Phase characteristics of positional isomers of 1,2-di(heptacosadiynoyl)-sn-glycero-3-phosphocholine; tubule-forming phosphatidylcholines.
We have examined the phase behavior of positional isomers of a polymerizable diacetylenic phospholipid, 1,2-di(heptacosadiynoyl)-sn-glycero-3-phosphocholine which has the diacetylene in varying position along the acyl chains. Upon cooling multilamellar vesicles (MLVs) through the liquid-crystalline to gel phase transition, all isomers examined spontaneously formed hollow, cylindrical microstructures (or tubules). Differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FTIR) have been used to characterize positional isomers of this lipid in an effort to understand the effect of diacetylenic position on the molecular characteristics of tubule formation. Calorimetric results indicate that moving the position of the diacetylene along the acyl chain results in the alternation of the exotherm observed for the hydrated transition temperature associated with tubule formation, with higher transition temperatures (Tm) observed from isomers with an even number of methylenes between the diacetylene groups and the glycerol backbone. As the diacetylene is moved toward either end of the acyl chain, even with the observed alternation, the Tm was observed to increase. Calorimetric results of dry members of this series reveal an exotherm during cooling, the same temperature at which fully hydrated samples form tubules. This suggests that there is little difference in the phase behavior observed upon cooling the hydrated tubules and the dry diacetylenic material. FTIR results support the high degree of conformational order observed in tubules of this isomer series as a very strong CH2 wagging progression is observed between 1375 and 1200 cm-1. In addition, the C-H stretch region (3000 cm-1 to 2800 cm-1) indicates tight acyl chain packing with many all-trans segments. These results provide further evidence that tubules are uniquely crystalline microstructures and that this inherent crystallinity, and the formation of tubules is not affected by diacetylenic position.
Rudolph
A S
AS
Biomolecular Engineering Branch, Naval Research Laboratory, Washington, DC 20375-5000.
Singh
B P
BP
Singh
A
A
Burke
T G
TG
D002152:Calorimetry, Differential Scanning; D055598:Chemical Phenomena; D002621:Chemistry; D003460:Crystallization; D005782:Gels; D006358:Hot Temperature; D007536:Isomerism; D008051:Lipid Bilayers / Q000378:metabolism*; D010713:Phosphatidylcholines / Q000378:metabolism*; D013055:Spectrophotometry, Infrared
D005782:Gels; D008051:Lipid Bilayers; D010713:Phosphatidylcholines; C057168:1,2-di-(heptacosadiynoyl)-sn-glycero-3-phosphocholine