Cis‐Platinum Induced Distortions in DNA

Proton NMR studies at 500 MHz in aqueous solution were carried out on the G‐G chelated deoxytrinucleosidediphosphate platinum complex cis ‐Pt(NH 3 ) 2 {d(GpCpG)}, on the uncoordinated trinucleotide d(GpCpG) and on the constituent monomers cis ‐Pt(NH 3 ) 2 {d(Gp)} 2 , cis ‐Pt(NH 3 ) 2 {d(pG)} 2 , d(Gp), d(pCp) and d(pG). Complete NMR spectral assignments are given and chemical shifts and coupling constants are analysed to obtain an impression of the detailed structure of d(GpCpG) and the distortion of the structure due to chelation with [ cis ‐Pt(NH 3 ) 2 ] 2+ . Platination of the guanosine monophosphates affects the sugar conformational equilibrium to favour the N conformation of the deoxyribose ring. This feature is also apparent in ribose mononucleotides and is possibly caused by an increased anomeric effect. In cis ‐Pt(NH 3 ) 2 {d(pG)} 2 the phase angle of pseudorotation of the S‐type sugar ring is 20° higher than in ‘free’ d(pG) which might be an indication for an ionic interaction between the positive platinum and the negatively charged phosphate. It appears that d(GpCpG) reverts from a predominantly random coil to a normal right‐handed B‐DNA‐like single‐helical structure at lower temperatures, whereas the conformational features of cis ‐Pt(NH 3 ) 2 {d(GpCpG)} are largely temperature‐independent. In the latter compound much conformational freedom along the backbone angles is seen. The cytosine protons and deoxyribose protons exhibit almost no shielding effect as should normally be exerted by the guanine bases in stacking positions. This is interpreted in terms of a ‘turning away’ of the cytosine residue from both chelating guanines. Conformational features of cis ‐Pt(NH 3 ) 2 {d(GpCpG)} are compared with the ‘bulge‐out’ of the ribose‐trinucleotide m 6 2 ApUpm 6 2 A.