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-α-D-glucosyl glycerols

Abstract.

The lamellar gel to lamellar liquid-crystalline (L_β/L_α) and lamellar liquid-crystalline to inverted hexagonal (L_α/H_II) phase transitions of a number of phosphatidylethanolamines (PEs) and diacyl-α-D-glucosyl-sn-glycerols (α-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 α-D-GlcDAGs, the L_β/L_α phase transition temperatures (T _m) increase and the L_α/H_II phase transition temperatures (T _h) decrease with increases in hydrocarbon chain length. The T _m and the especially the T _h values for the PEs are higher than those of the corresponding α-D-GlcDAGs. For PEs having the same effective hydrocarbon chain length but different chain configurations, the T _m and T _h values vary markedly but with an almost constant temperature interval (ΔT _L/NL) between the two phase transitions. Moreover, although the T _m and T _h values of the PEs and α-D-GlcDAGs are equally sensitive on the temperature scale to variations in the length and chemical configuration of the hydrocarbon chains, the ΔT _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 α-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 L_α phase is stable relative to the corresponding regions in the α-D-GlcDAGs. Overall, these findings indicate that the PEs have a smaller propensity to form the H_II phase than do the α-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 α-D-GlcDAGs) in both the L_α 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 L_α and H_II phases, but those changes are consistent between the PEs and α-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.