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A synthetic phospholipid, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), was studied by differential scanning calorimetry at different hydration levels. A glass transition was observed in the lamellar gel (Lβ) phase but was not detected in a more ordered lamellar crystalline phase. The glassy state of the Lβ phase of POPE has properties that are typical of other glass-forming materials, i.e., nonexponential relaxation behavior below the glass transition temperature, a broad distribution of relaxation times, and plasticization by water. The significance of the glass transition in biological membranes is discussed.

The glass transition of sucrose in 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE)/sucrose mixtures was studied by differential scanning calorimetry. Mole ratios of 1:2, 1:1, and 2:1 DOPE/sucrose mixtures were dehydrated at osmotic pressures ranging from 41 to 311 MPa through vapor-phase equilibration. Two glass transitions were observed in 1:2 DOPE/sucrose mixtures that were dehydrated at osmotic pressures >81 MPa. The first glass transition temperature (Tg) was lower than that of pure sucrose solutions that were dehydrated under identical conditions. The second Tg of the 1:2 DOPE/sucrose mixture was similar to the Tg of sucrose solutions that were dehydrated in the absence of DOPE. In the 2:1 and 1:1 DOPE/sucrose mixtures, only one Tg was observed at a temperature lower than that of pure sucrose. We propose that depression of the Tg of sucrose in the DOPE/sucrose mixtures is the result of a confinement effect that occurs in the sucrose solution that is sequestered in the aqueous cores of the h...

The authors present evidence that at acidic pH, liposomes composed of soybean lipids fuse with the plasma membrane of protoplasts isolated from rye leaves. Using the resonance energy transfer assay (RET), they determined the rate and extent of liposome and protoplast plasma membrane lipid mixing. The fluorescent donor-acceptor pair was N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidyl-ethanolamine (N-NBD-PE) and N-(lissamine rhodamine B sulfonyl)phosphatidylethanolamine (N-Rh-PE). Fusion was substantial below pH 5, and the half-time of lipid mixing was fast (t/sub 1/2/ on the order of minutes) and pH, concentration, and temperature dependent. The extent of liposome and protoplast fusion from the total amount of liposomes associated with the protoplasts was also determined by the RET assay. Protoplasts were incubated with fluorescent-labeled liposomes (5 min at 30/sup 0/C) at different pH values and then washed twice by centrifugation. The fluorescence spectra of the protoplast suspension permitted determination of the ratio of N-NBD-PE emission at 530 nm to the N-Rh-PE emission at 590 nm, which is a measure of the degree of lipid mixing. The amount of liposomes associated (fused and unfused) with protoplasts at pH 3.9 was approximately 9 times greater than that at pH 5.6. The transfer of liposome contents to the protoplast interior was studied withmore » a method based on the fluorescence enhancement of a solution of calcein, initially confined in the liposomes at self-quenching concentrations. The kinetics of calcein release were very similar to those of lipid mixing. Fluorescence microscopy showed that after fusion with liposomes containing calcein, the protoplasts exhibited a strong diffuse fluorescence in the interior.« less