Effect of cationic drugs on suprahelical organization of DNA

Interaction of a minor groove‐binding drug Hoechst‐33258, and an intercalating drug, proflavin, with the PSI (+) form of DNA, was studied using CD spectroscopy. Both drugs are shown to relax the suprahelical organization of DNA, leading to the formation of a B‐like structure, above a certain drug to phosphate ratio. However, unlike proflavin, Hoechst‐33258 brings about further structural changes after formation of the B‐like structure whereas proflavin does not. A reversal of the CD signal in the 300–450‐nm spectral region is also observed with Hoechst‐33258, indicating a change in the handedness of the suprahelical organization of DNA. To the best of our knowledge, drug‐mediated changes as presented in this paper have not been reported so far. © 1993 John Wiley & Sons, Inc.