Increase in molecular order of phospholipid membranes due to osmotic stress by polyethylene glycol

Abstract

We studied the effects of polyethylene glycol (PEG) on membrane molecular order of egg phosphatidylcholine liposomes by use of the birefringence technique. The intensity of interference light due to birefringence in the membranes was measured by polarized light transmission through the liposome suspension. The PEG-induced aggregation of liposomes was monitored by turbidity. We found that addition of PEG induced an abrupt increase in the interference light intensity at a concentration of 1 wt% PEG 8000 (average molecular weight: 8000), which is below a concentration required to induce liposome aggregation. The membrane molecular order was estimated to increase by about 25% from the change in interference light intensity, indicating that PEG caused a significant change in membrane molecular packing by an osmotic stress even at low concentrations without inducing liposome aggregation. The result suggests that liposome aggregation is caused not only by the dehydration in the water layer, but also by the tight packing in the membrane caused by the osmotic stress.