NMR study of micelles formed by monoalkylphosphoryl nucleosides

An NMR investigation of aqueous micelles obtained from surfactants bearing a nucleotide head attached to a linear hexadecyl hydrocarbon chain is presented. In particular. hexadecylphosphoryl-adenosine (C16-AMP) and hexadecylphosphoryl-uridine (C16-UMP) are studied by a combination of H-1-NMR techniques Such as NOESY, ROESY. and spin-lattice relaxation times. Both the intramolecular (i.e., within one surfactant monomer) and the intermolecular interactions (i.e., between neighboring surfactant molecules) are investigated. Relaxation measurements show that different groups of the surfactant molecule have distinct dynamic properties, the internal mobility decreases starting from the head group towards the Me terminal, while protons belonging to the base (which should be exposed to water) enjoy considerable freedom. The large upfield shift of the resonance of the terminal Me groups is evidence of a collective property of the micelle, an effect that, to the best of our knowledge, has not been reported so far. The micelles are studied both in water and salt solution, and the noticeable difference between the two cases is interpreted as a salt-induced stiffening effect. By mixing C16-AMP with C16-UMP, mixed micelles are obtained. i.e.. micelles that contain both surfactant monomers in each aggregate: our analysis shows that a significant interaction between the two complementary aromatic bases is present. All these results allow us to draw a picture of the surfactant in the micelle in which the plane of the aromatic ring lies parallel to the surface of the micelle and towards the aqueous medium. There are no basic structural differences between C16-AMP and C16-UMP micelles, or C16-AMP/C16-UMP mixed micelles.