The effect of solvent viscosity and temperature on DNA viscoelastic behavior

The effect of solvent viscosity (η s ) and temperature ( T ) on the shape of the concentration dependence of the principal and total recoils in creep‐recovery viscoelastometry experiments has been studied for T4 DNA solutions. The range of DNA concentration ( c ) was 2 – 40 μg/ml; glycerol, 70–80% v/v, sucrose, 60% v/v; NaCl, 5 m M – 1 M ; and T , 275 – 323 K. A linear proportionality between recoil and c was obtained at high η s / T . At low η s / T , the c ‐dependence was nonlinear, approaching saturation at higher c . At low c , the slope of both curves was the same. Transition between “linear” and “nonlinear” values occurred over a narrow range of η s / T (a width of 1–5 K if η s / T was changed by varying T ). (η s / T ) tr , the midpoint of the transition, was independent of solvent properties other than viscosity. Also, (η s / T ) tr increased with c . For a given c , η s / T values above this transitional value yield linear behavior; below this, nonlinear behavior. The ratio of linear to nonlinear recoil values is a linear function of c with K c , the slope of this dependence, independent of η s and T . A kinetic model for the observed nonlinearity of recoil with c is presented. It explains the independence of K c on η s and T . An attempt has been made to explain the linear–nonlinear transitions by comparison of τ 1 and T R , the lifetime of the contact points of the polymer network in the de Gennes theory. The nonlinear values are consistent with a pseudogel that exists when τ 1 < T R . At τ 1 > T R , the DNA behavior is similar to that in dilute solutions (linear values). Thus, the condition for transition is τ 1 = T R . However, some unsolved problems remain.