Diastereomeric DNA-binding geometries of intercalated ruthenium(II) trischelates probed by linear dichroism: [Ru(phen)(2)DPPZ](2+) and [Ru(phen)(2)BDPPZ](2+)

Linear dichroism (LD) has been used to probe the binding geometries of the diastereomeric adducts between DNA and the Delta and Lambda enantiomers of [Ru(phen)(2)L](2+) where the chelate ligand L is either dipyrido[3,2-a:2',3'-c]-phenazine (DPPZ) or its benzologue benzodipyrido[a:3,2-h:2'3'-j]phenazine (BDPPZ). By combined use of LD and emission anisotropy excitation spectra measured in highly viscous solution the absorption envelope of the metal-to-ligand charge transfer (MLCT) region has been fur the first time resolved in a complex of this type bound to DNA. The absorption can be described by four effective polarization directions (three almost orthogonal), along one of which any transition is polarized, assignments which are in good agreement with molecular orbital (INDO/S) calculations. The analysis of the LD spectrum provides the orientation of the complex in terms of several angles. Near perpendicularity relative to the DNA helix axis, found for the in-plane long and short axes of BDPPZ and DPPZ, together with extensive hypochromicity of the corresponding intraligand transitions, supports intercalation of the (B)DPPZ ligands between the DNA bases as earlier studies have indicated. MLCT transition moments that make an oblique angle to the molecular plane of the (B)DPPZ chromophores confirm this orientation and, in addition, provide strong evidence for a rotation (roll) of the complex around the pseudo-dyad axis, The roll is small (varying between 5 and 15 degrees), but significant, and has the same sign (clockwise) for both enantiomeric forms of the two complexes studied and shows only minor variations between calf-thymus DNA and alternating GC or AT homopolymer duplexes, It may reflect a property intrinsic of DNA (tilt of bases) or be a result of steric interference of the two phenanthroline ''propeller blades'' with a groove. The roll provides the first example of an angle in a DNA system that has been determined with its sign from LD spectroscopy.