Concentrated DNA rheology and microrheology

We present mechanical measurements of the frequency-dependent linear viscoelastic storage and loss moduli, G′(ω) and G″(ω), and the yield stress, τ_y, and yield strain, γ_y, for calf thymus DNA (13 kbp) over a range of mitotically relevant concentrations from C_DNA = 1 to 10 mg/ml. For large C_DNA, we find a dominant plateau elasticity, G′_p, at high ω. As ω decreases, G′ falls until it is equal to G″ at the crossover frequency, ω-c$/, below which G″ dominates. We measure G′_p approx. C_DNA^2.25 and ω-c$/ approx. C_DNA^-2.4, consistent with scaling exponents for classical polymer solutions. The mechanical |G^*(ω)| agree well with those measured using a new microrheological technique based on video tracking microscopy of thermally-driven fluorescent colloidal spheres and a frequency-dependent Stokes-Einstein equation. We have developed this technique to probe how enzymes, typically available in small quantities, can affect the rheology of the DNA. Using it, we report preliminary measurements of a higher ω-c$/ for a DNA network in which the ATP-powered enzyme Topoisomerase II transiently cuts and rebinds the DNA, thereby relaxing entanglements.