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Diacetylenic lipid microstructures: structural characterization by X-ray diffraction and comparison with the
M Caffrey1, J Hogan, A S Rudolph
1Department of Chemistry, Ohio State University, Columbus 43210.
Biochemistry
|February 26, 1991
Summary
This study investigates the thermotropic and lyotropic mesomorphism of a polymerizable lecithin and its saturated analog using X-ray diffraction and calorimetry. Findings reveal unique microstructures and chain packing crucial for polymerization and tubule formation.
Area of Science:
- Lipid chemistry and physical chemistry
- Materials science and polymer science
Background:
- Investigates polymerizable lecithin (1,2-ditricosa-10,12-diynoyl-sn-glycero-3-phosphocholine) and its saturated analog.
- Focuses on thermotropic and lyotropic mesomorphism, crucial for understanding lipid behavior and self-assembly.
Purpose of the Study:
- To investigate the mesomorphism of a polymerizable lecithin and its saturated analogue.
- To elucidate the structural and conformational properties of lipid microstructures, including tubules and stacked bilayer sheets.
- To understand the factors influencing polymerization and tubule formation in lipid systems.
Main Methods:
- Wide- and low-angle X-ray diffraction on powder and oriented samples.
- Differential scanning calorimetry (DSC).
- Analysis of thermotropic and lyotropic phase behavior.
Main Results:
- Hydrated diacetylenic lipid forms tubules and stacked sheets with a lamellar repeat size (d001 = 66.4 A) and crystalline acyl chain packing.
- A model involving extended acyl chains, a 32-degree long-axis tilt, and minimal interdigitation explains the polymerizable lecithin's structure and supports polymerization.
- Above the chain melting transition, the lamellar repeat increases to 74 A; calorimetry confirms conformational similarity between different phases and suggests order alone doesn't drive tubule formation.
- Saturated analog shows typical lamellar gel phase behavior, with distinct high-temperature phases (P delta for saturated, R alpha for diacetylenic lipid).
Conclusions:
- The structural model for the polymerizable lecithin is consistent with diffraction data and energetic considerations, facilitating polymerization.
- Tubule formation is influenced by conformational order but requires additional factors.
- Distinct thermotropic phase behaviors are observed for the polymerizable diacetylenic lipid and its saturated counterpart.