Phase transitions of phospholipid single-wall vesicles and multilayers. Measurement by vibrational Raman...

Raman spectroscopic frequency differences between selected carbon-carbon stretching modes of lipid hydrocarbon chains were determined as a function of temperature for use in monitoring lipid phase transition behavior and acyl chain disorder in both multilamellar and single-wall vesicles. Transition temperatures detected by this procedure for pure dipalmitoyl phosphatidylcholine and dimyristoyl phosphatidylcholine multilayers were observed at 39 +/- 1 degrees C and 23 +/- 1 degrees C, respectively. Although the phase transition for unilamellar vesicles of dipalmitoyl phosphatidylcholine occurred at nearly the same temperature as the multilayers, the crystal-liquid crystalline transition for the single-shell vesicles appeared to span a slightly broader temperature range, a characteristic consistent with irregularities in the packing arrangement of the hydrocarbon chains. Within the precision of the Raman spectroscopic method, however, the temperature behavior of both the multilamellar and the unilamellar dimyristoyl phosphatidylcholine assemblies appeared nearly identical. The temperature profile for the Raman frequency differences of an excess water sonicate of 25 mol percent cholesterol in dipalmitoyl phosphatidylcholine served as an example of the effect upon lipid phase transition characteristics of a bilayer component intercalated between the acyl chains. For this particular cholesterol-lipid system the phase transition was broadened over a 30 degrees C temperature range, in contrast to the marrow 2-4 degrees C range observed for pure multilayer and single-shell vesicle particles.