CLARIFICATION OF NATIVE DNA SOLUTIONS BY FILTRATION

Low angle light scattering studies on whole, half, and quarter molecules of T2 bacteriophage DNA

Static light scattering measurements have been made at angles as low as 8 degrees on whole, half, and quarter molecules of native, T2 bacteriophage DNA in 0.195 M Na+. The fragments were obtained by high-speed stirring of the native DNA, and fractionated on methylated-albumin-kieselguhr columns. Accompanying measurements of sedimentation coefficients and intrinsic viscosities were made. Because linear extrapolations of light scattering data above 8 degrees for these samples were suspect, the measurements were analyzed by fitting curves calculated from the theory of wormlike coils to experimental curves at c = 0. Results showed that the excluded volume parameter, epsilon, must be used in analyzing the scattering curves; a reasonable value of epsilon was 0.08, in agreement with that found for T7 DNA (Harpst, J. A. 1980. Biophys. Chem. 11:295-302). The persistence length of all three DNAs in this paper was 50 +/- 5 nm, showed no dependence on molecular weight, but was somewhat below that reported previously for T7 DNA (60 nm). Theoretical curves calculated with the preceding parameters had a clear upward curvature in scattering envelopes below 8 degrees for quarter and half molecules, but such curvature was minimal for whole T2 DNA, so that linear extrapolations of experimental data above 8 degrees gave a molecular weight and root-mean-square radius which were nearly the same as those from theory. The molecular weight and radius for whole T2, derived from the comparison of theory and experiment, were 115 X 10(6) and 1,224 nm, respectively. The measurements on T2 DNA were clearly at the upper limit of current techniques.

Dynamic light-scattering studies on thermal motions of native DNAs in solution

The basic character of dynamic light-scattering properties of native DNA was investigated on two DNA samples. The degree of non-single exponentiality of photocount correlation functions, C(t), and its dependence on K are quantitatively characterized by two methods. The spectral linewidth, Ts, determined from C(t) exhibits a K dependence near to but significantly different from the prediction for Rouse-Zimm (RZ) chains by Dubois-Violette and De Gennes: It is inferred from data on lambda-DNA that the exponent in the K dependence of the spectral linewidth for native DNA takes a value larger than 3 in the K region corresponding to the 'K3' region for RZ chains. These results are in good agreement with the prediction from the dynamic theory of semiflexible chains presented by one of us (K.S.). The apparent diffusion coefficients are fairly insensitive to DNA concentration and ionic strength at large K. On the other hand, it is indicated that the stiffness of native DNA may vary with temperature even in a temperature range substantially lower than that of melting.