Interaction of $\mathrm{TOEPyP4}$ porphyrin with $\mathrm{A}$ form of DNA

The peculiarities of binding the water-soluble meso-tetra-(4N-oxyethylpyridyl) porphyrin ($\mathrm{TOEPyP4}$) with $\mathrm{B}$-DNA and $\mathrm{A}$-DNA have been studied by means of UV-VIS spectrophotometry and circular dichroism method. The binding constant ($K_b$) and stoichiometry ($n$) were determined based on the absorbance spectra for each DNA–porphyrin complex. The free energy, enthalpy and entropy of binding also were calculated using the values of $K_b$. A comparative analysis with $\mathrm{TOEPyP4-B}$-DNA complex was performed. The obtained results show that the porphyrin interacts with $\mathrm{B}$-DNA by way of intercalation at low relative concentrations ($r = C_{porf}/C_{DNA}$) and external binding at large values of $r$, whereas the interaction of $\mathrm{TOEPyP4}$ with $\mathrm{A}$-DNA occurs via the outside binding mode only.