Temperature-induced fusion of small unilamellar vesicles formed from saturated long-chain lecithins and diheptanoylphosphatidylcholine.
Small unilamellar vesicles which form when gel-state long-chain phosphatidylcholines are mixed with micellar short-chain lecithins undergo an increase in size as the long-chain species melts to its liquid-crystalline form. Analysis of the vesicle population with quasi-elastic light scattering shows that the particle size increases from 90-A radius to greater than 5000-A radius. Resonance energy transfer experiments show total mixing of lipid probes with unlabeled vesicles only when the Tm of the long-chain phosphatidylcholine is exceeded. This implies that the large size change represents a fusion process. Aqueous compartments are also mixed during this transition. 31P NMR analysis of the vesicle mixtures above the phase transition shows a great degree of heterogeneity with large unilamellar particles coexisting with oligo- and multilamellar structures. Upon cooling the vesicles below the Tm, the original size distribution (e.g., small unilamellar vesicles) is obtained, as monitored by both quasi-elastic light scattering and 31P NMR spectroscopy. This temperature-induced fusion of unilamellar vesicles is concentration dependent and can be abolished at lower total phospholipid concentrations. It occurs over a wide range of long-chain to short-chain ratios and occurs with 1-palmitoyl-2-stearoylphosphatidylcholine and dimyristoylphosphatidylcholine as well. Characterization of this fusion event is used to understand the anomalous kinetics of water-soluble phospholipases toward these unusual vesicles.
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K M
KM
Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02167.
Riedy
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G
Langley
K H
KH
Roberts
M F
MF
D007700:Kinetics; D008027:Light; D009682:Magnetic Resonance Spectroscopy; D008567:Membranes, Artificial; D010713:Phosphatidylcholines / Q000032:analysis*; D010740:Phospholipases / Q000378:metabolism; D012542:Scattering, Radiation; D013050:Spectrometry, Fluorescence; D013696:Temperature
D008567:Membranes, Artificial; D010713:Phosphatidylcholines; C038470:1,2-heptanoylphosphatidylcholine; D010740:Phospholipases