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The present work is part of a systematic study of the structures of Ag+ and CH 3Hg+ complexes with the purines and pyrimidines found in nucleic acids. we intend to clarify the role played by these metal ions when added to solutions of polynucleotides or nucleic acids and to provide basic chemical informations 1vhich could be extrapolated to the interactions of Pt anticancer drugs with DNA. Chelate formation between N7 and the carbonyl oxygen 06 of guanine is one of the model mechanisms used to explain the attack of Pt complexes on DNA. In order to assess the reactivity of 06 toward soft metal ions, silver complexes with 9-methylhypoxanthine (HMPx), a guanine-like ligand, were prepared. With AgClO~, a complex of the formyl a [ (Ht~Px) 2Ag] Cl04 .H 20 \vas · obtained, in which AgT is linearly bonded to N7 atoms of two ligands, whereas 06 is involved only in H-bonding. The nitrate has the stoichiometry [ (HMPx)Ag] N0 3.H20. The solid consists of infinite chains in which the 1 i gand bridges tlvo Ag+ ions a 1 ong the chain via N7 and the uncommon endocyclic N3 atom. The carbonyl group is free. The preference for the strained N3 position over the unhindered 06group is an indication of the poor basicity of the latter site in the non-deprotonated form of the ligand. CH3HgN03 forms 1vith 1-methyl cytosine (MCy) a compound [ CH 3Hg(MCy)] N0 3 in which the Hg· atom is bonded to the usual N3 site. By reacting an excess of CH 3HgN0 3 in basic or slightly acidic solutions, one amino proton is displaced by mercury, 1vhile the second CH 3Hg+ ion is found on N3. The most peculiar feature of this structure is the~ configuration of the CH 3Hg groups bonded to N3 and N4, respectively, instead of the presumably less constrained anti arrangement. ---With 1-methylthymine (HMT), the substitution of H3 by CH 3Hg+ yields the neutral 1:1 compound [ CH 3Hg(MT)l. tH 20. The carbonyl groups 02 and 04 interact only weakly the Hgatom of adjacent molecules and with the water molecule. This neutral complex forms "addition compounds" with a number of simple salts of alkaline or alkaline-earth cations. The structure of [ CH 3 Hg(MT)] .tNaN0 3 was determined. The [ CH 3 Hg(i~T)] unit has the same structure as in the hemihydrate, but the residual basicity of,02 and 04 is now fulfilled by interactions with the NaT ions, which assume an octahedral environment of carbonyl and nitrate oxygen atoms. 03.2-07 RESTRICTION OF PEPTIDE CONFOR}~TION BY aMETHYL SUBSTITUTION. Patrick Van Roey, G. David Smith and Hilliam L. Duax, Medical Fndn. of Buffalo, Inc., 73 High St., Buffalo, NY 14203 and T. M. Balasubramanian and G. R. Marshall, Dept. of Physiology and Biophysics, \o/ashington University, St. Louis, MO 63110.

Restriction of peptide conformation by α-methyl substitution