The effects of cobalt-hexammine and cobalt-pentammine cations on the solution structure of calf-thymus DNA. DNA condensation and structural features studied by FTIR difference spectroscopy.

The interaction of calf-thymus DNA with cobalt-hexammine and cobalt-pentammine cations was investigated, in aqueous solution at pH 6-7 with cation/DNA(phosphate) molar ratios r = 1/80, 1/40, 1/20, 1/10, 1/4, 1/2 and 1, using Fourier Transform infrared (FTIR) difference spectroscopy. Correlations between spectral changes, DNA condensation and helical stabilization due to the cation interaction as well as conformational features are established. At a very low cation concentration (r = 1/80), the binding of cobalt-hexammine cation with DNA is through the H-bond formation between cation NH3 groups and the PO2 groups of the backbone, resulting in duplex stability. As the cation concentration increases, hydrogen bonding expands towards guanine N-7 and O-6 atoms. At r > 1/20, DNA condensation occurs with major reduction in the intensity of several DNA in-plane vibrations and that of the phosphate group. The cobalt-pentammine cation binding is via the PO2 groups (directly) at very low metal cation concentration (r = 1/80) and the guanine N-7 and the O-6 groups (indirectly) at higher ratios. At r > 1/10, DNA condensation begins with some degree of direct cation-base binding. No major conformational changes from the B-family structure were observed before and after DNA collapse, in the presence of cobalt-ammine cations.