It was found recently that Hoechst 33258, a dsDNA fluorescent dye used in cytological studies, is an efficient inhibitor of the interaction of TATA-box-binding protein with DNA, DNA topoisomerase I, and DNA helicases. In addition it proved to be a radioprotector. Biological activity of Hoechst 33258 may be associated with dsDNA complexes of not only monomeric, but also dimeric type. In this work, the Hoechst 33258 interaction with poly(dG-dC)·poly(dG-dC) was studied using UV-vis and fluorescent spectroscopy, circular and flow-type linear dichroism. It was found that Hoechst 33258 formed with poly(dG-dC)·poly(dG-dC) complexes of three types, namely, monomeric, dimeric, and, apparently, tetrameric, and their spectral properties were studied. Complexes of monomeric and dimeric types competed with distamycin A, a minor groove ligand, for binding to poly(dG-dC)·poly(dG-dC). We proposed that Hoechst 33258 both monomers and dimers form complexes of the external type with poly(dG-dC)·poly(dG-dC) from the side of the minor groove.

Key words: Hoechst 33258; bis-Hoechst; UV-Vis absorbance; Fluorescence; Circular and Flow linear dichroism; Competition with distamycin A.