the viscoelasticity on the bead motion. In the absence of external stress (passive microrheology), subdiffusion appears in the timescale of 0.1–10 s, and the normal diffusion recovers in longer timescales. We evaluated the apparent viscosity and elasticity, which yields a simple relaxation time for the viscoelastic medium. We found that the absence of DNA-length dependence for the time-dependent diffusion is explained by the simple relaxation of the viscoelastic media rather than the reptation dynamics, including the disentanglement. On the other hand, in the presence of a small external stress in active microrheology, the bead motion showed clear length dependence owing to the viscoelasticity. These results suggest that the viscoelasticity of the entangled DNA is highly sensitive to the external stress, even in the linear response regime.
Highlights
DNA has been widely investigated, to elucidate its biological function and its polymer dynamics,[1] including the rheological properties[2,3,4,5] and the self-diffusion of DNA depending on its length and topology.[6,7] As the polymer concentration increases, the polymers are overlapped and entangled, which yields non-trivial viscoelasticity
To explain the timescale-dependent diffusion, we evaluated the apparent viscosity and elasticity, which yields a simple relaxation time for the viscoelastic medium
We observed the diffusive motion of a micron-sized bead in an entangled-DNA solution in the absence of external stress
Summary
DNA has been widely investigated, to elucidate its biological function and its polymer dynamics,[1] including the rheological properties[2,3,4,5] and the self-diffusion of DNA depending on its length and topology (linear/circular).[6,7] As the polymer concentration increases, the polymers are overlapped and entangled, which yields non-trivial viscoelasticity. The subdiffusion is due to elasticity caused by entangled DNA, and the recovery to normal diffusion is explained by the relaxation of the reptation dynamics of entangled DNA.[28] Similar subdiffusion has been observed in dense F-actin; the exponent α depends on the ratio of the radius of the probe particle to the mesh size of the polymer network, owing to hopping between pores in the network.[16] Recently, a hopping mechanism for the diffusion of a particle in a polymer network was theoretically proposed, showing timescale-dependent diffusion;[29,30] the normal diffusion recovers in a timescale long enough to rearrange the entangled network
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