On the Use of Chiral Compounds for Probing the DNA Handedness: Z to B Conversion in Poly(dGm5dC) Upon Binding of Fe(phen)32+and Ru(phen)32+

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Abstract
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 a 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 Δ-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.