Stereospecific DNA binding of luminescent atropisomeric viologens

Binding to B-DNA of one enantiomer of dq3pyp, --a member of a novel family of intercalating luminescent viologens derived from N,N'-dialkyl-6-(2-pyridyl)phenanthridine, in which dialkyl is -(CH2)2- (dq2pyp), -(CH2)3- (dq3pyp), or (-CH3)2 (Me2pyp)--, is strongly favored compared to its estable atropisomer. Evidence of the stereospecific interaction has been gained by resolution of a rac-dq3pyp mixture upon dialysis in the presence of DNA, monitored by circular dichroism. Molecular modeling suggests that the S enantiomer of both dq3pyp and dq2pyp is the one preferred for the binding. No similar resolution has been achieved with rac-dq2pyp due to the lower barrier to interconversion of its atropisomers, as shown by 1H-NMR. The identical binding energy of the two modeled diastereoisomeric Me2pyp-DNA complexes discard enantiospecific interaction of this drug. Affinity chromatography on DNA-cellulose allows isolation of the pure enantiomers of dq3pyp.