Phase behaviour of membrane lipids containing polyenoic acyl chains.
The low-temperature thermal behaviour of di-18:2 phosphatidylethanolamine (di-18:2 PE) is shown to be characterized by similar broad low-enthalpy transitions to those previously reported for polyenoic samples of phosphatidylcholines (Keough and Kariel (1987) Biochim. Biophys. Acta 902, 11-18), and monogalactosyldiacylglycerol (Sanderson and Williams (1992) Biochim. Biophys. Acta. 1107, 77-85). Real-time X-ray diffraction measurements indicate that these transitions correspond to transitions between the gel (L beta) and liquid-crystal (L alpha) phases of the lipids. The gel phase of these lipids is, however, much more loosely packed than the corresponding phases of membrane lipids containing monoenoic or fully-saturated acyl chains. The low enthalpy and reduced co-operativity of the L alpha--> L beta phase transitions of the polyenoic lipids is attributed to the reduced contribution of van der Waals interactions between their acyl chains in the gel-state of these lipids. Comparison with the earlier results obtained for MGDG suggest that the acyl chains of polyenoic lipids can form well-ordered lattices but require the additional energy input associated with the formation of a hydrogen bond network between the lipid headgroups in order to do so.
Williams
W P
WP
Life Sciences Division, King's College London, Campden Hill, UK.
Sanderson
P W
PW
Cunningham
B A
BA
Wolfe
D H
DH
Lis
L J
LJ
D002152:Calorimetry, Differential Scanning; D004075:Diglycerides / Q000737:chemistry*; D038983:Galactolipids; D006017:Glycolipids / Q000737:chemistry*; D008563:Membrane Lipids / Q000737:chemistry*; D013696:Temperature; D014961:X-Ray Diffraction
D004075:Diglycerides; D038983:Galactolipids; D006017:Glycolipids; D008563:Membrane Lipids; C007388:digalactosyldiacylglycerol; C009909:monogalactosyldiacylglycerol