X-ray conformational study of the DNA duplex in solution.

Earlier x-ray studies on dissolved linear DNA molecules were interpreted on the assumption that the molecules scattered as rigid rods. Improvement in equipment and advances in theory of the scattering from randomly oriented helices prompted us into a reinvestigation of this problem. Careful measurements were made on the scattering from both linear calf thymus DNA and from circular plasmid COP608 superhelical DNA. Contrary to the earlier work, we find that the scattering patterns show a helical character, with maxima corresponding to those of a helix with pitch angle of 62", close to that of the C-W helix. The patterns for both types of DNA, although similar, show a 5% displacement of the maximum in the superhelical form, just that expected when the C-W helix is superimposed on a superhelix axis. Introduction of intercalators (PtTS) causes a progressive extension of the helix, as shown by a shift to larger angles and a fading out of the maximum. In the concentration range of 40 mg/mL, interference peaks develop, the result of an apparent stacking of the chains, with an interchain distance of -35 A.