Nucleosome and dna-protein condensed structures in solution from flow birefringence and intrinsic viscosity

Abstract

Highly sensitive streaming birefringence measurements combined with intrinsic viscosity are used to characterize the shape anisometry and optical anisotropy of nucleosomes over a range of salt concentration > 30 mM KCl and of structures obtained by the condensation of high molecular weight DNA with polylysine. These measurements appear useful for several reasons. Both streaming birefringence and intrinsic viscosity are hydrodynamic properties based upon the rotational diffusion of macromolecular particles and hence are inherently more sensitive to details of particle anisometry than are hydrodynamic properties based upon translational diffusion. An established body of both hydrodynamic and continuum dielectric optical theory is available with which to interpret streaming birefringence results. Extinction angles (i.e., mean orientation angles of particles in a velocity gradient) are entirely hydrodynamic properties, and hence can be interpreted through the rotational coefficient to characterize particle anisometry and to estimate absolute dimensions. The ratio of Maxwell coefficient to intrinsic viscosity is proportional to the absolute particle anisotropy. The high optical anisotropy of DNA relative to that of associated protein permits certain details of tertiary structure and shape anisometry to be estimated from the observed optical anisotropy compared to optical models involving the DNA alone. The method is essentially independent ofmore » solvent.« less