Aggregation of small unilamellar vesicles of polyunsaturated phosphatidylcholines under the influence of polyethylene glycol

Several biophysical methods have been used to study the influence of polyet hylene glycol (PEG) on physicochemical properties of bilayer lipid membrane s (BLM) and aggregation of small unilamellar vesicles: 1-palmitoyl-2-oleoyl -sn-glycero-3-phosphocholine (POPC), 1-straroyl-2-linoleoyl-sn-glycero-3-ph osphocholine (SLPC) and 1-stearoyl-2-linolenoyl-sn-glycero-3-phosphocholine (SLnPC). Measurements of elasticity modulus E-perpendicular to of solvent- free BLM perpendicular to the membrane plane revealed an increase of bilaye r compressibility, when the number of double bonds of phosphatidylcholine h ydrocarbon chains increased, indicating a decrease of ordering of the hydro phobic part of lipid bilayer. An increase in unsaturation of the sn-2-acyl chain resulted in an increased tendency of vesicle aggregation in the prese nce at PEG as revealed by turbidimetry. Measurements of changes of ultrasou nd velocity provided evidence that PEG induced dehydration of the vesicle s urface. Thus, measurement of mechanical properties of membranes led to a po sitive correlation between compressibility of lipid bilayers and aggregatio n of vesicles.