surfactant aggregates on the DNA surface

S. Morrissey, E. Kudryashov, K Dawson and V. Buckin.

Biophysical Journal, 76(1), pp. 321 (1999)

Binding of the cationic surfactants to DNA plays an important role in the, construction of gene delivery systems. Previously we have found that the compressibility effect of the binding of the cationic surfactants to DNA is similar to the compressibility change in the micelle formation of the same surfactant [Progr. Colloid Polym. Sci., 110, pp. 214 (1998)]. This fact was interpreted as a formation of micelle-like aggregates of surfactant molecules on the DNA surface, which have a highly compressible internal hydrophobic core. In the present work we have used a combination of optical spectroscopy,fluorescence, isothermal titration calorimetry and surfactant selective electrode techniques combined with the high-resolution ultrasonic velocity and density measurements to study the thermodynamic and structural parameters of the binding of the cationic surfactant, dodeclytrimethylammonium bromide, to short fragments of DNA. We found two stages of co-operative binding. At the first stage the surfactant forms small aggregates on the DNA surface. This binding is accompanied by a significant change in the DNA secondary structure. At the second stage, we observed the formation of large aggregates of surfactant on the DNA surface. Migration of the fluorescence probe into the hydrophobic core of these aggregates was detected at the second stage. Compressibility of these aggregations is close to the compressibility of micelles.

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