Sidered though certainly not established that the job

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sidered, though certainly not established, that the job of the metal ion may be simpler than with DNA, since the ribose provides a nearby nucleophile already in the 2'-hydroxide. The reaction of tRNA with Pb(II) nonetheless illustrates how a metal ion may be utilized in promoting highly specific chemistry on a nucleic-acid polymer. Last, it must be mentioned that metal coordination to the purine N7 position can also indirectly promote strand cleavage, although not through direct hydro- lytic reaction on the sugar-phosphate backbone. Metal ions such as Pd 2+ and Cu 2+ , through coordination at N7, promote depurination. The depurinated site then becomes easily susceptible to hydrolysis upon treatment with mild base.
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(A) en = ethylene diamine OR 0 "--7 p H / "- /0)0 ? (en)2Co CO(en)2 "- / o 0 "- / 0-7~ --+ + ROH 467 (B) 2+ (C) \:r /0 OH O-""p /~ o 0 \;1" o OH HO-'~~_'r o OH ~ Figure 8.6 Hydrolysis reactions catalyzed by metal ions and complexes. (A) Illustration of a phosphate es- ter hydrolysis in a binuclear model complex catalyzed by coordinated cobaltic ions, with one metal ion functioning as a Lewis acid and the other functioning to deliver the coordinated hy- droxide. 34 (B) Ru(DIPhMacro, a metal complex constructed to contain a central DNA-binding domain (Ru(DIP)32+) with two tethered amine arms to chelate additional metal ions (Zn 2 +) to deliver to the sugar-phosphate backbone and promote hydrolytic strand cleavage. 36 (C) RNA site-specificially hydrolyzed by lead ion. Diagram of the proposed mechanism of sugar-phos- phate backbone cleavage between residue D 17 and G I8 in yeast RNA Ph e.38
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468 8 / METAUNUCLEIC-ACID INTERACTIONS III. A CASE STUDY: TRIS(PHENANTHROLlNE) METAL COMPLEXES Now we may examine in detail the interaction of one class of metal complexes with nucleic acids, how these complexes bind to polynucleotides, the techniques used to explore these binding interactions, and various applications of the com- plexes to probe biological structure and function. Tris(phenanthroline) metal complexes represent quite simple, well-defined examples of coordination com- plexes that associate with nucleic acids. Their examination should offer a useful illustration of the range of binding modes, reactivity, techniques for study, and applications that are currently being exploited and explored. In addition, we may contrast these interactions with those of other transition-metal complexes, both derivatives of the tris(phenanthroline) family and also some complexes that differ substantially in structure or reactivity. A. Binding Interactions with DNA Tris(phenanthroline) complexes of ruthenium(II), cobalt(III), and rhodium(III) are octahedral, substitutionally inert complexes, and as a result of this coordina- tive saturation the complexes bind to double-helical DNA through a mixture of noncovalent interactions. Tris(phenanthroline) metal complexes bind to the dou- ble helix both by intercalation in the major groove and through hydrophobic association in the minor groove. ll b,40 Intercalation and minor groove-binding are, in fact, the two most common modes of noncovalent association of small molecules with nucleic acids. In addition,
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