THE INTERACTION OF METAL-IONS WITH SYNTHETIC DNA - INDUCTION OF CONFORMATIONAL AND STRUCTURAL TRANSITIONS

The propensity of a large number of metal ions to induce cooperative c onformational or structural transitions in double-stranded poly d(G-C) was assessed primarily by UV and CD spectrometry. This ability was se en to be an intrinsic property of most metal ions. The observed (metal ion)/(polydeoxynucleotide) mole ratio calculated per G-C base pair an d corresponding to the midpoints of the principal transition ranged fr om 0.3 (Ag(II)) to 100 (Al(III)). A strong correlation was seen [y = - 1.01(log x) + 3.26, r = 0.95, n = 20] between the (metal ion)/(poly d (G-C)) mole ratio required for the transition midpoint (x) and a coval ent index to complex stability (y) of the metal ions. This relationshi p was independent of the types of transitions observed (monophasic or biphasic) or of specific conformations (e.g., B, Z, psi). The y index measures the ability of metal ions to bind to nitrogen and/or sulphur donor atoms in ligands compared to oxygen centers; equilibrium analysi s indicates that the mole-ratio x decreases; with increasing affinity of metal ions for poly d(G-C). Thus the observed relationship suggests that base-nitrogen binding facilitates the induced transitions. In ge neral, metal ions designated as Class B or nitrogen/sulphur seeking (A g(I), Hg(II), and Ru(III)) induced monophasic transitions, whereas Cla ss A or oxygen seeking ions (La(III), Ce(III), Tb(III), Dy(III)) induc ed biphasic transitions. Transitions generated by ions of more ambival ent ligand preference (Border-line ions) were either monophasic (Mn(II ), Fe(III), Cu(II), Cd(II), In(III), and Pb(II)) or biphasic (Cr(III), Co(II), Ni(II) and Zn(II)). Poorly defined transition-curve profiles were observed for Pt(II), Pd(II), and Al(III). Specific conformational assignments were made for some of the observed transitions. For a lim ited number of metal ions (Ni(II), Cu(II), Cd(II), Ag(I), Hg(II)), int eraction with calf thymus DNA was similarly examined. In these instanc es, the susceptibility to DNase I digestion of both the DNA and polyde oxynucleotide complexes was assessed.