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Disproportionation of [(py)(NH3)4RuIII] at the N7 of Guanine Nucleosides:  Severing the N-Glycosidic Bond

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journal contribution
posted on 09.10.1996 by K. J. LaChance-Galang, M. Zhao, M. J. Clarke
Under optimal conditions, the N7-coordinated complexes trans-[L(py)(NH3)4RuIII] (L = Guo, dGuo, 1MeGuo) disproportionate to give ∼50% trans-[Guo(py)(NH3)4RuII] and a putative RuIV species that yields ∼50% [Gua(py)(NH3)4RuIII]. Disproportionation follows the rate law d[RuII]/dt = ko[RuIII] + k1[OH-][RuIII] (for L = Guo:  ko = 2.9 × 10-4 s-1, k1 = 6.4 M-1 s-1) so that the rate-limiting step in the dominant, hydroxide-dependent pathway is not electron transfer between RuIII's, but probably deprotonation of an ammine. Consistent with the ordering of k1's for the ligands (1MeGuo > Guo ∼ dGuo > 9MeGua ≫ Gua), ionization of the purine at N1 or N9 slows the disproportionation by suppressing ammine ionization. Activation parameters for k1 (pH = 11.50) with L = Guo are as follows:  ΔH = 17.4 ± 0.8 kcal/mol (Ea = 18.0 ± 0.8 kcal/mol), and ΔS = 2.4 ± 0.1 cal/(mol K). Following disproportionation, the appearance of trans-[Gua(py)(NH3)4RuIII] and free ribose is consistent with general acid hydrolysis of the glycosidic bond induced by RuIV, which is subsequently reduced. The rate of appearance of trans-[Gua(py)(NH3)4RuIII] (pH 9.2−11.9) is complicated by purine loss, anation and possibly redox reactions, so that a net hydroxide dependence of approximately [OH-]1/2 was observed. Activation parameters for kobs (pH 11.90) with L = Guo are as follows:  ΔH = 24.6 ± 1.6 kcal/mol (Ea = 25.2 ± 1.6 kcal/mol), ΔS = 8.9 ± 0.8 cal/(mol K). In the presence of oxygen, trans-[8-OGuo(py)(NH3)4RuIII] was detected as a minor product, but neither 8-oxoguanine nor complexes involving it were observed.