Guanine binding of a cytotoxic platinum-acridin-9-ylthiourea conjugate monitored by 1-D 1H and 2-D [1H,15N] NMR spectroscopy: hydrolysis is not the rate-determining step.

The reaction of PtCl(en)(ACRAMTU)(2) (PT-ACRAMTU, 1; ACRAMTU=1-[2-(acridin-9-ylamino)ethyl]-1,3-dimethylthiourea, en=ethane-1,2-diamine) and the [(15)N]-en labeled analogue, 1', with 2'-deoxyguanosine (dG) was studied by (1)H NMR and two-dimensional [(1)H,(15)N] HSQC (heteronuclear single quantum coherence) spectroscopy. Reactions were performed in phosphate buffered solution at 37 degrees C at various ratios and total concentrations of reactants. The (1)H NMR data suggest that the hydrolyzed form of the drug, Pt(H(2)O)(en)(ACRAMTU) (1a), forms at a rate (k(1)) similar to that observed in classical platinum chloroam(m)ines but to only a minor extent ( approximately 15%). Attempts to detect and characterize 1'a by two-dimensional NMR spectroscopy, however, were unsuccessful, and 1' and dG( *) were the only species observed in the HSQC spectra. Reaction of the putative aqua intermediate 1a with dG to yield Pt(en)(dG-N7)(ACRAMTU) (dG( *)) is slow and is highly dependent on the initial concentrations of the reactants. This unusual observation is consistent with a mechanism in which a second-order term becomes rate-determining (k(2)<k(1)) (the opposite situation is usually observed in cisplatin-type complexes). The possibility of direct substitution of chloride in 1 by dG-N7 (k(3)) and the implications of the data acquired in this model system for the binding of the conjugate to double-stranded DNA are discussed.