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recently quantified as a downfield shift of 0.48 f 0.07 ppm (M. H. Fouchet, D. Lemaire, J. Kozelka and J.-C. Chottard, unpublished results); (b) the ring-current effect of one GpG guanine on the H8 resonance of the other guanine, which is negative (shielding) for the 5'-H8 and positive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...

The three diguanosine phosphates GpG (4 X 10(-4) M), d(GpG) (10(-5) M), and d(pGpG) (10(-5) M) have been reacted with cis-[Pt(NH3)2(H2O)2](NO3)2 (1 Pt/dinucleotide) in water at pH 5.5 and 37 degrees C. In each case a single product is formed. The three complexes have been characterized by proton nuclear magnetic resonance (1H NMR) and circular dichroism (CD) analyses. They are N(7)-N(7) chelates of the metal with an anti-anti configuration of the bases. They present a conformational change upon deprotonation of guanine N(1)H whose pKa is ca. 8.7 (D2O). Their CD spectra, compared to those of the free dinucleotides, exhibit an increase of ellipticity in the 275-nm region, which can be qualitatively related to the characteristic increase reported for platinated DNA and poly(dG) . poly(dC). These results are in favor of the hypothesis of intrastrand cross-linking of adjacent guanines, by the cis-PtII(NH3)2 moiety, after a local denaturation of DNA.

A 500, 400 and 300 MHz proton NMR study of the reaction product of cis-Pt(NH3)2Cl2 or cis-[Pt(NH3)2 (H2O)2] (NO3)2 with the deoxydinucleotide d(GpG): cis-[Pt(NH3)2 d(GpG)] was carried out. Complete assignment of the proton resonances by decoupling experiments and computer simulation of the high field part of the spectrum yield proton-proton and proton-phosphorus coupling constants of high precision. Analysis of these coupling constants reveal a 100% N (C3'-endo) conformation for the deoxyribose ring at the 5'-terminal part of the chelated d(GpG) moiety. In contrast, the 3'-terminal -pG part of the molecule displays the normal behaviour for deoxyriboses: the sugar ring prefers to adopt an S (C2'-endo) conformation (about 70%). Extrapolating from this model compound, it is suggested that Pt chelation by a -dGpdG- sequence of DNA would require a S to N conformational change of one deoxyribose moiety as the main conformational alteration and lead to a kink in one strand of the double-helical structure of DNA.

A conformational study of the double‐stranded decanucleotide d(GCCG*G*ATCGC) · d(GCGATCCGGC), with the G* guanines chelating a cis‐Pt(NH3)2 moiety, has been accomplished using 1H and 31P NMR, and molecular mechanics. Correlation of the NMR data with molecular models has disclosed an equilibrium between several kinked conformations and has ruled out an unkinked structure. The deformation is localized at the CG*G*· CCG trinucleotide where the helix is kinked by approximately 60° towards the major groove and unwound by 12–19°. The models revealed an unexpected mobility of the cytosine complementary to the 5′‐G*. This cytosine can stack on either branch of the kinked complementary strand. The energy barrier between the two positions has been calculated to be ≤ 12 kJ/mol. The NMR data are in support of rapid flip‐flopping of this cytosine. An explanation for the strong downfield shift observed in the 31P resonance of the G*pG* phosphate is given.

A bis(sulfenato-S)Co III complex and its corresponding bis(sulfinato-S) complex were prepared. Their catalytic nitrile hydration activity was tested. Whereas the former cata-

No abstract

Postcoordination oxidation by dioxygen of one of the thiolate groups in a pentadentate N(2)S(3) ligand results in an iron(III) complex with two N-carboxamido, two thiolato, and one O-sulfinato ligands (see the CAMERON representation). This novel mixed coordination is similar to that determined for the inactive form of the nitrile hydratase from Rhodococcus sp. N-771, but differs by the O versus S binding of the sulfinato ligand.

Detailed studies of the kinetics of platination of the single‐stranded 14‐base DNA oligonucleotide d(ATACATGGTACATA) and the corresponding duplex by cis‐[Pt(NH3)2(H2O)2]2+ show that HPLC and NMR are complementary methods which provide similar results. The 5'‐G and 3'‐G monofunctional intermediates were trapped, separated and characterized by NMR (via 15NH3 labeling) and enzymatic digestion followed by mass spectrometry. The kinetic data are compared with those for the corresponding reactions of cis‐[PtCl2(NH3)2] (cisplatin) and its monohydrolysed analogue. For both single and double strands of the oligonucleotide, the aqua complex shows little selectivity for the 5'‐G or the 3'‐G in the initial platination step, whereas the chloro‐complex preferentially platinates the 3'‐G. The base on the 3' side of the GG sequence appears to play an important role in controlling this selectivity; replacement of T by C increases the selectivity of duplex platination by the diaqua complex by a factor of about 6, and the selectivity of chelation of the 3'‐G monofunctional adduct by a factor of about 3. In general the reactivity of the 5'‐G in a GG sequence appears to be enhanced in a duplex compared with a single‐strand. For both the aqua‐monoadduct and chloro‐monoadduct, cis‐[Pt(NH3)2(NG)(H2O or Cl)], the 5'‐G monoadduct is much longer lived (t½≈ 4 h at 288 K for aqua, 80 h at 298 K for chloro) than the 3'‐G monoadduct (t½≤ 45 min at 288 K for aqua, 6 h at 298 K for chloro). Inspection of molecular mechanics models of the end states of various monofunctional adducts provided insight into H‐bonding and destacking interactions in these adducts and the sequence selectivity observed in their formation. Such adducts may play an important role in the mechanism of action of platinum anticancer drugs.