A study of the flocculation of phospholipids as a model of membrane interactions

Dispersion of the pure phospholipids phosphatidylcholine (lecithin) and phosphatidyiserine were prepared in solutions of different ionic type and concentration. The dispersions were prepared so as to give particies, spherules, of the phospholipids which were of a fairly constant diameter, i.e. they were quasi-monodisperse suspensions. The dispersions were subjected to a laminar flow shear gradient was to affect collisions between the particles of the dispersions and thus to bring about flocculation or aggregation of the suspensions. By measuring the total number of particles in the dispersions at timed intervals a parameter was calculated for each aggregation, the collision efficiency. The collision efficiency is a measure of the rate of aggregation and can also be used to calculate the enrgy of the adhesive interaction of the particles. The values of the collision efficiency for the flocculation of lecithin spherules showed that for the monovalent cations the order of increasing efficacy was Potassium: Sodium: Lithium which is the order of decreasing ionic radius. The results for divalent cations showed that these were more effective by at least two orders of magnitude. Further divalent cations exhibited a reversal of charge behaviour at concentrations above about 10-3 molar. The order of increasing efficacy did not reflect the order of decreasing ionic radius and was Magnesium: Strontium: Barium: Calcium, which suggest that lecithin may have a specific affinity for Calcium. The trivalent cation Lanthanum was more effective than the divalent cations and showed a charge reversal at a lower concentration. Temperature was found to have little effect on the rate of flocculation of lecithin dispersion. For the flocculation of phosphatidylserine dispersions higher concentrations of sodium and calcium ions were found to be necessary, than for the flocculation of lecithin dispersions. This was to be expected because phosphatidylserine spherules are considered to bear a considerable negative surface charge, whereas those of lecitihin are considered to be uncharged. The collision efficiency, measured in ionic conditions where the spherules are probably uncharged, was used to calculate a value for the London-Hamaker constant. Values in the range 7x10.;-15 - 2.45x10.;-14ergs wereobtained for lecithin spherules and values of 4.7x10.;-15 - 1x10.;-14ergs forphosphatidylserine. These values were used to calculate the adhesive energy of the particle interactions, assuming that the particles adhered in the primary minimum with a separation of 5A. Values in excess of 200kT were obtained, indicating stable adhesions. These values are comparable with those derived experimentally and theoretically for other lipid systems. As the spherules are considered to be made of structures which are similar to the cell plasmalemma and the spherules used in the flocculations have a size similar to that cells, the result may be of interest in a study of cell membrane interactions, in particular cell to cell adhesion. The values of the London-Hamaker constent measured, would allow adhesion both in the primary and secondary minimumas proposed in the lyophobic colloid theory adhesion.