The influence of ionic strength on the binding of a water soluble porphyrin to nucleic acids

The ionic strength dependences of the binding of tetrakis (4-N-methypyridyl)porphine (H2TMpyP) to poly(dG-dC) and calf thymus DNA have been determined. For the former system the results are typical of other Intercalators, i.e., a plot of log K vs log [Na+] is linear albeit with a slope which suggests that the “effective charge” of the porphyrin is closer to two than the formal charge of +4. For calf thymus DNA, the binding profile is not completely compatible with the predictions of condensation theory. Whereas the avidity of binding does decrease with increasing [Na+] as predicted, of greater interest is the relocation of the porphyrin from GC-rich regions to AT-rich regions as the ionic strength increases.