The energetic stabilization in direct coordination of

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isomer), covalent binding dominates the interaction. The energetic stabilization in direct coordination of the ruthenium(II) center is certainly more substantial than the weaker stabilization derived from intercalation. Rh(phenhCl 2 + and its derivatives have also been shown to bind covalently to DNA but only upon photoactivation, since light is needed to promote dissociation of the coordinated chloride and substitution of the nucleic acid base as a ligand. 49 B. Techniques to Monitor Binding Many of the same techniques employed in studying the basic chemistry of co- ordination complexes can be be used in following the binding of transition-metal complexes to nucleic acids, but biochemical methods, with their often exquisite sensitivity, become valuable aids as well in delineating specific binding inter- actions. Tris(phenanthroline) metal complexes are particularly useful to illustrate this point, since here the metal center in the complex is selected in terms of the technique used for examination. Coordination complexes are often visibly colored, and these colorations pro- vide a useful and sensitive spectroscopic handle in following fundamental reac- tions. This notion holds as well with tris(phenanthroline) metal complexes in their interactions with nucleic acids. Ru(phenh 2 + and its derivatives are highly colored because of an intense metal-to-ligand charge-transfer band (A max = 447 nm, E = 1.9 X 10 4 M -I cm -I). Furthermore, the complexes are highly photolumi- nescent (A em = 610 nm, T = 0.6 f.LS in aerated aqueous solution). On binding
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III. A CASE STUDY: TRIS(PHENANTHROLlNEj METAL COMPLEXES 473 to nucleic acids these transitions are perturbed. Hypochromism is observed in the charge-transfer band, and intercalation leads to an increase in lifetime of the charge-transfer excited state. 43 ,46 Indeed, single-photon counting experiments show a biexponential decay in emission from Ru(phenh 2 + bound to double-helical DNA. The longer-lived component (T = 2 fLS) has been assigned as the inter- calated component and the shorter-lived 0.6 fLS component has been attributed to a mixture of free and groove-bound species. These spectroscopic perturba- tions permit one to define equilibrium-binding affinities for the different com- ponents of the interaction as a function of metal-center chirality and under dif- ferent solution conditions. 41 One can also follow the polarization of emitted light from the complexes after excitation with polarized light, and these studies have been helpful in describing the dynamics of association of the complexes on the helix. 41 ,43 Mixed-ligand complexes of ruthenium(II) show similar spec- troscopic perturbations, and these have been used to characterize binding affin- ities and chiral preferences, as well as the extent of intercalation versus groove binding as a function of ligand substitution on the metal center. 42 The spectro- scopic handle of the metal center therefore affords a range of experiments to monitor and characterize the binding of the metal complexes to polynucleotides.
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