MECHANISTIC ASPECTS OF THE ADSORPTION OF PHOSPHOLIPIDS ONTO LAURIC ACID STABILIZED FE3O4 NANOCOLLOIDS

The mode of adsorption of different phosphatidylglycerol (PG) analogues onto nanometer-sized magnetite (Fe3O4) particles, which are originally stabilized with lauric acid, is investigated by analyzing the kinetic features of the binding process. In the presence of a sufficient amount of vesicles (mmol of PG/g of Fe3O4 ratio almost-equal-to 4.1), made of dimyristoyl- or dipentadecanoylPG, the lipids adopt a bilayer configuration around the iron oxide core. The kinetic profiles, describing the construction of the inner layer during the initial adsorption phase, obey first-order reaction rules; the t1/2 values equal 11 and 116 min, respectively. The generation of the outer leaflet, which occurs later on, is considerably retarded by back fluxes. Furthermore, the overall kinetic binding curves are unaltered upon changing the Fe3O4 and vesicle concentrations at constant ratio. In the presence of DMSO however, the adsorption rate is dramatically enhanced. The longer chain length dipalmitoylPG, and in particular distearoylPG, bind at a much lower rate. In conditions where only a monolayer is constructed (starting PG/Fe3O4 ratio almost-equal-to 0.3), a similar chain length dependency is noticed. Also with mixed vesicles, composed of equimolar amounts of dimyristoyl- and distearoylPG, the shorter chain length homologue becomes preferentially attached. All these findings are consistent with an "aqueous transfer mechanism" which involves a rate-limiting escape of the phospholipids from the vesicles, followed by a rapid passage through the water phase and uptake by the magnetic colloid. With regard to this final uptake step, the important contribution of the apolar part of the PG molecules is highlighted. Multilayer adsorption of phosphatidylglycerols beyond the bilayer stage is not observed.