Thermal stability of complexes of diaminoacridines with deoxyribonucleic acids of varying base content

Abstract

1. 1. The dissociation of proflavine when heating its complexes with native DNA's of varying base content in the media of 1 mM and 10 mM salt concentration was followed spectrophotometrically. It was found that the dye bound more weakly to the surface (complex II) dissociated in a wide temperature interval, whereas dissociation of the dye bound more strongly (complex I) occurred in the temperature region of the helix-coil transition of the complex and was of a cooperative character. The midpoints of this “dissociation transition” T r (ref. 25) were identical with the denaturation temperatures T m for the complexes of r 25 > 0.1 (where r t is the ratio of the number of binding sites occupied by the dye to the total number of binding sites on DNA at temperature t). For the complexes of r 25 ≦ 0.1, the T r values were placed at temperatures higher than T m; in this case, however, an extension of the temperature interval of the helix-coil transition has occurred, for at a lower frequency of regions containing dye bound in complex I, which are more stable against heat denaturation than the regions not containing any dye, the heterogeneity of DNA increased. The results obtained are in agreement with the intercalation model for complex I. 2. 2. The cooperative “dissociation transition” was not found when heating proflavine complexes with denatured DNA. 3. 3. The binding of proflavine and acridine orange to DNA increases its stability against heat denaturation. The shift of T m in the complexes of a particular dye towards higher temperatures is, at constant ionic strength, a function of r 25 (complex I stabilizing DNA much more markedly than complex II) and of the base composition of DNA. If r 25 is constant, the shift of T m is proportionate to the adenine-thymine content in DNA. In the stabilization process, specific interactions between pairs of DNA bases and the dye molecules bound in complex I come into play besides the non-specific electrostatic forces between the negatively charged phosphate groups of DNA and the cations of the dyes.