The interaction of calf thymus DNA with mercuric acetate and 3,6-bis-(acetatomercurimethyl)-dioxane. Small-angle X-ray scattering and viscosity studies.

The binding of Hg2+ and 3,6-bis-(acetatomercurimethyl)-dioxane (BAMD) to sonicated calf thymus DNA was studied by small-angle X-ray scattering and viscosity measurements. The scattering experiments with DNA complexed by different amounts of mercurials (for Hg2+ rb=0-0.79, for BAMD rb=0-0.86 mol of mercurial bound per mol of base pairs) established that the rod-like character of the DNA molecules is maintained up to high binding ratios. They revealed further a steady decrease of the cross-section radius of gyration Rc for the DNA X Hg2+ complex and a similar decrease of Rc for the DNA X BAMD complex up to Rb=0.35. This behaviour is certainly caused by the incorporation of both mercurials near the axis of the DNA helix. Binding of BAMD at rb greater than 0.35 led to an increase of Rc which behaviour obviously reflects the location of mercury atoms at large distances from the axis, possibly on the surface of the helix. The increase of the mass per unit length Mc upon binding of the mercurials was found to be much higher than expected. This finding established that the length of the DNA helix decreases by 0.10 +/- 0.01 nm per bound mercurial at low binding ratios (i.e. up to rb=1/3 for BAMD, up to possibly rb=0.5 for Hg2+). A similar conclusion was also drawn from the observed decrease of intrinsic viscosity [n] with increasing rb. The analysis of Mc at high binding ratios suggests that every BAMD molecule bound beyond rb=1/3 decreases the length of the DNA by 0.21 +/- 0.05 nm whereas Hg2+ when bound beyond rb=0.5 causes no change of the length.