Neglected bidentate sp2 N-donor carrier ligands with triazine nitrogen lone pairs: platinum complexes retromodeling cisplatin guanine nucleobase adducts

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Rapid rotation of guanine base derivatives about Pt-N7 bonds results in fluxional behavior of models of the key DNA intrastrand G-G cross-link leading to anticancer activity of Pt(II) drugs (G = deoxyguanosine). This behavior impedes the characterization of LPtG2 models (L = one bidentate or two cis-unidentate carrier ligands; G = guanine derivative not linked by a phosphodiester group). We have examined the formation of LPtG2 adducts with G = 5'- and 3'-GMP and L = sp(2) N-donor bidentate carrier ligands [5,5'-dimethyl-2,2'-bipyridine (5,5'-Me2bipy), 3-(4'-methylpyridin-2'-yl)-5,6-dimethyl-1,2,4-triazine) (MepyMe2t), and bis-3,3'-(5,6-dialkyl-1,2,4-triazine) (R4dt)]. NMR spectroscopy provided conclusive evidence that these LPt(5'-GMP)2 complexes exist as interconverting mixtures of head-to-tail (HT) and head-to-head (HH) conformers. For a given G, the rates of G base rotation about the Pt-N7 bonds of LPtG2 models decrease in the order Me4dt > Et4dt > MepyMe2t > 5,5'-Me2bipy. This order reveals that the pyridyl ring C6 atom + H atom grouping is large enough to impede the rotation, but the equivalently placed triazine ring N atom + N lone pair grouping is sterically less impeding. For the first time, the two possible HH conformers (HHa and HHb) in the case of an unsymmetrical L have been identified in our study of (MepyMe2t)Pt(5'-GMP)2. Although O6-O6 clashes involving the two cis G bases favor the HT over the HH arrangement for most LPtG2-type complexes, the HH conformer of (R4dt)Pt(5'-GMP)2 adducts has a high abundance (approximately 50%). We attribute this high abundance to a reduction in O6-O6 steric clashes permitted by the overall low steric effects of R4dt ligands. Under the reaction conditions used, 3'-GMP forms a higher abundance of the LPt(GMP)2 adduct than does 5'-GMP, a result attributable to more favorable second-sphere communication in the LPt(3'-GMP)2 adduct than in the LPt(5'-GMP)2 adduct.

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Inorganic chemistry

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