Thermodynamic studies of DNA‐dye complexes, 2. DNA‐methylene blue and DNA‐thionine systems

The interaction between DNA and various dyes (methylene blue ( 1 ), and thionine ( 2 )) was studied by means of the heat of mixing. From the results, the thermodynamic quantities were estimated for the DNA‐dye system by an intercalation process. The free energy change of the system DNA‐ 2 was found to be nearly equal to that of the system DNA‐ 1 , but the absolute value of the heat of interaction of the system DNA‐ 2 , which has amino groups, is higher than that of the system DNA‐ 1 . These facts are comparable with those previously found for the systems DNA‐proflavine and DNA‐acridine orange. However, the locations of 1 and 2 intercalated into DNA may be different from those of acridine orange and proflavine because the enthalpy changes for the systems DNA‐ 1 and DNA‐ 2 are higher than those of the systems DNA‐acridine orange and DNA‐proflavine, respectively. These facts showed that the location of the intercalated dye depends on the presence of amino groups in 3‐position of the dye.