Concentrated DNA-amphiphiles Structure, Hydration, and Dynamics

The structure, hydration, and dynamics of concentrated DNA-amphiphile complexes were investigated by small-angle X-ray diffraction, sorption calorimetry, and nuclear magnetic resonance. A concentrated, electroneutral, DNA-amphiphile complex can be formed by mixing charge-stoichiometric solutions. The effect is a strong association of DNA with the oppositely charged amphiphile resulting in the formation of a precipitate that contains the DNA-amphiphile aggregate in equilibrium with the aqueous phase to which small counterions are released.

In the complex, DNA and the amphiphilic aggregate acquire new characteristics reflecting the balance between electrostatic attraction, short range repulsion, and hydrophobic interactions. The supramolecular structure of the complexes resembles those of the amphiphilic assemblies per se. 2D hexagonal and lamellar structures can be found. Different systems were investigated in this thesis: DNA-cationic surfactant, DNAlipid mixture, and DNA-cationic surfactant-alcohols.
In comparison to DNA, the amphiphilic aggregate is more responsive with respect to external conditions and upon association changes shape to modulate the electrostatic interactions with DNA. The outcome is a higher degree of disorder in the hydrophobic part of the amphiphiles. Other modulations take place when the amphiphilic aggregate is comprised of charged and uncharged molecules; here a local demixing occurs with the charged molecules preferentially located in the vicinity of the DNA chains. The DNA-amphiphile complex can take up water to a limiting value between 30-40 water molecules per DNA-amphiphile pair. This value increases with the addition of a screening agent. During hydration, the distortion of the amphiphilic aggregate decreases while there is an increase in the rate of conformational changes. Water is rather mobile in the confined DNA-amphiphile assembly even at low water content. The hydration of the complexes can be predicted by a thermodynamic model where water is treated as a medium of constant dielectric permittivity. The existence of a thin layer of water separating the strongly attracted DNA and amphiphile can be explained by a short range repulsive force.