Quasielastic light scattering by biopolymers. IV. Tertiary collapse of calf thymus DNA in 5.5M LiCl

Circular dichroism has been commonly employed to infer the conformation of DNA in solution. The basis of the conformational assignments is the work of Tunis‐Schneider and Maestre, wherein CD spectra of DNA were obtained under conditions comparable to those employed in the x‐ray diffraction studies of A‐, B‐, and C‐DNA. It has recently been suggested that the CD spectrum of DNA in chromatin, which is similar to the CD spectrum of the C‐form DNA, is a superposition of the normal B‐DNA spectrum and a single negative band, centered at 275 nm. This negative band is qualitatively identical to the spectrum for condensed Ψ‐form DNA. We have employed the hydrodynamic methods of quasielastic light scattering and sedimentation velocity to determine the extent of DNA tertiary structural alteration in 5.5M LiCl as a possible explanation of the C‐form CD spectrum. These studies suggest an eightfold contraction of the Stokes hydrodynamic volume for calf thymus DNA in going from 0.4M NH₄Ac to 5.5M LiCl, with no change in molecular weight. The estimated maximum presistence length of DNA in 5.5M LiCl is estimated to be 20.0 nm compared to the “minimum” value of 44.7 nm in NaCl solutions. The value 20.0 nm corresponds to a maximum radius of 16.7 nm for a “continuously coiled” cylinder of DNA, which compares with the value 5.0 nm of DNA in the nucleosome unit of chromatin.