Kinetic studies of the interaction of fatty acids with phosphatidylcholine vesicles (liposomes).

The kinetics of addition of fatty acids (as alkaline solutions of the fatty acid anions) to pre-existing unilamellar phospholipid vesicles (mean diameter 100 nm) has been studied. The phospholipid DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) has been mainly used, together with three fatty acids, oleic acid (cis-9-octadecenoic acid), linoleic acid (cis,cis-9,12-octadecadienoic acid) and capric acid (decanoic acid). Experiments were performed above as well as below the main phase transition temperature (Tm) of DMPC vesicles. The pH chosen to study the fatty acid vesicle interaction (after fatty acid and vesicle mixing) was 8.5 in the case of oleic acid and linoleic acid and 7.4 for capric acid. In the absence of any pre-existing phospholipid vesicles, the addition of alkaline solutions of the fatty acid anions to corresponding buffer solutions of pH 8.5 or 7.4 leads to a partial protonation of the fatty acid anions again resulting in the formation of fatty acid vesicles. This process is rather slow, taking place over a period of hours/days, and the vesicles formed are very polydisperse and include a range of vesicle sizes/shapes. However, in the presence of pre-existing phospholipid vesicles the added fatty acids equilibrate readily within a few minutes and the size of the vesicles that form are then closely related to the size of the originally present phospholipid vesicles; the vesicles formed being generally somewhat larger than the pre-existing vesicles. In the case of the phospholipid DMPC, the mixed fatty acid/phospholipid vesicle system is often formed rather rapidly (particularly above Tm), so that stopped-flow methods have been applied to follow the kinetics of the process. It is proposed that most of the fatty acid molecules are initially rapidly incorporated into the bilayers of the pre-exisiting phospholipid vesicles as monomers, rather than that the added fatty acids form separate fatty acid vesicles. The mean vesicle sizes formed in the systems investigated have been analysed by using dynamic light scattering measurements. The behaviour of the DMPC system was found to be slightly different from the POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) system studied before, but the results are consistent with a model that involves growth and subsequent fission of the mixed vesicles. The study provides further support of the "matrix effect" in this type of system [S. Lonchin, P.L. Luisi, P. Walde, B.H. Robinson, J. Phys. Chem. B 103 (1999) 10910-10916]. The pre-existing DMPC vesicles act as a kind of seed to control the behavior of the system in the presence of added fatty acid anions.