A comparison of van't Hoff and calorimetric heats of binding to DNA using an ethidium selective electrode

A liquid membrane electrode selective for ethidium ion was used to measure free ethidium in mixtures with calf thymus DNA. Electrode response was unaffected by variation in ionic strength from 1 mM to 0.5 M, and was not degraded over the temperature range studied. DNA-ethidium binding isotherms obtained with the electrode at 17.4, 25.4, 30.1, and 40.6 degrees C were fitted to a single class of excluded sites model for v ranging from 0.01 to 0.16. van't Hoff analysis of these data yielded delta H = -8300 cal/mol ethidium bound (in 0.5 M KCl, 10 mM Tris buffer, pH 10, 1 mM EDTA). Direct calorimetric measurements of the heat of complex formation led to a value of -7600 cal/mol at 25 degrees C in the same medium; the two results were not significantly different at the 95% confidence level. The agreement supports the validity of the ethidium selective electrode, and illustrates its utility in the study of ligand binding to nucleic acids and related materials.