NMR Studies of Molecular Mobility in a DNA−Amphiphile Complex

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The molecular mobility in a hexagonal DNA-cationic surfactant complex is studied using H-1 and C-13 nuclear magnetic resonance spectroscopy. The charge-compensated complex can swell in water up to a content of approximately seven water molecules per charge. The NMR measurements show that in the dry state the alkyl chains of the surfactant have the properties of a disordered solid with internal motions of sufficient amplitude to substantially narrow the H-1 resonance line from the rigid lattice limit. As water is introduced, there is an increase in molecular motion resulting in further narrowing of the signal. In the fully swollen system, the signal is narrower than that observed for a normal hexagonal liquid crystalline phase with the same surfactant. This shows that the alkyl chains are packed with a degree of disorder that is higher than in the corresponding liquid crystalline surfactant system, reflecting the aggregate deformations induced by the requirement of charge matching with DNA. Furthermore, the translational diffusional motion of the surfactant molecule is slower than D < 10(-13) m(2)/s, while for the water molecules we observe D going from 1 x 10(-11)m(2)/ s at 5 water molecules per base pair to 2 x 10(-10) m(2)/s at the swelling limit of 27 waters per base pair. The DNA remains solid throughout the hydration range. By combining the NMR observations with the thermodynamic characterization of the system by Leal et al.(1) we arrive at a detailed description of the molecular organization in the complex between DNA and the single chain cationic surfactant hexadecyltrimethylammonium, CTA. (Less)