New Concepts Relevant to Cisplatin Anticancer Activity from Unique Spectral Features Providing Evidence That Adjacent Guanines in d(GpG), Intrastrand-Cross-Linked at N7 by a cis-Platinum(II) Moiety, Can Adopt a Head-to-Tail Arrangement

N7−Pt−N7 d(GpG) intrastrand cross-link adducts are formed in DNA by the anticancer drug, cisplatin. By creating adducts with slow dynamic motion, we have identified a new abundant conformer with the guanine bases in a head-to-tail (HT) arrangement and with both sugars in the N pucker of A-form DNA instead of the S pucker of B-form DNA. Both features are unprecedented for such cross-links. The HT form is one of two abundant thermodynamic and kinetic products formed by addition of d(GpG) to [(S,R,R,S)-BipPt(H2O)2]2+ (Bip = 2,2‘-bipiperidine with S, R, R, and S configurations at the asymmetric N, C, C, and N chelate ring atoms). The second form has the common head-to-head conformation (HH1) with the backbone propagating in the normal direction, both G's anti and with N and S puckers for the 5‘- and 3‘-G residues, respectively. This form has a typical NMR shift pattern: upfield 5‘-G H8, downfield 3‘-G H8, and downfield 31P NMR signal. In contrast, the HT (S,R,R,S)-BipPt(d(GpG)) form has several unusual or unique NMR spectral features, including pronounced upfield shifts of both G H8 signals, unusually shifted 5‘-G H3‘ and 3‘-G H2‘ signals, and an unexpectedly upfield-shifted 31P NMR signal. A strong 3‘-G H8−H1‘ NOE cross-peak, the absence of an H8−H8 NOE cross-peak, and H1‘ couplings establish that this form is an HT conformer with a syn 3‘-G, an anti 5‘-G, and both sugars having mainly N pucker. The HT base orientation in cross-links introduces chirality, and two conformers, ΔHT1 and ΛHT2, are possible with normal and opposite directions of backbone propagation, respectively. NMR-restrained molecular mechanics and dynamics calculations show that the ΔHT1 conformer has the lower energy. Of some interest, rules in the literature cannot account for the G H8 shifts of any BipPt(d(GpG)) form reported by us here and previously. We advance new rules that allow successful G H8 shift predictions for these cross-links and also for cisplatin oligonucleotide adducts; these rules are consistent with the solid-state structure of cis-Pt(NH3)2(d(pGpG)) (Sherman, S. E.; Gibson, D.; Wang, A. H.-J.; Lippard, S. J. J. Am. Chem. Soc. 1988, 110, 7368−7381), but they conflict with widely held interpretations of cis-Pt(NH3)2(d(GpG)) solution structure and dynamics. Finally, our results suggest that, in the future, the effect of the carrier ligands on the HH vs HT cross-link conformation must be considered in drug design.