Solvation thermodynamics of ethidium bromide in mixed solvents

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Abstract
The spectroscopy of the heteroaromatic cationic dye ethidium bromide (EB), a well known optical probe for DNA, has been investigated here in alcohol–water (methanol, ethanol and isopropyl, n-propyl and t-butyl alcohol). Following a suggestion on the role of solvation processes upon the EB–DNA interaction, the fluorescence of the dye has been measured. A statistical–thermodynamic model has been developed in order to account for the changing optical response of EB when the solvent composition is varied. The model assumes that the dye, in mixed solvents, is surrounded by a solvation layer made of cells (lattice model) which can be filled by solvent molecules. Furthermore, a linear dependence of the dye optical response vs. solvation layer composition is assumed. When activities rather than concentrations are employed, in agreement with the thermodynamics of real solvents, a remarkable fit of the optical data is obtained in the whole range of alcohol: water ratios. A dominant group of hydrophobic EB solvation cells with negative free energy –ΔG1≳kT in favour of alcohols is clearly established; there is some evidence of a second group of hydrophilic cells, with preference for water and ΔG2≳kT.