A clarification of the complex spectrum observed with the ultraviolet circular dichroism of ethidium bromide bound to DNA

Ethidium bromide intercalation strongly affects the circular dichroism spectrum of DNA in the region of 230-300 mμ, in a complex manner. In this report we present a study that quantitizes the relationships of the circular dichroism spectrum in the region of 230-300 mμ and the ethidium bromide induced optical activity centered around 308 mμ. We present evidence of two hidden cooperative bands that are probably the negative counterparts of the 308 my band and 330 my shoulder positive cooperative bands. The hidden band is quantitatively characterized. We confirm that the direct effect of ethidium bromide on the DNA spectrum is simply linearly proportional to the amount of intercalated dye. We also observe that the ethidium bromide enters freely when there is a molecule intercalated for every 3 sites, but that the intercalation is more difficult when the molecule intercalates at every second site.