Salt Effects on Equilibrium and Kinetics of the Reaction of Tetrakis(1-methyl-4-pyridinio)porphine with DNA

The association reaction of tetrakis(1-methyl-4-pyridinio)porphine (TMpyP) with salmon testes DNA has been investigated spectrophotometrically, statically, and kinetically in buffers containing different concentrations of Na+. Free TMpyP had an absorption maximum at 424 nm, which decreased with increasing DNA concentration. At the low Na+ concentrations of less than 1.20×10−1 mol dm−3 the minimal absorbance at the wavelength was reached by less than 1×104 mol dm−3 DNA. On the other hand, near the end of the titration at the high DNA concentrations at 2.20×101 mol dm−3 Na+, the absorbance no longer continued to decrease but displayed a small absorbance increase. The dependence of the association constant on the Na+ concentration indicated that less than one ion-pair contributed to a formation of the complex between TMpyP and DNA if there was the same reaction mechanism at the Na+ concentration range used in the experiment. These results of the spectra and the association constants suggested that a mechanism of the reaction might be different at the low and high salt concentrations. Kinetic results by a micro stopped-flow method showed that the reaction was a single step at the low Na+ concentration, while at the high Na+ concentration it consisted of two steps.