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

Abstract

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.

DNA Binding by a New Metallointercalator that Contains a Proflavine Group Bearing a Hanging Chelating Unit

“…Their possible anticancer activity has made them attractive synthetic targets. [10,19,20] The intercalative binding of these complexes to nucleic acids constitute a prerequisite for the consequent slow attack of Pt II on the base nitrogen atom. [21,22] On the basis of the above-mentioned arguments, we found it interesting to synthesise a new fluorescent metallointercalator and to investigate its interaction with double-stranded calf thymus DNA.…”

DNA fragment conformations in adducts with Kiteplatin

“…1) are possible. 17,46,[58][59][60][61][62][63] These major conformers, in turn, can have several sub-forms differing in backbone conformation and base canting (R or L).…”

“…17,[46][47][48]53,55,59,64 The stereochemistry of the Bip carrier ligand influences both the conformer distribution and the cross-link handedness. For example, ((R,S,S,R)-Bip)Pt(d(GpG)) was shown to have two abundant right-handed conformers (HH1 R, HH2 R), and ((S,R,R,S)-Bip)Pt(d(GpG)) was shown to have two abundant lefthanded conformers (HH1 L, ΔHT1 L).…”

“…For example, ((R,S,S,R)-Bip)Pt(d(GpG)) was shown to have two abundant right-handed conformers (HH1 R, HH2 R), and ((S,R,R,S)-Bip)Pt(d(GpG)) was shown to have two abundant lefthanded conformers (HH1 L, ΔHT1 L). 17,46,59 In these conformers at least one guanine base is oriented such that its O6 is on the same side of the coordination plane as a Bip NH proton; thus, a G O6-NH hydrogen bond could form. In each case, the observed base canting direction of that G residue permits such H-bonding.…”

“…For the ΔHT1 conformer (which is related to the HH1 conformer by having a syn conformation of the 3′-G residue having the base flipped with respect to the 5′-G residue), we use the chemical shifts observed for (S,R,R,S-Bip)Pt(d(GpG)) (7.91 and 7.77 ppm for 3′-G and 5′-G, respectively). 59 This leads to estimated high temp- 60 Moreover, the chemical shifts are expected to be very little affected by the composition of the solvent in the case of a water and methanol mixture. Therefore, we can be confident that the high temperature spectrum reflects the coalescence set of signals of two dominating conformers, HH1 and ΔHT1 (and possibly some minor conformers).…”