DNA-helix bending, stiffening and elongation on ligand binding; analysis for several DNA-drug systems, general viscometric DNA response and stereochemical implications.

For several DNA-ligand systems the DNA helix bending, stiffening and elongation behaviour is treated quantitatively. The experimental basis are viscosity data from literature as a function of r, the ratio of drug molecules bound per DNA monomer unit. If the relative viscosity changes delta y1(r) and delta yh(r) for DNA of low and high molar mass, respectively, are known, the relative changes of contour length, delta L/L degrees, and of persistence length, delta a/a degrees, can be evaluated as a function of r, as repeatedly demonstrated. For random sequence-independent interactions, helix-bending is reflected by a helix-typical increment of delta a/a degrees (r), being zero at r = 0 and also at DNA saturation by bound ligand molecules [Reinert, Biophysical Chemistry 13, 1-14 (1981)]. This characteristic DNA behaviour often enables us to separate the bending and the stiffening increment of delta a/a degrees. The theoretical treatment of this problem (Schütz and Reinert, J. Biomolec. Struct. & Dynam. 9, 315-329, 1991) now permits a more detailed study of the ligand-induced DNA bending. The ligand-DNA systems treated here concern the following drugs (in parentheses DNA bending angle at low r-values): proflavin (8 degrees), daunomycin (11 degrees), aclacinomycin A (9.7 degrees, on cooperative interaction), actinomycin D (16 degrees), mitomycin C (16 degrees), a double intercalating bisphenantridine (12 degrees), 9-deacetyl-daunomycin (8 degrees) and 9-epi-deacetyl-daunomycin (12-18 degrees). We also demonstrate that the consideration of the DNA flexibility and its change on interaction of short DNA molecules with intercalating drugs delivers helix elongation values in better accord with the theoretical value. In the Appendix, a catalogue of simulated delta y(r)-dependences is given for both short and long DNA molecules. It systematically describes the DNA viscosity response upon typical DNA stiffening, elongation, and helix-bending effects.