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A Very Rare Example of a Structurally Characterized 3′-GMP Metal Complex. NMR and Synthetic Assessment of Adducts Formed by Guanine Derivatives with [Pt(Ltri)Cl]Cl Complexes with an N,N′,N″ Tridentate Ligand (Ltri) Terminated by Imidazole Rings

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journal contribution
posted on 06.07.2017, 13:38 by Kokila Ranasinghe, Svetlana Pakhomova, Patricia A. Marzilli, Luigi G. Marzilli
[Pt­(N(R)-1,1′-Me2dma)­Cl]Cl complexes with tridentate ligands (bis­(1-methyl-2-methylimidazolyl)­amine, R = H; N-(methyl)­bis­(1-methyl-2-methylimidazolyl)­amine, R = Me) were prepared in order to investigate Pt­(N(R)-1,1′-Me2dma)G adducts (G = monodentate N9-substituted guanine or hypoxanthine derivative). Solution NMR spectroscopy is the primary tool for studying metal complexes of nucleosides and nucleotides because such adducts rarely crystallize. However, [Pt­(N(H)-1,1′-Me2dma)­(3′-GMPH)]­NO3·5H2O (5) was crystallized, allowing, to our knowledge, the first crystallographic molecular structure determination for a 3′-GMP platinum complex. The structure is one of only a very few structures of a 3′-GMP complex with any metal. Complex 5 has the syn rotamer conformation, with 3′-GMP bound by N7. All Pt­(N(R)-1,1′-Me2dma)G adducts exhibit two new downfield-shifted G H8 signals, consistent with G bound to platinum by N7 and a syn/anti rotamer mixture. Anticancer-active monofunctional platinum­(II) complexes have bulky carrier ligands that cause DNA adducts to be distorted. Hence, understanding carrier-ligand steric effects is key in designing new platinum drugs. Ligand bulk can be correlated with the degree of impeded rotation of the G nucleobase about the Pt–N7 bond, as assessed by the observation of rotamers. The signals of syn and anti rotamers are connected by EXSY cross-peaks in 2D ROESY spectra of Pt­(N(H)-1,1′-Me2dma)G adducts but not in spectra of Pt­(N(H)­dpa)G adducts (N(H)­dpa = bis­(2-picolyl)­amine), indicating that rotamer interchange is more facile and carrier-ligand bulk is lower in Pt­(N(H)-1,1′-Me2dma)G than in Pt­(N(H)­dpa)G adducts. The lower steric hindrance is a direct consequence of the greater distance of the G nucleobase from the H4/4′ protons in the N(R)-1,1′-Me2dma carrier ligand in comparison to that from the H6/6′ protons in the N(H)­dpa carrier ligand. Although in 5 the nucleotide is 3′-GMP (not the usual 5′-GMP) and the N(H)-1,1′-Me2dma carrier ligand is very different from those typically present in structurally characterized Pt­(II) G complexes, the rocking and canting angles in 5 adhere to long-recognized trends.

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