Effects of geometric isomerism in dinuclear platinum antitumor complexes on aquation reactions in the presence of perchlorate, acetate and phosphate

Abstract

The aquation and subsequent reactions of the dinuclear Pt antitumor complexes [{trans-PtCl(NH(3))(2)}(2)(mu-NH(2)(CH(2))(6)NH(2))](2+) (1,1/t,t) and [{cis-PtCl(NH(3))(2)}(2)(mu-NH(2)(CH(2))(6)NH(2))](2+) (1,1/c,c) in 15 mM perchlorate, acetate or phosphate solutions were followed at 298 K by [(1)H,(15)N] HSQC 2D NMR spectroscopy. Rate and equilibrium constants for the initial reversible aquation and the subsequent reversible reaction with phosphate or acetate are reported. The rate constant for the first aquation step is two-fold lower for 1,1/c,c than 1,1/t,t but the anation rate constants are similar so that the equilibrium lies further towards the chloro form for the 1,1/c,c compound. A pK (a) value of 6.01+/-0.03 was determined for the diaquated species [{cis-Pt(NH(3))(2)(H(2)O)}(2)(mu-NH(2)(CH(2))(6)NH(2))](4+) (1,1/c,c-3) which is 0.4 units higher than that of the 1,1/t,t compound. The rate constants for the binding of acetate and phosphate to 1,1/t,t are similar, but the rate constant for the reverse reaction is close to ten-fold higher in the case of phosphate so that equilibrium conditions are attained more rapidly (12 h compared with 64 h). On the other hand, for 1,1/c,c the rate constants for the forward and reverse reactions with acetate and phosphate are quite similar so that equilibrium conditions are reached very slowly (80-100 h) and a greater proportion of phosphate-bound species are present. The reduced lability of the bound phosphate for 1,1/c,c is attributed to the formation of a macrochelate phosphate-bridged species which was characterized by (31)P NMR and ESI-MS. The speciation profiles of 1,1/t,t and 1,1/c,c under physiological conditions are explored.

Effects of Geometric Isomerism and Anions on the Kinetics and Mechanism of the Stepwise Formation of Long‐Range DNA Interstrand Cross‐Links by Dinuclear Platinum Antitumor Complexes

“…[26] For this reason we chose to follow the platination reaction of [ 15 N]1 with the identical self-complementary 12-mer duplex 5′-{d(ATATGTACATAT) 2 } under conditions (15 mM sodium phosphate buffer, pH 5.9, 298 K) that allow best comparison with the results of that study. [26] The use of a higher pH than that employed in the reaction of 1,1/t,t (pH 5.4) takes into account the higher pK a of the aquated form of 1,1/c,c (6.01) [35] than 1,1/t,t (5.62), [36] so that both reactions were carried out at a pH just below the pK a value. A higher ionic strength was used in this study, with the addition of 80 mM NaClO 4 required to form a stable duplex.…”

“…In this case, the reactions were complicated by competing reactions involving the coordination of phosphate (Scheme 1), which were not observed previously. [26,28] Assignment of the species observed during the reactions was made by reference to our recent [ 1 H, 15 N] HSQC NMR spectroscopy study of the aquation of [ 15 N]1 in 15 mM phosphate buffer [35] in which several different aquated and phosphate-bound species were identified. The chemical shifts of all intermediate and bifunctional product species observed during the reaction are summarized in Table 1, representative [ 1 H, 15 N] HSQC NMR spectra are shown in Figure 1, and plots showing changes in the aromatic and imino regions of the 1 H NMR spectra during the reactions are shown in Figure 2.…”

“…[35] No similar peak was resolved in this reaction, which suggests that this species reacts rapidly to form the monofunctionally bound adduct 7 and never achieves a concentration high enough to be detected. Distinct peaks possibly assignable to the Pt-NH 3 groups of 6 were observed during the reaction in 100 mM phosphate, which was carried out at a slightly different pH (see Figure S2b).…”

“…The chemical shifts for all the NH 3 and NH 2 groups are all almost identical to those observed during the aquation reaction of [ 15 N]1 under similar conditions in the absence of DNA. [35] The partner peaks of the {PtN 3 Cl} group are all concealed by the peaks of 1. At later time points it is evident that these three peaks must also correspond to the {PtN 3 PO 4 } group of monofunctionally bound adduct(s) 7 because they are still visible in the spectra after peaks from the {PtN 3 Cl} groups in all species (1, 2, and 5) have disappeared ( Figure 1d).…”

“…In neither reaction (15 or 100 mM phosphate) was a peak observed for the macrochelate phosphate-bridged species, which is recognizable by a distinct trans-NH 3 peak (δ=4.22/-87.1 ppm) and accounted for 25% of the species present at equilibrium for the aquation reaction of [ 15 N]1 in 15 mM phosphate. [35] …”