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.

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The reactions of five dinucleoside monophosphates (Ipl, GpG, ApA, GpC, and ApC) with cA-[Pt(NH3)2(H20)2](N03)2, with both reactants ca. 4 X 10™4 M in water (pH 5.5), have been studied. Sephadex chromatography and LC, NMR, and CD analyses have been used to characterize the complexes formed. The three homodinucleotides have a geometry leading to N(7)-N(7) chelation of the metal. Ipl and GpG give a single N(7)-N(7) chelated complex while ApA also gives other products due to competitive N(l) binding to the metal. GpC and ApC lead to mixtures of several complexes and in both cases cytosine appears to have more affinity for the platinum than do guanine and adenine. In the case of GpC one of the complexes appears as an N(7)G-N(3)C platinum chelate, the CD of which supports a left-handed helical arrangement of the bases. As far as the perturbation of the DNA structure upon binding of the m-(NH3)2Ptn moiety is concerned, these results bring further support to the hypothesis of intrastrand cross-linking of adjacent guanines occurring after a local denaturation or premelting of the DNA. They also suggest that cross-linking of adjacent guanine and cytosine could occur for a left-handed sequence of the polynucleotide.The mechanism of action of the antitumor m-dichlorodiammineplatinum(II) complex3 is still the subject of active in-vestigations.4,5 There is much evidence which suggests that DNA is the primary target of the platinum(II) drug.6™9 Several studies

Poplars were grown from cuttings under air containing CO 2 with C at about 20% I3 C, in a growth chamber during three months. 13 C-labelled lignin fractions were isolated and characterized by 13 C NMR spectroscopy. Advantage was taken of the 13 C enrichment to perform 13 C NMR resolution enhancement and editing experiments. Some effects of LiCl, added to the DMSO-d 6 Solutions, on the I3 C NMR spectra are discussed. It was also attempted to assign more accurately the polysaccharidic peaks found in the I3 C NMR spectra of lignin fractions in Order to study the lignin-carbohydrate relationships. Lignines de Peuplier Enrichies Photosynthetiquement en Carbone 13: Experiences de RMN I3 C ResumeDes Peupliers ont cultives trois mois sous atmosphere contenant du CO 2 enrichi jusqu'ä 20% 13 C, en chambre de culture. Des preparations de lignines enrichies en I3 C ont ete isolees et caracterisees par RMN 13 C. L'enrichissement a permis de realiser des experiences d'augmentation de la resolution des spectres de RMN 13 C et des experiences de determination de la multiplicite des carbones. Quelques spectres de RMN I3 C ont ete enregistres avant et apres addition de LiCl dans le DMSO-d 6 et les variations observees ont et£ discutees. Les signaux des polyosides contaminant les preparations de lignine ont ete examinos afin d'etudier la nature des relations entre lignines et polyosides.Photosynthetisch mit I3 C markierte Pappelholzlignine: I3 C NMR Versuchsergebnisse

1 3) Hydride formation mechanisms in other Co(1) polypyridine systems are currently under study as are the routes via which the hydrides react with water to give H2. Hydride formation via H-atom abstraction from organic radicals finds precedent in the C~(CN),~-/alkyl halide systems (Chock, P. B.; Halpern, J. H. J . Am. Chem. Soc. 1969, 91, 582).

The solution structure of alpha-cobratoxin, a neurotoxin purified from the venom of the snake Naja naja siamensis, at pH 3.2 is reported. Sequence-specific assignments of the NMR resonances was attained by a combination of a generalized main-chain-directed strategy and of the sequential method. The NMR data show the presence of a triple-stranded beta-sheet (residues 19-25, 36-41, and 52-57), a short helix, and turns. An extensive number of NOE cross peaks were identified in the NOESY NMR maps. These were applied as distance constraints in a molecular modeling protocol which includes distance geometry and dynamical simulated annealing calculations. A single family of structures is observed which fold in such a way that three major loops emerge from a globular head. The solution and crystal structures of alpha-cobratoxin are very similar. This is in clear contrast to results reported for alpha-bungarotoxin where significant differences exist.

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