Double-Helix Melting of Octamers of Deoxyriboadenylic and Deoxyribothymidylic Acids in the Presence of Ethidium

The effect of ethidium (ED) on the double-helix melting of octamers of deoxyriboadenylic and deoxyribothymidylic acids (dA8 and dT8) in a phosphate buffer containing 1 mol dm−3 NaCl has been studied quantitatively. The results indicate that the enthalpy and entropy changes for the dA8·dT8 melting in the presence of ED are much larger than those in the absence of the drug; that is, ED increases the stability of the duplex.