The electrostatic binding of polycations with DNA‚EB complex results in displacement of intercalated cationic dye ethidium bromide (EB) from DNA double helix to the solution which is accompanied by a quenching of EB fluorescence. On the basis of this phenomenon, the fluorescence assay of DNA-containing polyelectrolyte complexes was recently developed. Data obtained in the current work demonstrate the applicability and advantages of this approach for monitoring both an interaction of DNA with cationic surfactants (CS) and stability of DNA-CS complexes. The comprehensive study was carried out with cationic detergents having different C12-C16 "tails" and "heads" with pyridinium or amino groups. In parallel, the similar experiments were performed with pyrenyl-tagged poly(methacrylate) anion (PMA*), in which the complex formation was monitored by quenching of PMA* fluorescence with pyridinium or nonquaternary amino groups of the detergents. The fluorescence titration curves of DNA‚EB or PMA* with CS consisted of two parts, with negligible quenching on the initial stage followed by the pronounced quenching. The critical aggregation concentration (CAC) determined from the intersection points of the curves decreased substantially with the length of "tail". CAC values measured in DNA-CS mixtures proved to be noticeably higher than those from PMA*-CS mixtures. This finding suggests that DNAinduced self-assembly of CS molecules in the intramacromolecular aggregates is hindered due to rigidity of the double helix. Dissociation of DNA-CS complexes in salt (NaCl) solutions was monitored by the increase of fluorescence intensity of EB intercalated in free sites of DNA. Inasmuch as the addition of salt resulted in increase of CAC and decrease of critical micelle concentration (CMC), two regimes of destruction of DNA-CS complexes dependent on CS concentration were revealed. The regime of noncooperative destruction was realized if CS concentration was lower than CMC at the ionic strength of the complex dissociation, otherwise the second regime of the cooperative destruction took place. In the latter case, the salt concentration corresponding to the destruction virtually did not depend on the length of "tail" but markedly decreased with increase of a number of N-methyl groups in the "head" in the series C12NH2 > C12NHMe > C12NMe2 > C12N + Me3Br -. It implies that distance between charges in the ion pairs is the dominant factor determining the stability of the complexes. In the case of a DNA-C12NMe2 complex, the destruction was rather pH sensitive and occurred at pH and ionic strength that were close to physiological conditions. The results might create the basis for design of DNA-CS complexes with controlled stability which could be of particular promise for DNA delivery to the target cell.

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