Ternary palladium(II)–glycylmethionine-nucleobase complexes: solution studies and crystal structure of the 9-methylguanine compound

Ternary complex formation between model nucleobases and [Pd(gly-L-met)] 1 (gly-L-met = dianion of glycylmethionine, deprotonated at the amide N and at the met carboxylate terminus) has been studied by 1 H NMR spectroscopy and potentiometry. A representative example, [Pd(gly-L-metH)(Hmgua)]NO 3 ·H 2 O 3 (Hmgua = 9-methylguanine), has been characterized by X-ray crystallography. Co-ordination of Pd is through the terminal amino group of the glycyl entity, the deprotonated amide nitrogen, S of the methionine, and N 7 of the purine nucleobase. The carboxylic acid group of methionine is protonated. In aqueous solution binding of Pd to other nucleobases occurs via N 3 in the case of 1-methylcytosine (Hmcyt) and of deprotonated 1-methyluracil, N 7 of 9-ethylguanine (Hegua) or N 1 , N 7 /N 1 of the guanine anion (egua). Discrete rotamers form on binding of Pd to N 3 of the pyrimidine nucleobases and to N 1 of the guanine. The gly-L-metH resonances provide no evidence for the existence of stable diastereomers, suggesting that inversion at the chiral S atom of met is fast. In contrast, both [Pt(gly-L-metH)Cl] and its ternary derivative with Hmcyt appear to be present in solution as diastereomeric mixtures. Stability constants have been determined for ternary complexes formed from 1 and [Pd(gly-L-metH)Cl], respectively, with nucleobases.