The binding of enantiomers of Ru(bpy)2(ppz)2+ (bpy = 2,2'-bipyridine; ppz = 4,7-phenanthrolino[6,5-b]-pyrazine) to calf thymus DNA is investigated using absorption, fluorescence and resonance enhanced Raman spectroscopies. Both isomers show absorption hypochromicity, steady-state fluorescence increase, reduced accessibility to an anionic quencher, and fluorescence lifetime increase associated with binding to B-form DNA, though the effect for the A isomer is always less. However, the maximal fluorescence polarization for both isomers is the same, indicating a similarly rigid binding to DNA. These findings are consistent with a major groove binding in which the ppz ligand is partially intercalated for both enantiomers. The bpy ligands are relatively small and not competent for inducing intercalative binding, but enantioselectivity is maintained by the differential interactions of these bpy ligands with the surface of the major groove as the ppz ligand partially intercalates.The binding to DNA, RNA, and synthetic polynucleotides of cationic tris-chelatesof ruthenium(I1) and rhodium(II1) in which the ligands are bidentate diimines with aromatic ring structures has been extensively studied'-'2J7J9 in recent years. An intercalative mode of binding, in which a portion of one of the three chelated aromatic rings inserts between adjacent base pairs of the native DNA structure via major groove access, is a key component of models which have been p r~p o s e d~,~*~, * J~J~ to describe the binding of such complex ions to polynucleotides. In particular, intercalative major groove binding is consistent with and provides a rational basis for the observation that enantioselectivity is a feature of the binding of many such complex ions to these polynucleotides.Although numerous complexes of this type have been shown to bind enantioselectively, only a few reports have appeared3-sJ3 which detail the interactions of individual enantiomers with DNA or other polynucleotides, principally for R~( p h e n )~~+ (phen = 1,lO-phenanthroline) and trisphenanthroline complexes of Rh3+, C03+, N?+, and Cr3+. Barton et al. have studiedl-3Jv6 the binding of A-and A -R~( p h e n )~~+ to various DNA's and homopolynucleotides, mainly poly(dG4C) in both B-and Z-forms. An analysis of spectroscopic and photophysical evidence leads them to the conclusion that the favored binding of the A isomer within the major groove of B-form DNA is explained by an intercalative binding mode of one of the phen ligands with the other two coordinated phens establishing a pattern of van der Waals contacts with atoms lining the major groove, providing a better "fit" for the A isomer. Similar binding of the A isomer within the major groove is more sterically disfavored. In binding studies3 of the enantiomers of Ru(DIP)3Z+ (DIP = 4,7-diphenylphenanthroline), the same group concluded that binding of the A isomer of this complex to the left-handed helical Z-form poly(dGC) was favored via a similar (complementary) mechanism. NMR studies by the Barton group6 have not provided co...
