Specific cation effects on conformational transitions of DNA in aqueous solutions

The circular dichroism (CD) and absorption spectral properties of calf thymus DNA have been examined in aqueous solutions containing varying concentrations of MgCl2, CaCl2, MnCl2, ZnCl2 and CoCl2. When the CD spectra were analyzed by methods previously described (Hanlon, S., Brundno, S., Wu, T.T. and Wolf, B. (1975) Biochemistry 14, 1648--1660), it was found that the spectral changes observed between 0 and 0.1 molal concentrations of each salt could be satisfactorily accounted for as a linear combination of two independent spectral components. One of these components had the spectrum which we had previously obtained in dilute solutions of NaCl (0.01-0.04 molal) and in 0.2 M tetramethyl ammonium chloride and attributed to a B-like structure. The other spectral endpoint was similar but not identical to the C spectrum obtained in the monovalent series. These differences were particularly pronounced for the transition metal ions, Mn2+, Zn2+ anc Co2+. Since significant band shifts were also observed in the absorption spectra of DNA in the presence of these transition metal ions, we have concluded that the differences observed between the C spectrum previously reported and that found for these ions is attributable to changes in the absorption properties of the monomers due to cation association, rather than dramatic differences in comformational properties of the C secondary structure of DNA in the presence of these ions. The greater efficiency of these divalent ions compared to the monovalent ions previously studied in effecting the B leads to C transconformational reaction is attributed to the more effective screening of electrostatic interactions, both along the phosphate backbone, and between sites on the bases which bear partial negative charges and the negatively charged phosphate groups. In the light of these present results, we believe that it is this electrostatic factor which is primarily involved in effecting the transition from a B to a more C-like state. The role of dehydration is viewed as a subsidiary one which facilitates better ion association, especially at the weaker base sites, thus effecting better damping of the repulsive electrostatic interactions.