Solution structural studies of the Ag(I)-DNA complex

We report equilibrium dialysis and electric dichroism studies of the two strong complexes (I and II) of silver ion with DNA. Cooperative conversion of DNA to the stronger type I complex results in a 9% length decrease, and a structure in which intercalated ethidium is perpendicular to the helix axis. Upon addition of more Ag+ to form the type II complex, the DNA length reverts to its original value and bound ethidium once again becomes tilted from the plane perpendicular to the helix axis. In both type I and type II Ag (I) - DNA complexes, ethidium binding is mildly cooperative. We interpret the results in terms of a sequence of silver-induced cooperative switches of DNA from its B-form structure with propeller twisted base pairs to a structure with flat base pairs in the type I complex, and back again to propellered base pairs in the type II complex.