Steady-state opticohydrodynamic properties of DNA: molecular weight dependence and the internal viscosity problem.

Abstract

AbstractExtinction angles, flow birefringence, and intrinsic viscosities are compared for linear, bihelical DNAs from viral and other sources that span a range in molecular weight from ∼105 to 1.3 × 108. This range effectively spans the region over which transition from rigid‐rod to expanded‐coil hydrodynamic property behavior occurs. All DNAs are in identical, phosphate–EDTA, neutral‐pH buffers, 0.1M in NaCl. The extinction angle is a hydrodynamic property only and is thus particularly sensitive to effects of kinetic chain rigidity or internal viscosity. Our extinction angle results cannot be interpreted by any simple, single‐function theoretical expression. Rather, they must be divided into distinct high‐ and low‐molecular‐weight domains. The low‐molecular‐weight region is typical of rigid‐particle opticohydrodynamic property behaviour characterized primarily by particle orientation. The high‐molecular‐weight domain shows evidence for a finite internal viscosity effect, however, which can be interpreted as very nearly Kuhnian using Cerf's amplification of the Gaussian subchain model to include internal viscosity. It is found that the high‐molecular‐weight, monodisperse viral DNAs from T7, T5, and T2 bacteriophage show an internal viscosity contribution to the limiting extinction angle–shear rate ratio of ∼3 × 10−3 s. An effect of this magnitude may be marginally important in interpreting extinction angle and certain other hydrodynamic property data for high‐molecular‐weight DNA systems. Internal viscosity effects do not appear to be manifest in the ratio of flow birefringence to intrinsic viscosity, however, and the persistence length of the high‐molecular‐weight DNAs is found to be independent of molecular weight to within estimated experimental uncertainty.