Concentrated DNA Rheology and Microrheology

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Abstract
We present mechanical measurements of the frequency-dependent linear viscoelastic storage and loss moduli,G′(ω) and G″(ω), and the yield stress, τy, and yield strain, γy, for calf thymus DNA (13 kbp) over a range of mitotically relevant concentrations fromCDNA= 1 to 10 mg/ml. For largeCDNA, we find a dominant plateau elasticity,G′p, at high ω. As ω decreases,G′ falls until it is equal toG′ at the crossover frequency, ωc, below whichG″ dominates. We measureG′p∼CDNA2.25and ωc∼CDNA−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 ωcfor a DNA network in which the ATP-powered enzyme Topoisomerase II transiently cuts and rebinds the DNA, thereby relaxing entanglements.