Chiral probes for the handedness of DNA helices: enantiomers of tris(4,7-diphenylphenanthroline)ruthenium(II).

The chiral complexes tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) (RuDIP) are shown to be specific chemical probes with which to distinguish right- and left-handed DNA helices in solution. In spectrophotometric titrations of racemic RuDIP with both B-form calf thymus DNA and Z-form poly[d(G-C)], hypochromicity in the intense metal-to-ligand charge-transfer band is found and enhancement in luminescence is observed. The spectrophotometric assay of DNA binding to the well-resolved enantiomers of RuDIP provides a means to determine the helical conformation. Strong chiral specificity is seen in binding experiments with right-handed B-DNA and, on this basis, the absolute configurations are assigned. Although delta-RuDIP can bind by intercalation into the right-handed helix, steric constraints imposed by the helix asymmetry preclude completely binding by the lambda enantiomer. Both isomers, however, are found to bind equally to Z-DNA. Left-handed helices that are more similar structurally to B-DNA would be predicted to display a stereospecific preference for this lambda isomer.