STRUCTURE OF DNA METAL-COMPLEXES IN SOLUTION STUDIES BY LINEAR AND CIRCULAR-DICHROISM - [PT(II)(ETHYLENEDIAMINE)(2,2'-DIPYRIDINE)]2+ BINDS STRONGLY TO DNA BY INTERCALATION - [CU(II)(2,2'-DIPYRIDINE)2]2+ IS NOT INTERCALATED

We have studied the interaction between B-form DNA in dilute aqueous solution and [Pt(ethylenediamine) (2,2′ -dipyridine)]2+ (I) and[Cu(2,2′-dipyridine)x]2+, x = 1 (II) and 2 (III), by means of linear dichroism (LD) and circular dichroism (CD). LD and two-phase partition equilibrium analysis reveal a strong DNA complex with I (0.005 M NaCl: K = 2 × 106 M-1, n = O.17 Ligands per nucleotide residue; 0.1 M NaCl: K = 1O5 M-1, n = 0.08). No spectroscopic evidence was obtained for any complex with II. A fairly strong complex is formed with III (0.005 M NaCl: K = 105 M-1, n = 0.2; 0.1 M NaCl: K = 1O5 M-1, n = 0.05). Conclusive structural evidence is provided by the specific linear and circular dichroisms of these complexes.[Pt(en)(dipy)]2+ is oriented with its plane parallel to the planes of the DNA bases. Together with the stability data this strongly indicates a binding according to the intercalation model of Lerman. The non-planar (Cu(dipy)2]2+ behaves quite differently with an average of 40° between the Cu dipy planes and the base planes. This complex is not intercalated but is most probably bound in the large groove by a Cu-base nitrogen coordination. © 1978.