Photoaddition of thienocoumarin derivatives to DNA: stoichiometry and kinetics of binding

Photoreaction of the 6,9-dimethyl-4-methoxymethyl-2H-thieno[3,2-g]-1-benzopyran-2-one (compound I) and 4-acetoxymethyl-6,9-dimethyl-2H-thieno[3,2-g]-1-benzopyran-2-one (compound II) to DNA was studied. The quantitative evaluation of the photobound molecules was performed by means of inductively coupled plasma atomic emission spectrometry (ICP-AES), exploiting the presence of the sulphur atom inside the tricyclic chromophore. The concurrent estimation of the phosphorus atom, present exclusively in the macromolecule, allowed possible intercalation sites to be identified and their involvement in the photoaddition reaction to be determined. The development of a kinetic model made it possible to discriminate and evaluate the single kinetic events that constitute the overall photoaddition process of I and II to DNA.