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RNA crystal structures have provided a wealth of information on localized metal ions that are bound to specific sites, such as the RNA deep groove, the Hoogsteen face of guanine nucleotides and anionic phosphate oxygens. With a number of crystal structures being solved with heavy metal derivatives and other “reporter” ions, sufficient information is available to estimate global similarities and differences in ion binding properties and to begin determining the influence of RNA and ions on each other. Here we will discuss the ions that are observed bound to RNA, their coordination properties, and the roles they play in RNA structural studies. Analysis of the crystallographic data reinforces the fact that ion interactions with nucleic acids are not easily interchanged between similarly charged ions. The physiological relevance of RNA-ion interactions, mainly involving K+ and Mg2+ cations, needs to be analyzed with care as different structures are solved under very diverse ionic conditions. The analysis is complicated by the fact that the assignment is not always accurate, often done under sub-optimal conditions, which further limits the generalization about the types of interactions these ions can establish.

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The interactions of Run(tpy)(bpy)OH22+ (I), RuII(tpy)(phen)OH22+ (2), and Ruu(tpy)(tmen)OH22+ (3) (tpy = 2,2',2"-terpyridine, bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, tmen = A',A',Ar',N'-tetramethylethylenediamine) with DNA have been investigated by cyclic voltammetry. The addition of DNA to solutions of these complexes causes a dramatic decrease in current for the Ru(IV/III) and Ru(III/II) couples indicative of binding of the complexes to DNA. From the decrease in current, binding constants 15 x 101 23 M'1, 78 X 103 M'1, and 5.3 X 103 M"1 can be estimated for 1-3, respectively. Thus, the binding affinity appears to be partly a function of the extended planarity and aromaticity of the bidentate ligands. The previously reported activity of the Ru(IV) state toward DNA cleavage is apparent as a current enhancement in the Ru(IV/III) oxidation wave. As a result

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