A NMR study of the ionization of fatty acids, fatty amines and N-acylamino acids incorporated in phosphatidylcholine vesicles.

The ionization of fatty acids, fatty amines and N-acylamino acids incorporated in phosphatidylcholine single-walled vesicles has been measured. The guest molecules have been specifically enriched with 13C and titrated by using NMR spectroscopy. The apparent pKa of fatty acids in phosphatidylcholine bilayers if 7.2-7.4 and those of fatty amines are approx. 9.5. These pKa values depend on many different parameters related to the structure of the lipid/solution interface, to the composition of the aqueous medium and to the localization of the ionizable groups. A special sensitivity to the ionic strength and to the surface charge has been found. A positive surface charge decreases the pKa value whereas a negative one increases it, the total range of variation being 2.5-3 units. In a qualitative macroscopic interpretation, it is proposed that pKa is essentially determined by the low polarity of the lipidic matrix.