The thermotropic phase behaviour and phase structure of a homologous series of racemic β-d-galactosyl dialkylglycerols studied by differential scanning calorimetry and X-ray diffraction

The thermotropic phase behaviour of aqueous dispersions of some synthetic 1,2-di-O-alkyl-3-O-(β-d-galactosyl)-rac-glycerols (rac-β-d-GalDAGs) with both odd and even hydrocarbon chain lengths was studied by differential scanning calorimetry (DSC), small-angle (SAXS) and wide-angle (WAXS) X-ray diffraction. DSC heating curves show a complex pattern of lamellar (L) and nonlamellar (NL) phase polymorphism dependent on the sample's thermal history. On cooling from 95 °C and immediate reheating, rac-β-d-GalDAGs typically show a single, strongly energetic phase transition, corresponding to either a lamellar gel/liquid-crystalline (Lβ/Lα) phase transition (N ≤ 15 carbon atoms) or a lamellar gel/inverted hexagonal (Lβ/HII) phase transition (N ≥ 16). At higher temperatures, some shorter chain compounds (N = 10–13) exhibit additional endothermic phase transitions, identified as L/NL phase transitions using SAXS/WAXS. The NL morphology and the number of associated intermediate transitions vary with hydrocarbon chain length. Typically, at temperatures just above the Lα phase boundary, a region of phase coexistence consisting of two inverted cubic (QII) phases are observed. The space group of the cubic phase seen on initial heating has not been determined; however, on further heating, this QII phase disappears, enabling the identification of the second QII phase as Pn3¯m (space group Q224). Only the Pn3¯m phase is seen on cooling.Under suitable annealing conditions, rac-β-d-GalDAGs rapidly form highly ordered lamellar-crystalline (Lc) phases at temperatures above (N ≤ 15) or below (N = 16–18) the Lβ/Lα phase transition temperature (Tm). In the N ≤ 15 chain length lipids, DSC heating curves show two overlapping, highly energetic, endothermic peaks on heating above Tm; corresponding changes in the first-order spacings are observed by SAXS, accompanied by two different, complex patterns of reflections in the WAXS region. The WAXS data show that there is a difference in hydrocarbon chain packing, but no difference in bilayer dimensions or hydrocarbon chain tilt for these two Lc phases (termed Lc1 and Lc2, respectively). Continued heating of suitably annealed, shorter chain rac-β-d-GalDAGs from the Lc2 phase results in a phase transition to an Lα phase and, on further heating, to the same QII or HII phases observed on first heating.On reheating annealed samples with longer chain lengths, a subgel phase is formed. This is characterized by a single, poorly energetic endotherm visible below the Tm. SAXS/WAXS identifies this event as an Lc/Lβ phase transition. However, the WAXS reflections in the di-16:0 lipid do not entirely correspond to the reflections seen for either the Lc1 or Lc2 phases present in the shorter chain rac-β-d-GalDAGs; rather these consist of a combination of Lc1, Lc2 and Lβ reflections, consistent with DSC data where all three phase transitions occur within a span of 5 °C. At very long chain lengths (N ≥ 19), the Lβ/Lc conversion process is so slow that no Lc phases are formed over the time scale of our experiments. The Lβ/Lc phase conversion process is significantly faster than that seen in the corresponding rac-β-d-GlcDAGs, but is slower than in the 1,2-sn-β-d-GalDAGs already studied. The Lα/NL phase transition temperatures are also higher in the rac-β-d-GalDAGs than in the corresponding rac-β-d-GlcDAGs, suggesting that the orientation of the hydroxyl at position 4 and the chirality of the glycerol molecule in the lipid/water interface influence both the Lc and NL phase properties of these lipids, probably by controlling the relative positions of hydrogen bond donors and acceptors in the polar region of the membrane.