Further insight into the Zn2+-mediated binding of streptonigrin to DNA

The interaction of double- and single-stranded DNA with the antitumor antibiotic streptonigrin (STN) in the presence of zinc ions has been examined by spectrophotometric and circular dichroism techniques. Very little interaction occurs in the absence of the metal ion. When Zn2+ is present, the binding process exhibits a bell-shaped dependence upon the ion concentration, consistent with the formation of a ternary complex, splitting into two binary complexes at high content of the metal ion. Evidence is presented for the participation of two STN molecules bridged by Zn2+ in the complex with DNA.