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 amid e N and at the met carboxylate terminus) has been studied by H-1 NMR s pectroscopy and potentiometry. A representative example, [Pd(gly-L-met H)(Hmgua)]NO3 . H2O 3 (Hmgua = 9-methylguanine), has been characterize d by X-ray crystallography. Co-ordination of Pd is through the termina l amino group of the glycyl entity, the deprotonated amide nitrogen, S of the methionine, and N-7 Of the purine nucleobase. The carboxylic a cid 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 (He gua) or N-1, N-7/N-1 of the guanine anion (egua). Discrete rotamers fo rm 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 ex istence of stable diastereomers suggesting that inversion at the chira l 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 dias tereomeric mixtures. Stability constants have been determined for tern ary complexes formed from 1 and [Pd(gly-L-metH)Cl], respectively, with nucleobases.