Guanine-Ru

With these caveats in mind, a number of points support a relatively short distance for guanine-Ru(bpy)3 electron transfer in our system. First, as shown in Fig. 3, when the guanines are scarce in the sequence, the binding of Ru(bpy)3 at sites on the helix to which electron transfer is not efficient must be considered [19]. Thus, the oxidant can bind to the duplex at sites too far from a guanine for electron transfer. Quantitative analysis of these data suggests that electron transfer occurs within distances that are consistent with those observed in studies of covalently bound donors and acceptors and therefore with values of the tunneling parameter fi consistent with those observed in other systems [38, 50, 98-100]. 

Fig. 20. Molecular models of two conformers of 14-mer duplex d(ATACATGGTACATA) ruthenated at N7 of one of the guanine residues with [Ru(ri6-bip)Cl(en)]+ (10). Conformer (a) shows the intercalation of the arene (b) shows the nonintercalated phenyl ring of the arene stacked on a thymine residue.

Both Ru(II) and Ru(III) complexes are known to bind DNA preferentially at N7 of G but also to A and C bases (182, 183). Although most ruthenium antitumor agents have two reactive coordination sites, GG intrastrand cross-links on DNA do not appear to form readily. The only example appears to be the adduct of 27 with GpG, which has been structurally characterized by NMR spectroscopy (184). In this complex, the two N7-coordinated guanines adopt a head-to-head conformation and the two bases are strongly destacked. 

H-atom abstraction has been demonstrated to be the mechanism of action of excited uranyl ions, and in this case negligible base oxidation is found. Nucleo-base (especially guanine) oxidation is the principal reaction caused by singlet oxygen and this reactive species can be generated by a number of the complexes (e.g. many Ru(II)polypyridyls and porphyrins). It is worth pointing out, however, that the yield of may be lower when the sensitiser is bound to DNA, and it is the authors view that some of the reactions claimed to proceed via 62 may be caused by direct reaction of the photo-oxidised sensitiser with the DNA. 

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