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

Differential scanning calorimetry was used to examine the structure-function relationship of the phospholipids on the L alpha-phase stabilization of phosphatidylethanolamine (PE). Phosphatidylglycerol (PG) was chosen as a model stabilizer. Dielaidoylphosphatidylethanolamine (DEPE) was mixed with various PGs to study the effects of (i) chain length, (ii) chain unsaturation, and (iii) chain number of the stabilizer on the L alpha-phase stabilization. At low concentrations of stabilizer, both bilayer stabilization and destabilization were observed. Phase separations also were seen, as revealed by split peaks of the L beta----L alpha transition; these were particularly prone to occur in the destabilization cases. When saturated PGs were compared, shorter chains (C12:0 and C14:0) promoted bilayer stabilization whereas longer chains (C16:0 and C18:0) promoted bilayer destabilization. Unsaturated PG with larger hydrophobic volumes (C18:2) favored bilayer destabilization, relative to unsaturated PG with smaller hydrophobic volumes (C18:1). Lyso-PG (C14:0) showed higher bilayer stabilization activity than their double-chain counterparts. Thus, at low concentrations of stabilizer, the acyl chain composition plays a vital role in bilayer-phase stabilization. However, at higher concentrations (greater than or equal to 8 mol %), all PGs become active bilayer stabilizers. This is probably because the increased head-group hydration becomes the dominant factor in the stabilization. The effect of acyl chain composition of the stabilizer was also studied by using small unilamellar vesicles composed of dioleoylphosphatidylethanolamine (DOPE). Fluorescence quenching of calcein entrapped in liposomes was used to monitor the stability of the liposomes. Similar acyl chain effects on liposomal stabilization were obtained.(ABSTRACT TRUNCATED AT 250 WORDS)