Trisphenanthroline metal complex two phenanthroline

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tris(phenanthroline) metal complex, two phenanthroline ligands are likely bound against the right-handed helical groove, stabilized through hydrophobic associ- ation. For the A-isomer, bound in this fashion, the ligands lie against and com- plement the right-handed groove; with the ~-isomer, the ligands oppose the groove, and no close surface contacts are made. Intercalation of metal complexes in DNA is not uncommon. Lippard and coworkers first established metallointercalation by Pt(II) complexes in the 1970s. 18 . 20 Square-planar platinum(II) complexes containing the terpyridyl li- gand were shown to intercalate into DNA. In an elegant series of x-ray diffrac- tion experiments on DNA fibers, Lippard illustrated the requirement for planar- ity in the complex. 18,44 Although (phen)Pt(en)2+ and (bpy)pt(en)2+ were shown to intercalate into the helix, (pyrhPt(en)2+ , with pyridine ligands rotated out of the coordination plane, could not. Complex planarity is in itself insufficient to promote intercalation, however. Cis-(NH 3 hPtCI 2 or even cis-(NH 3 hPt(en)2+ does not appear to intercalate into a helix, despite full planarity. Instead, aromatic heterocyclic ligands must be included in order to promote dipole-dipole inter- actions with the heterocyclic bases stacked in the helix. Indeed, planarity of the full complex is not required. Intercalation is not restricted to coordination com- plexes that are square planar. The tris(phenanthroline) complexes represented the first examples of "three-dimensional intercalators" and illustrated that oc- tahedral metal complexes could also intercalate into the helix. 40 ,45,46 Here one can consider the partial intercalation of one ligand into the helix, providing the remaining ligands on the complex an opportunity to enhance specificity or reac- tivity at a given site. Curiously, one unique and apparently general characteristic of metallointer- calators is their preference for intercalation from the major groove of the helix. Most small molecules associate with DNA from the minor groove, but metal- lointercalators, both those that are square planar, such as (terpyri- dyl)platinum(II) complexes, and those that are octahedral, such as the tris(phenanthroline) metal complexes, appear to intercalate into the major groove. This then mimics quite well the association of much larger DNA-binding pro- teins with the helix; DNA regulatory proteins generally appear to target the major groove. The reason why metallointercalators favor major groove associa- tion is still unclear.
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+ [Ru(phen)2 phi]2+ Cu(phen)/ I~ h N I " N-Pt-N ;; \\ /; I ~ II S" /CH 2 CH 2 " OH [(terpy)pt(HEt)j+ M-T4MPyP 471 Figure 8.8 Some metal complexes that bind DNA noncovalently primarily through intercalation (top) or binding in the minor groove (bottom). Some metalloporphyrins also primarily associate via intercalation.
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