Long-range distance constraints in platinated nucleotides: Structure determination of the 5′ orientational isomer of cis-[Pt(NH3)(4-aminoTEMPO){d(GpG)}]+ from combined paramagnetic and diamagnetic NMR constraints with molecular modeling

Article abstract of DOI:10.1021/ja00148a012

The compound cis-[Pt(NH₃)(4-aminoTEMPO)C1I] (7) is a paramagnetic analogue of the anticancer drug cisplatin and of cis-[Pt(NH₃)(C₆H₁₁]NH₂)Cl ₂] (1), a major metabolite of a recently developed, orally administered derivative. The bifunctional mixed amine complex 7 and a monofunctional triamine complex, trarts-[Pt(NH₃)₂(4-aminoTEMPO)Cl]NO₃ (8), were synthesized to provide localized unpaired electron spin density for use in NMR spectral studies of their polynucleotide adducts. Compounds 7 and 8 readily coordinate to the N(7) positions of guanosine nucleosides, as revealed by ¹H, ³¹P, and ¹⁹⁵Pt NMR spectroscopy. The NMR spectra were selectively broadened owing to distance-dependent relaxation from the unpaired electron localized on the nitroxyl radical of the 4-aminoTEMPO ligand. Platination of d(GpG) by the mixed amine complex 7 afforded two orientational isomers which differed with respect to the positioning of the 4-aminoTEMPO group toward either the 3′ or 5′ side of the phosphodiester linkage. The purified orientational isomers were readily distinguished by selective broadening of the ¹H NMR resonances of the 3′ and 5′ deoxyribose rings. The minimum energy solution structure for the 5′ orientational isomer of the platinated dinucleotide cis-[Pt(NH₃)(4-aminoTEMPO){d(GpG)}]⁺ (13) was determined by NMR methods including combined diamagnetic (J coupling constants) and paramagnetic (electron-¹H, ³¹P distances) constraints. Moreover, with the paramagnetic spin probe, we have been able to obtain the first observable NMR distance constraints for determining the configuration of the ζ or α torsion angles in any oligonucleotide. Dynamics trajectories (200 ps) for 13 demonstrated that only computations including paramagnetic distance constraints could determine the ζ^-, α^- conformation of the phosphodiester linkage and the conformation of the 4-aminoTEMPO ligand. These NMR data and computational methods demonstrate the utility of long-range paramagnetic distance constraints in elucidating the NMR solution structures of DNA modified by cisplatin analogues.