Electron spin resonance spectroscopic and spectrophotometric investigation of the binding of tetracationic porphyrins and porphyrazines with calf thymus DNA. Unequivocal evidence for intercalation

The water-soluble largely planar 5,10,15,20-tetra-(4-methylpyridinium)porphyrin and 5,10,15,20-tetra-(3-methylpyridinium)porphyrin as well as their copper(II) chelates and copper(II)tetra(2,3-N-methylpyridinium)porphyrazine bind to calf thymus DNA in aqueous electrolyte solution. Evidence for the intercalative binding of the copper(II) chelates of the dye molecules has been obtained from e.s.r. spectral data on pressed films of DNA containing the copper(II) chelates. The result is diagnostic for intercalation of the planar tetracationic metallo-macrocyclic dye into calf thymus DNA. Metachromic shifts in the electronic spectra of the dye molecules in the region of visible light are observed. Quantitative treatment of the electronic spectral curves obtained from aqueous equilibrium mixtures containing DNA, the dyes and electrolyte leads to an evaluation of n, the number of base pairs involved in the binding site, and K, the equilibrium binding quotient. When the electrolyte concentration is 1.0 mol dm–3 the values of n obtained from the spectrophotometric data seem unusually high, but are consistent with base sequence specificity. At the lower electrolyte concentration of 0.1 mol dm–3 the values of n are consistent with some of the dye binding facially to outside sites on DNA.