[Formation of complexes between DNA and cationic amphiphile molecules by the fluorescent probe method]

The formation of complexes of DNA with dodecylamine, dodecyltrimethylammonium, tetradecyltrimethylammonium, and hexadecyltrimethylammonium was studied using a fluorescent probe pyrene. The dependences of the spectral parameters of the hydrophobic pyrene probe on the concentration of the cationic amphiphile in the presence and absence of DNA were obtained and analyzed. It is shown that, in the absence of DNA, these dependences exhibit only one S-shaped region, which corresponds to the micelle formation of the amphiphile, whereas in the presence of DNA there are two S-shaped regions, which indicates the cooperative formation of two types of DNA-cationic amphiphile complexes. For each of the four cationic amphiphiles, the critical concentrations for the micelle formation in the absence of DNA (C0) and the concentrations at which the first (Cd1) and the second complex with DNA are formed were determined. It was found that the Cd1 value is 15-40 times lower than C0. The Cd1 value does not depend on DNA concentration and is determined only by the length of the hydrocarbon chain and the structure of the amphiphile ionic fragment. The Cd1 value increases as the length of the aliphatic chain decreases and upon replacement of mobile hydrogen atoms in the ammonium fragment by methyl groups. It was shown that hydrophobic clusters of amphiphile arising upon complex formation with DNA play the role of cross-links promoting DNA aggregation, or DNA compactization in the case of dilute solution of high-molecular weight DNA. The structures of the first and second DNA-cationic amphiphile complexes are proposed, and the mechanism and nature of interactions that determine their formation are discussed.