STRUCTURAL AND SOLUTION STUDY ON BINARY PEPTIDE AND TERNARY PEPTIDE NUCLEOBASE COMPLEXES OF PALLADIUM(II)

The dipeptide complex [Pd(gly-L-hisNalpha)Cl].1.5H2O 1 (gly-L-hisN(alp ha) = monoanion of glycylhistidine, deprotonated at the amide N) has b een prepared and structurally characterized. Co-ordination of Pd is th rough the terminal amino group of the glycyl entity, N(pi) of the imid azole ring of the histidine, and the deprotonated amide nitrogen. Reac tions of 1 with the model nucleobases 1-methyluracil (Hmura), 1-methyl cytosine (mcyt), 9-methyladenine (made), 9-ethylguanine (Hegua) and 6- methoxy-9-methylguanine (momgua) have been studied in solution by H-1 NMR spectroscopy. The mcyt complex, [Pd(gly-L-hisNalpha,O)(mcyt)].3.5H 2O (gly-L-hisNalpha,O = dianion of glycylhistidine, deprotonated at th e amide N and the carboxylic acid group) has been structurally charact erized. Nucleobase co-ordination is via N3. Proton NMR spectra of comp lexes of 1 with all model bases are indicative of rotamer formation an d, with the purine bases, of linkage isomerism (N7, N1 and simultaneou sly with N7/N1). In weakly acidic, neutral or slightly alkaline media or with an excess of 1, Hegua and, most likely, also made form complex es of Pd3(nucleobase) stoichiometry with Pd binding through N1 and N7 and the third Pd linked to one of these Pd(gly-L-his) entities. As a c onsequence, the resonances of some of the aromatic protons of the hist idine imidazole ring are shifted upfield by as much as 1 ppm. Complex stability constants have been determined by means of H-1 NMR spectrosc opy for a number of 1:1 complexes of 1 with model nucleobases. Competi tion experiments carried out at pD 7.1 indicate that 1 binds to the fo ur model nucleobases in the following preference: Hegua-N7 almost-equa l-to mcyt-N3 much greater than made-N1 > made-N7 > mura-N3.