Lipid-alginate interactions render changes in phospholipid bilayer permeability.

Lipid vesicles, e.g. liposomes, generally release their contents in a continuous manner. However, when these vesicles are entrapped in Ca-alginate and coated with poly(L-lysine), they release their contents in an unusual fashion, in 'bursts'. Molecular-level studies indicated that lipid-alginate interactions are responsible for changes in the barrier properties of lipid vesicles. Differential scanning calorimetry revealed that exposure of liposomes to alginate resulted in a 4-fold reduction in the phase transition enthalpy, with no change in the melting temperature. Size-exclusion chromatography of liposomes-in-alginate gave an additional liposomal peak with a smaller elution volume. These studies suggested that alginate is inserted into the lipid bilayer of vesicles. Lipid-alginate interactions were highly dependent on phospholipid head group charge and the phase transition temperature of the phospholipid. Based on these interactions, a mechanism to explain the 'burst' from these entrapped liposomes is suggested.