Influence of surface shape on DNA binding of bimetallo helicates.

In order to probe the DNA-helicate interactions responsible for the DNA binding and remarkable changes of the DNA secondary structure induced by a tetracationic bi-metallo helicate [Fe(2)(L(1))(3)](4+) (L(1)=C(25)H(20)N(4)), we have designed and synthesised derivatives with hydrophobic methyl groups at different positions on the ligand backbone. Two dimetallo helicates [Fe(2)(L(i))(3)](4+) were prepared using ligands L(3) and L(5) with the methyl substituent on, respectively, the 3 and 5 positions of the pyridyl ring thus producing a wider or slightly longer tetracationic DNA binder. UV/visible absorbance, circular and linear dichroism spectroscopies have been used to characterize the interactions of the cylinders with DNA with the aim of investigating any sequence preference or selectivity upon binding. Competitive binding studies using fluorescent dyes Hoechst 33258 (a minor groove binder), ethidium bromide (an intercalator) and a major groove binding cation (cobalt (III) hexammine) which induces the B-->Z transition have been employed to determine the binding geometries of the enantiomers of two methylated helicates (L(3) and L(5)) to DNA and compare with the data obtained previously for the unmethylated analogue (L(1)). The results demonstrate that the racemic mixtures and the resolved enantiomers of all helicates bind to DNA inducing structural changes. The overall conclusion from the effect of adding these groups to the surface of the parent helicate is that increasing the width (L(3)) reduces the DNA binding strength, the bending and coiling effect and the groove selectivity of the enantiomers compared with the parent compound. There is limited evidence to suggest a slight GC sequence preference. Lengthening the helicate (L(5)) results in DNA interactions similar to those of the parent compounds, with an increased preference of the P enantiomer for the minor groove indicating an enhancement of mode selectivity.

Enantiomeric resolution of supramolecular helicates with different surface topographies.

The enantiomeric resolution of an extended range of di-metallo supramolecular triple-helical molecules are reported. The ligands for all complexes are symmetric with two units containing an aryl group linked via an imine bond to a pyridine. Alkyl substituents have been attached in different positions on the ligand backbone. Previous work on the parent compound, whose molecular formula is [Fe(2)(C(25)H(20)N(4))(3)]Cl4, showed that it could be resolved into enantiomerically pure solutions using cellulose and 20 mM aqueous sodium chloride. In this work a range of mobile phases have been investigated to see if the separation and speed of elution could be increased and the amount of NaCl co-eluted with the compounds decreased. Methanol, ethanol and acetonitrile were considered, together with aqueous NaCl : organic mixtures. Effective separation was most often achieved when using 90% acetonitrile : 10% 20 mM NaCl (aq) w/v, which gives scope for scaling up to incorporate the use of HPLC. The overall most efficient (i.e. fastest) separation was generally achieved where the cellulose column was packed with 20 mM NaCl (aq) and the column first eluted with 100% acetonitrile, then with 75% ethanol : 25% 20 mM NaCl (aq) until the M enantiomer had fully eluted and finally with 90% acetonitrile : 10% 20 mM NaCl (aq) until the P enantiomer had been collected. The sequence of eluents ensured minimum NaCl accompanying the enantiomers and minimum total solvent being required to elute the enantiomers, especially the second one, from the column. No helicate with a methyl group on the imine bond could be resolved and methyl groups on the pyridine rings also have an adverse effect on resolution.

Intramolecular DNA coiling mediated by metallo-supramolecular cylinders: differential binding of P and M helical enantiomers.

We have designed a synthetic tetracationic metallo-supramolecular cylinder that targets the major groove of DNA with a binding constant in excess of 10(7) M(-1) and induces DNA bending and intramolecular coiling. The two enantiomers of the helical molecule bind differently to DNA and have different structural effects. We report the characterization of the interactions by a range of biophysical techniques. The M helical cylinder binds to the major groove and induces dramatic intramolecular coiling. The DNA bending is less dramatic for the P enantiomer.