Structure and function relationship of phosphatidylglycerol in the stabilization of phosphatidylethanolamine bilayer

Differential scanning calorimetry was used to examine the structure-function relationship of the stabilizer on the La phase stabilization of phosphatidylethanolamine (PE). Phosphatidylglycerol (PG) was chosen as a model stabilizer. Dielaidoylphosphatidylethanolamine (DEPE) was mixed with various PG to study the effects of: (1) chain length, (ii) chain unsaturation and (iii) chain number of the stabilizer on the La phase stabilization. At low concentration of stabilizer, both bilayer stabilization and destabilization were observed. Phase separation revealed as split peaks of the L%beta;+Lα transition, which was particularly prone in the destabilization cases, were also seen. When saturated PG were compared, shorter chains (012:0 & 014:0) promoted bilayer stabilization whereas longer chains (016:0 & 018:0) promoted bilayer destabilization. Unsaturated PG with larger volume of the hydrophobic moiety (018:2) was found to more favor bilayer destabilization than the PG with smaller volume (018:1). Lyso PG (014:0) showed higher bilayer stabilization activity than their double-chain counterparts. Thus at low concentrations of the stabilizer, the acyl chain composition played a vital role in PE bilayer phase stabilization. However, at higher concentration (≥8 mole %), all PG became active bilayer stabilizers, which was probably due to the head group hydration effect being the dominant factor in the bilayer stabilization activity. The effect of acyl chain composition of the stabilizer was also used studied by using dioleoylphosphatidylethanolamine (DOPE) liposomes prepared by sonication. Fluorescence V quenching of calcein entrapped in liposomes was used to monitor the stability of the liposomes. Similar acyl chain effects on liposomal stabilization were obtained. However, a higher mole % of the stabilizer is required to stabilize DOPE liposomes than that required to stabilize the L&alpha phase of DEPE. Furthermore, lyso PG which was active DEPE bilayer stabilizers were found to be poor DOPE liposomal stabilizers. This is probably due to the coexistence of La and isotropic, or micellar, phase in the binary mixtures of DOPE and lyso PG, or due to a phase separation in the fluid liposome membrane.