Ethidium bromide binding to native and denatured poly(dA)poly(dT)

Isotherms of the EtBr adsorption on native and denatured poly(dA)poly(dT) i n the temperature: interval 20-70 degrees C were obtained. The EtBr binding constants and the number of binding sites were determined. The thermodynam ic parameters of the EtBr intercalation complex upon changes of solution te mperature 20-48 degrees C were calculated: 1.0 . 10(6) M-1 less than or equ al to K less than or equal to 1.4 .10(6) M-1, free energy Delta G degrees = -8.7 +/- 0.3 kcal/mol, enthalpy Delta H degrees congruent to 0, and entrop y Delta S degrees = 28 +/- 0.5 cal/(mol deg). UV melting has shown that the melting temperature (T-m) of EtBr-poly(dA)poly(dT) complexes (mu = 0.022, 4.16 . 10(-5) M EtBr) increased by 17 degrees C as compared with the Delta T-m of free homopolymer, whereas the half-width of the transition (T-m) is not changed. It was shown for the first time that EtBr forms complexes of t wo types on single-stranded regions of poly(dA)poly(dT) denatured at 70 deg rees C: strong (K-1 = 1.7 . 10(5) M-1; Delta G degrees = -8.10 +/- 0.03 kca l/mol) and weak (K-2 = 2.9 . 10(3) M-1; Delta G degrees = -6.0 +/- 0.3 kcal /mol). The Delta G degrees of the strong and weak complexes was independent of the solution ionic strength, 0.0022 less than or equal to mu less than or equal to 0.022. A model of EtBr binding with single-stranded regions of poly(dA)poly(dT) is discussed.