The origin of the anomalous H8 chemical shifts observed in 'H-NMR spectra of oligonucleotides cross-linked at a GpG sequence with c i .~-[ P t ( N H~)~1~ ' has been investigated and clarified. The main contributions that distinguish the H8 resonances of the two platinum-ligating guanines from other GH8 signals and from each other are: (a) the inductive effect of platinum binding which we have recently quantified as a downfield shift of 0.48 f 0.07 ppm (M. H. Fouchet, D. Lemaire, J. Kozelka and J.-C. Chottard, unpublished results); (b) the ring-current effect of one GpG guanine on the H8 resonance of the other guanine, which is negative (shielding) for the 5'-H8 and positive (deshielding) for the 3'-H8 in single-stranded adducts, but has the opposite sign in double-stranded adducts; (c) a deshielding polarization effect of the phosphate 5' to the GpG unit. The different signs of the ringcurrent effects in single-stranded and double-stranded oligonucleotides originate from the orientation of the guanines in the cis-[Pt(NH3),(Gua),]~ ' moiety (Gua, guanine), which is left-handed helicoidal in single strands and right-handed helicoidal in double strands. In the platinated dinucleotides (cis-[Pt(NH,),(GpG)]+, cis-[Pt(NH3),{d(GpG)}]+ and cis-[Pt(NH,),{d(pCpG))I), the guanines assume either the left-handed or the right-handed arrangement, depending on the sugar moiety (ribose or deoxyribose), protonation state at N1 and, in the solid state, on crystal forces. This work shows that chemical shifts contain valuable structural information which is complementary to that extracted from correlated spectroscopy and nuclear Overhauser spectroscopy data.The antitumor drug cis-diamminedichloroplatinum(I1) binds to DNA, the preferential binding sites being GpG and ApG sequences [l]. These dinucleotides form, with the platinum residue, a macrochelate in which the two N7 atoms are covalently bound to platinum. NMR studies on oligonucleotides (as models for DNA) have been used in order to elucidate the structural changes imposed on DNA by platinum binding [l]. In these studies, spectral changes were interpreted in terms of structural deformation of the helicoidal oligonucleotide geometry. Unfortunately, the most spectacular and easily detectable spectral changes observed upon platination, large shifts of some proton and phosphorus resonances, could only be interpreted to a limited extent [2-41, because the origin of these shifts is not easy to discern.The most striking change in the 'H-NMR spectra of oligonucleotides upon platinum binding to GpG or ApG sequences is a downfield shift of the GpG and ApG H8 protons. These downfield shifts are different in single-stranded and double-stranded adducts, and for single \Wands the\ are dependent on whether or not the GpG or ApG dinuclcotide is preceeded by a free phosphate group or by another nucleotide (Table 1). There is no obvious relationship between thechemical shift (6) values for HX and the adduct types listed in Table 1, yet they must contain valuable structural information. In this w...

An alternative to classical? In square‐planar d8 complexes the metal ion can interact with axial H2O molecules either as a Lewis acid or as a Lewis base. Ab initio calculations predicted that uncharged PtII complexes form a hydrogen‐bond‐like interaction with H2O, in which PtII acts as a Lewis base. Such a nonclassical OH⋅⋅⋅Pt hydrogen bond has now been identified in crystals of trans‐[PtCl2(NH3)(N‐glycine)]⋅H2O by neutron diffraction.