ИСТИНА

Cationic dye ethidium bromide (EB) is used for studying the interaction between native DNA and polycations. Formation of the polyelectrolyte complex leads to the squeezing out of the dye followed by a decrease of EB fluorescence, whereas destruction of the complex allows re-intercalation of EB in the free sites of DNA resulting in an ignition of fluorescence. Based on this phenomenon the fluorescence assay of the complexes was developed [1] and the factors influencing on stability of the complexes in water-salt solutions were revealed [2]. The goal of the present work was to elucidate the applicability of the same approach for monitoring the interaction between DNA and cationic surfactants (CS). A large number of CS differed by length of the hydrophobic “tail” and nature of the hydrophilic “head” was used to carry out the comparative studying of (DNA-CS) complexes. In parallel, the same experiments were done with CS and pyrenyl-tagged poly(methacrylate) anion (PMA*) using the ability of amino groups of the “heads” to quench a fluorescence of PMA*. The data obtained strongly suggest that the assay based on EB fluorescence adequately describes DNA-CS interactions and demonstrate the advantages of this approach. Thus, as expected, the efficiency of quenching of fluorescence of (DNA•EB) complex by added surfactants increased in the row C12N+Me3 < C14N+Me3 < C16N+Me3. Fluorescence titration curves of (DNA•EB) complex by CS solutions consisted of two approximately linear parts. The values of critical concentration of aggregation (CCA) calculated from the points of the intersection were in agreement with the content of methyl groups in the surfactant. C12N+Me3 with bulky quaternary amine group in the molecule formed with both DNA and PMA* complexes least tolerant to destructive action of added salt (NaCl). On the contrary, complexes composed of C12NH2 bearing primary amine group and hence forming the most closed ion pairs revealed the best stability. Complexes of C12NMe2 and C12NHMe with tertiary and secondary amine groups respectively were characterized by moderate stability. These data are in good agreement with the results obtained on studying the stability of DNA-containing polyelectrolyte complexes with polycations carrying amine groups of different structure [2]. It is noteworthy that CCA values estimated by the developed approach for mixtures of DNA and CS proved to be noticeably lower than CCA determined by quenching of PMA* with the same CS. This finding could be the result of pronounced difference in conformation of the polyanions. Self-assembly of CS micelles on PMA* is evidently facilitated by flexibility of the chain, whereas rigidity of DNA double helix hinders formation of the micelle. 1. V. A. Izumrudov, M. V. Zhiryakova, Macromol. Chem. Phys. 200(11), 2533-2540 (1999). 2. V. A. Izumrudov, M. V. Zhiryakova, S. E. Kudaibergenov, Biopolymers (Nucleic Acid Sciences) 52, 94-108 (1999).