Three point turn: A metallosupramolecular helicate with trigonal‐antiprismatic geometry fits perfectly into the central hydrophobic cavity of a three‐way DNA junction allowing a new mode of DNA recognition.

The molecular structure of the DNA A‐tract dodecamer d(CGCAAATTTGCG) complexed with the drug Hoechst 33258 has been determined by X‐ray diffraction analysis. The Hoechst molecule binds in the DNA minor groove covering the sequence AATTT of the central A‐tract, with the piperazine group close to one of the GC regions. The drug molecule makes two three‐centered hydrogen bonds from the nitrogen atoms of the benzimidazole rings to the N3 and O2 atoms of the DNA bases. Although a high propeller twist is observed in the A‐tract, only one unsymmetrical three‐centered hydrogen bond is present in the DNA major groove. The structure is compared with other minor‐groove‐binding drug complexes and the influence of these drugs on DNA A‐tracts is discussed.

No abstract

Ellipticine is a natural plant product that has been found to be a powerful anticancer drug. Although still unclear, its mechanism of action is considered to be mainly based on DNA intercalation and/or the inhibition of topoisomerase II. Many experimental data suggest an intercalation based on stacking interactions along the major base‐pair axis, but alternative binding modes have been proposed, in particular for ellipticine derivatives. The 1.5 Å resolution structure of ellipticine complexed to a 6 bp oligonucleotide unveils its mode of binding and enables a detailed analysis of the distorting effects of the drug on the DNA.