An analysis of the relationship between fatty acid composition and the lamellar gel to liquid-crystalline and the lamellar to inverted nonlamellar phase transition temperatures of phosphatidylethanolamines and diacyl-alpha-D-glucosyl glycerols.
The lamellar gel to lamellar liquid-crystalline (Lbeta/Lalpha) and lamellar liquid-crystalline to inverted hexagonal (Lalpha/H(II)) phase transitions of a number of phosphatidylethanolamines (PEs) and diacyl-alpha-D-glucosyl-sn-glycerols (alpha-D-GlcDAGs) containing linear saturated, linear unsaturated, branched or alicyclic hydrocarbon chains of various lengths were examined by differential scanning calorimetry and low-angle X-ray diffraction. As reported previously, for each homologous series of PEs or alpha-D-GlcDAGs, the Lbeta/Lalpha phase transition temperatures (Tm) increase and the Lalpha/H(II) phase transition temperatures (Th) decrease with increases in hydrocarbon chain length. The Tm and the especially the Th values for the PEs are higher than those of the corresponding alpha-D-GlcDAGs. For PEs having the same effective hydrocarbon chain length but different chain configurations, the Tm and Th values vary markedly but with an almost constant temperature interval (deltaT(L/NL)) between the two phase transitions. Moreover, although the Tm and Th values of the PEs and alpha-D-GlcDAGs are equally sensitive on the temperature scale to variations in the length and chemical configuration of the hydrocarbon chains, the deltaT(L/NL) values are generally larger in the PEs and vary less with the hydrocarbon chain structure. This suggests that the PE headgroup has a greater ability to counteract variations in the packing properties of different hydrocarbon chain structures than does the alpha-D-GlcDAG headgroup. With decreasing chain length, this ability of the PE headgroup to counteract the hydrocarbon chain packing properties increases, significantly expanding the temperature interval over which the Lalpha phase is stable relative to the corresponding regions in the alpha-D-GlcDAGs. Overall, these findings indicate that the PEs have a smaller propensity to form the H(II) phase than do the alpha-D-GlcDAGs with an identical fatty acid composition. In contrast to our previous report, there is some variation in the d-spacings of these various PEs (and alpha-D-GlcDAGs) in both the Lalpha and H(II) phases when the hydrocarbon chain structure is changed while the effective chain length is kept constant. These hydrocarbon chain structural modifications produce different d-spacings in the Lalpha and H(II) phases, but those changes are consistent between the PEs and alpha-D-GlcDAGs, probably reflecting differences in the hydrocarbon chain packing constraints in these two phases. Overall, our experimental observations can be rationalized to a first approximation by a simple lateral stress model in which the primary bilayer strain results from a mismatch between the actual and optimal headgroup areas and the primary strain in the H(II) phase arises from a simple hydrocarbon chain packing term.
Mannock
D A
DA
Department of Biochemistry, University of Alberta, Edmonton, Canada.
Lewis
R N
RN
McElhaney
R N
RN
Harper
P E
PE
Turner
D C
DC
Gruner
S M
SM
D000127:Acholeplasma / Q000378:metabolism; D055592:Biophysical Phenomena; D001703:Biophysics; D002152:Calorimetry, Differential Scanning; D005227:Fatty Acids / Q000737:chemistry*; D005782:Gels; D005990:Glycerol / Q000031:analogs & derivatives* / Q000737:chemistry*; D006017:Glycolipids / Q000737:chemistry*; D006027:Glycosides / Q000737:chemistry*; D006838:Hydrocarbons / Q000737:chemistry; D008956:Models, Chemical; D010714:Phosphatidylethanolamines / Q000737:chemistry*; D013696:Temperature; D014961:X-Ray Diffraction
D005227:Fatty Acids; D005782:Gels; D006017:Glycolipids; D006027:Glycosides; D006838:Hydrocarbons; D010714:Phosphatidylethanolamines; D005990:Glycerol