The different binding modes of Hoechst 33258 to DNA studied by electric linear dichroism

The binding mode of the bisbenzimidazole derivative Hoechst 33258 to a series of DNAs and polynucleotides has been investigated by electric linear dichroism. Positive reduced dichroisms were measured for the poly(dA-dT).poly(dA-dT)- and poly(dA).poly(dT)-Hoechst complexes in agreement with a deep penetration of the drug into the minor groove. Similarly, the drug displays positive reduced dichroism in the presence of the DNAs from calf thymus, Clostridium perfringens and Coliphage T4. Conversely, negative reduced dichroisms were obtained when Hoechst 33258 was bound to poly(dG-dC).poly(dG-dC), poly(dA-dC).poly(dG-dT) and poly(dG).poly(dC) as well as with the GC-rich DNA from Micrococcus lysodeikticus indicating that in this case minor groove binding cannot occur. Substitution of guanosines for inosines induces a reversal of the reduced dichroism from negative to positive. Therefore, as anticipated it is the 2-amino group of guanines protruding in this groove which prevents Hoechst 33258 from getting access to the minor groove of GC sequences. The ELD data obtained with the GC-rich biopolymers are consistent with an intercalative binding. Competition experiments performed with the intercalating drug proflavine lend credence to the involvement of an intercalative binding rather than to an external or major groove binding of Hoechst 33258 at GC sequences.