The physical properties of glycosyl diacylglycerols. Calorimetric, X-ray diffraction and Fourier transform spectroscopic studies of a homologous series of 1,2-di-O-acyl-3-O-(beta-D-galactopyranosyl)-sn-glycerols

We have synthesized a homologous series of saturated 1,2-di-O-n-acyl-3-O-(b eta -D-galactopyranosyl)-sn-glycerols with odd- and even-numbered hydrocarb on chains ranging in length from 10 to 20 carbon atoms, and have investigat ed their physical properties using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy . The DSC results show a complex pattern of phase behaviour, which in a typ ical preheated sample consists of a lower temperature, moderately energetic lamellar gel/lamellar liquid-crystalline (L-beta/L-alpha) phase transition and a higher temperature, weakly energetic lamellar/nonlamellar phase tran sition. On annealing at a suitable temperature below the L-beta/L-alpha. ph ase transition, the L-beta phase converts to a lamellar crystalline (L-c1) phase which may undergo a highly energetic L-c1/L-alpha or L-c1/inverted he xagonal (H-II) phase transition at very high temperatures on subsequent hea ting or convert to a second L-c2 phase in certain long chain compounds on s torage at or below 4 degreesC. The transition temperatures and phase assign ments for these galactolipids are supported by our XRD and FTIR spectroscop ic measurements. The phase transition temperatures of all of these events a re higher than those of the comparable phase transitions exhibited by the c orresponding diacyl alpha- and beta -D-glucosyl glycerols. In contrast, the L-beta/L-alpha and lamellar/nonlamellar phase transition temperatures of t he beta -D-galactosyl glycerols are lower than those of the corresponding d iacyl phosphatidylethanolamines (PEs) and these glycolipids form inverted c ubic phases at temperatures between the lamellar and H-II phase regions. Ou r FTIR measurements indicate that in the L-beta phase, the hydrocarbon chai ns form a hexagonally packed structure in which the head-roup and interfaci al region are undergoing rapid motion, whereas the, L-c phase consists of a more highly ordered, hydrogen-bonded phase, in which the chains are packed in an orthorhombic subcell similar to that reported for the diacyl-beta -D -glucosyl-sn-glycerols. A comparison of the DSC data presented here with ou r earlier studies of other diacyl glycolipids shows that the rate of conver sion from the L-beta to the L-c phase in the beta -D-galactosyl glycerols i s slightly faster than that seen in the alpha -D-glucosyl glycerols and muc h faster than that seen in the corresponding beta -D-glucosyl glycerols. Th e similarities between the FTIR spectra and the first-order spacings for th e lamellar phases in both the beta -D-glucosyl and galactosyl glycerols sug gest that the headgroup orientations may be similar in both beta -anomers i n all of their lamellar phases. Thus, the differences in their L-beta/L-c c onversion kinetics and the lamellar/nonlamellar phase properties of these l ipids probably arise from subtly different hydration and H-bonding interact ions in the headgroup and interfacial regions of these phases. In the latte r case, such differences would be expected to alter the ability of the pola r headgroup to counterbalance the volume of the hydrocarbon chains. This pe rspective is discussed in the context of the mechanism for the L-infinity/H -II phase transition which we recently proposed, based on our X-ray diffrac tion measurements of a series of PEs. (C) 2001 Elsevier Science Ireland Ltd . All rights reserved.