Temperature dependence of enthalpy changes for ethidium and propidium binding to DNA: Effect of alkylamine chains

✍ Scribed by Harry P. Hopkins Jr.; Juan Fumero; W. David Wilson

• Publisher: Wiley (John Wiley & Sons)
• Year: 1990
• Tongue: English
• Weight: 890 KB
• Volume: 29
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.360290215

✦ Synopsis

Abstract

Calorimetric titrations have been performed on the binding of ethidium and propidium to calf thymus DNA at temperatures in the 15–60°C range. Enthalpy changes (Δ__H__~B~) derived from these experiments performed with the new Omega reaction calorimeter have a precision of ±0.10 kcal/mol or less at all temperatures. For ethidium (a monocation), Δ__H__~B~ varies little with temperature, and the heat capacity change (Δ__C__~P~) for the binding reaction derived from these parameters is 10 cal/deg/mol. In contrast, Δ__H__~B~ changes from −6.5 to −8.1 kcal/mol for DNA binding of propidium (a dication due to a charged amine group at the end of an alkyl chain attached to the phenanthridine ring nitrogen), and Δ__C__~P~ is −57 cal/deg/mol. At 21°C a plot of Δ__H__~B~ vs mole ratio is curved downward for propidium in the 0.08–0.25 range, whereas the same plot at 45°C is a straight line from 0.05 to 0.15 and sharply downward thereafter. Similar plots for ethidium follow the latter pattern between 25 and 50°C. These observations and our analyses of Δ__H__~B~ and Δ__S__~B~ are consistent with the hypothesis that the location in the DNA complex and the rotational motion of the alkylamine chain change substantially over the temperature range in this study. Only near 50°C is Δ__H__~B~ equal for the binding of these two cations to DNA, and caution must be used in analyses of enthalpic effects when the temperature dependence for Δ__H__~B~ is not available.