CONFORMATION OF PT(DIEN)[D(APGPA)-N7(2)] IN THE SOLID-STATE AND IN AQUEOUS-SOLUTION, AS DETERMINED WITH SINGLE-CRYSTAL X-RAY-DIFFRACTION AND HIGH-RESOLUTION NMR-SPECTROSCOPY IN SOLUTION

The structure in the solid state and in solution of the adduct between Pt(dien)2+ and the trinucleotide d(ApGpA), Pt(dien)[d(ApGpA)-N7(2)], has been studied with high-resolution NMR techniques and X-ray diffraction. This adduct is a model for the intermediate in the binding of the antitumor drug cis-PtCl2(NH3)2, cisplatin, to DNA. In aqueous solution the intramolecular interactions between the guanine residue and the 3' adenine are reduced upon platination, while the stacking between the 5' adenine and guanine remains intact. Furthermore, binding of the monofunctional platinum compound to the N7 of guanine results in a change in the sugar conformation of all three residues. Instead of the usual 80-100% S conformation, all three Pt(dien)[d(ApGpA)-N7(2)] deoxyribose rings are in an almost 50% N/S equilibrium. The change of the sugar conformation causes a change in the DNA backbone; this change seems to be the reason for the more easy Pt binding at the 5' side of the molecule. The conformational changes were found to be sequence dependent as seen from comparison with Pt(dien)[d(CpGpT)-N7(2)]. In that case the conformation of only the guanine residue is changed upon platinum binding. Crystallographic data of Pt(dien)[d(ApGpA)-N7(2)] are as follows: space group P2(1)2(1)2(1), Z = 8, a = 20.094 (5) angstrom, b = 21.418 (5) angstrom, c = 29.631 (6) angstrom, V = 12752 angstrom 3, resolution = 1.15 angstrom, R(w) = 0.087 for 4855 reflections and 776 variables. Apart from two Pt(dien)[d(ApGpA)-N7(2)] molecules 18 well-ordered water molecules are present in the asymmetric unit. The two independent molecules which have a similar conformation are held together in the unit cell by stacking interactions and hydrogen bonding of the bases. Characteristic features are unusual G.G base pairs (using N3 and N2, N ... N = 2.84-2.99 angstrom) and A.A base pairs (using N6 and N1 of A(1), i.e. the 5' adenine, N ... N distances are 2.80 and 3.04 angstrom; and also using N6 and N7 of A(3), N ... N = 2.96-2.99 angstrom). These G.G and A.A base pairs are stacked in the b direction of the crystal. The backbone of each d(ApGpA) molecule has an extended conformation. The only intramolecular interactions are relatively weak H bridges between an amine ligand and a phosphate oxygen and between another amine ligand and a guanine O6 (N ... O = 3.10-3.15 angstrom). Such an intramolecular interaction has been observed earlier in the crystal structure of cis-Pt(NH3)2[d(CpGpG-N7(2),N7(3)]. The intermolecular interactions (i.e. the crystal packing effects) are clearly stronger than the stacking forces within a molecule, resulting in different molecular structures and conformations in solution and in the crystal.