Structural basis for stabilization of Z-DNA by cobalt hexaammine and magnesium cations

In the equilibrium between B-DNA and Z-DNA in poly(dC-dG), the Co(NH3)63+ ion stabilizes the Z form 4 orders of magnitude more effectively than the Mg2+ ion. The structural basis of this difference is revealed in Z-DNA crystal structures of d(CpGpCpGpCpG) stabilized by either Na+/Mg2+ or Na+/Mg2+ plus Co(NH3)63+. The crystals diffract X-rays to high resolution, and the structures were refined at 1.25 A. The Co(NH3)63+ ion forms five hydrogen bonds onto the surface of Z-DNA, bonding to a guanine O6 and N7 as well as to a phosphate group in the ZII conformation. The Mg2+ ion binds through its hydration shell with up to three hydrogen bonds to guanine N7 and O6. Higher charge, specific fitting of more hydrogen bonds, and a more stable complex all contribute to the great effectiveness of Co(NH3)63+ in stabilizing Z-DNA.