Thermodynamic characterization of ethidium bromide binding to a unique site on a yeast tRNAPhe

Abstract

AbstractA self‐consistent thermodynamic characterization of the binding of ethidium to yeast phenylalanine‐specific tRNA at 25°C, pH 7.0, in 11 nM MgCl2, 375 nM NaCl, and 25 mM sodium phosphate has been obtained. Two ethidium molecules bind per tRNA under these conditions. The stronger site has a dissociation constant equal to 1.9 ± 0.5 μM and ΔHdis°′ = 12 ± 1 Kcal/mol, and the weaker sites has a dissociation constant equal to 24 ± 9 μM and ΔHdis°′ = 8.9 ± 1.5 Kcal/mol. The average calorimetric ΔHdis°′ for the to sites 10.6 ± 0.4 kcal/mol. The thermodynamics of binding to the stranger sites are most probably the thermodynamics of interaction between A·U (6) and A·U (7), the unique site identified by Jones and Kearns. The binding is enthalpically driven and classical hydrophobic interactions do not appear to be important in the binding reaction.