ON THE COMPLEX FORMATION OF ACRIDINE DYES WITH DNA‐IV. THE EQUILIBRIUM CONSTANTS OF SUBSTITUTED PROFLAVINE AND ACRIDINE ORANGE DERIVATIVES*

Abstract— The binding constants of the complex formation of several acridine dyes with calf thymus DNA are reported. The results indicate, that the binding tendency increases with the basicity of the dyes in the order acridine < 3–amino‐acridine < 3.6–bis‐monocetamino‐acridine < 3.6–diamino‐acridine ≅ 3.6–bis‐dimethylamino‐acndine < 3.6–diamino‐10–methyl‐acridine. The ring‐N alkylated amino‐acridines give in general the highest brinding constants, but there is no significant difference in the extrapolated values, K0, at diminishing dye concentration if the chain length of the alkyl groups is increased from methyl to amyl. Only substitution with phenyl seems to inhibit the binding. On the other hand, the weaker binding of the acndine itself is sensitive to the ring‐N alkylation and the binding tendency decreases in the order acridine > 10–methyl‐acridine > 10–propyl‐acridine. These findings could be explained, if the electrostatic binding involving the positively charged ring nitrogen, which seems to be more important for the more weakly interacting acridines, is complemented by other binding mechanisms, including possibly non‐electrostatic interactions in the case of those amino‐acridines having the strong binding effect.