Head group interaction in phosphatidylcholine micelles studied by 14N-nuclear magnetic resonance

Abstract

The polar head group of egg phosphatidylcholine (EPC) was studied by 14N-nuclear magnetic (NMR) relaxation methods in four organic solvents: chloroform, benzene, carbontetrachloride, and methanol. The 14N relaxation times in the choline head group were found to increase sharply with the addition of water. Similar changes have been reported by NMR of other nuclei, but the changes observed in 14N-NMR were much more dramatic. The results can be explained by the formation of inverted micelles in dry hydrophobic solvents and by micelles with a water core in the presence of water. The remarkable changes observed by the addition of water must be due to one or more of the following reasons: (1) A change in the motional freedom at the head group. (2) A change in electrostatic interactions between neighboring groups reflected in the value of 14N quadrupole coupling constant. (3) A change in the ordering of the N-C β bond in the micelles. In the hydrated micelles, it was found that micelle size in three hydrophobic solvents was similar and that the correlation time for the rapid local motion of head group was comparable to that in single unilamellar vesicles. The (H 2O-D 2O) isotope effect of local motion around N nucleus and on the size of hydrated micelles was small.