Structural and solution study on binary peptide and ternary peptide–nucleobase complexes of palladium(II)

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Abstract
The dipeptide complex [Pd(gly-L-hisNα)Cl]·1.5H2O 1(gly-L-hisNα= monoanion of glycylhistidine, deprotonated at the amide N) has been prepared and structurally characterized. Co-ordination of Pd is through the terminal amino group of the glycyl entity. N(π) of the imidazole ring of the histidine, and the deprotonated amide nitrogen. Reactions of 1 with the model nucleobases 1-methyluracil (Hmura), 1-methylcytosine (mcyt), 9-methyladenine (made), 9-ethylguanine (Hegua) and 6-methoxy-9-methyl-guanine (momgua) have been studied in solution by 1H NMR spectroscopy. The mcyt complex, [Pd(gly-L-hisNα,O)(Mcyt)]·3.5H2O (gly-L-hisNα,O = dianion of glycylhistidine, deprotonated at the amide N and the carboxylic acid group) has been structurally characterized. Nucleobase co-ordination is via N3. Proton NMR spectra of complexes of 1 with all model bases are indicative of rotamer formation and, with the purine bases, of linkage isomerism (N7, N1 and simultaneously with N7/N1). In weakly acidic, neutral or slightly alkaline media or with an excess of 1, Hegua and, most likely, also made form complexes 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 consequence, the resonances of some of the aromatic protons of the histidine imidazole ring are shifted upfield by as much as 1 ppm. Complex stability constants have been determined by means of 1H NMR spectroscopy for a number of 1:1 complexes of 1 with model nucleobases. Competition experiments carried out at pD 7.1 indicate that 1 binds to the four model nucleobases in the following preference: Hegua-N7≈ mcyt-N3 made-N1 > made-N7 > mura-N3.

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