Allosteric conversion of Z DNA to an intercalated right-handed conformation by daunomycin

On the use of chiral compounds for probing the DNA handedness: Z to B conversion in poly(dGm5dC) upon binding of Fe(phen)3(2+) and Ru(phen)3(2+)

In order to examine whether chiral metal complexes can be used to discriminate between right- and left-handed DNA conformational states we have studied the enantioselective interactions of Fe(phen)3(2+) and Ru(phen)3(2+) (phen = 1,10-phenanthroline) with poly(dGm5dC) under B- and Z-form conditions. With the inversion-labile Fe(phen)3(2+), enantioselectivity leads to shifts in the diastereomeric binding equilibria. This effect, known as the "Pfeiffer effect" (1-4), is monitored as slowly emerging circular dichroism of the solution, corresponding to a net excess of the favoured enantiomer. With Ru(phen)3(2+), which is stable to intramolecular inversion, the difference in DNA-binding strengths of the enantiomers results in an excess of the less favoured enantiomer in the bulk solution. This excess is detected in the dialysate of the DNA/metal complex solution. With both complexes we find that the delta-enantiomer is favoured when the polynucleotide adopts the B-form, as previously shown, but also when it initially adopts the Z-form conformational state. This observation, together with evidence from UV-circular dichroism and binding data, indicates that the binding of these metal complexes induces a Z- to B-form transition in Z-form poly(dGm5dC). Consequently, neither of the studied chiral DNA-binders can easily be used to discriminate the DNA handedness.

Inhibition of the thermally driven B to Z transition by intercalating drugs

Poly(dG-m5dC) in phosphate buffer containing 50 mM NaCl and Mg2+ will undergo a reversible thermally driven conversion from the B to the left-handed Z conformation. The temperature at the midpoint of the thermally driven B to Z transition (denoted Tz) is dependent upon the total Mg2+ concentration, with [d(1/Tz)]/(d ln [Mg]) = 0.0134 K-1. The Mg2+ concentration at the midpoint of the equilibrium B to Z transition curve, denoted [Mg]1/2, is dependent on temperature, with (d ln [Mg]1/2)/(d ln T) = -1.02. Binding of the anticancer drug daunomycin to the polymer results in a pronounced increase in Tz, dependent on the molar ratio of added drug. Tz is increased by 71.9 degrees C with nearly saturating amounts of drug bound. Transition profiles are biphasic at less than saturating amounts of bound drug. By experiments monitoring such biphasic curves at a visible wavelength sensitive to the binding of daunomycin, it may be demonstrated that no drug is released until the later phase of the transition. These results are analogous to the effects of intercalating drugs on the thermal denaturation of DNA and indicate that drug molecules preferentially interact with B-form DNA and are redistributed to regions in the B conformation over the course of the transition. Comparative studies show that some intercalators stabilize right-handed DNA more effectively than others. At similar initial binding ratios, the following order, from most to least effective, was experimentally observed: actinomycin greater than daunomycin greater than ethidium greater than proflavin.