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-( -D-galactopyranosyl)-sn-glycerols

We have synthesized a homologous series of saturated 1,2-di-O-n-acyl-3-O-( -D-galactopyranosyl)-sn-glycerols with oddand even-numbered hydrocarbon chains ranging in length from 10 to 20 carbon atoms, and have investigated 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 typical preheated sample consists of a lower temperature, moderately energetic lamellar gel/lamellar liquid-crystalline (L /L ) phase transition and a higher temperature, weakly energetic lamellar/nonlamellar phase transition. On annealing at a suitable temperature below the L /L phase transition, the L phase converts to a lamellar crystalline (Lc1) phase which may undergo a highly energetic Lc1/L or Lc1/inverted hexagonal (HII) phase transition at very high temperatures on subsequent heating or convert to a second Lc2 phase in certain long chain compounds on storage at or below 4°C. The transition temperatures and phase assignments for these galactolipids are supported by our XRD and FTIR spectroscopic measurements. The phase transition temperatures of all of these events are higher than those of the comparable phase transitions exhibited by the corresponding diacyl and -D-glucosyl glycerols. In contrast, the L /L and lamellar/nonlamellar phase transition temperatures of the -D-galactosyl glycerols are lower than those www.elsevier.com/locate/chemphyslip Abbre iations: -GlcDAG, -D-glucopyranosyl diacylglycerol; -GalDAG, -D-galactopyranosyl diacylglycerol; -GlcDAG, -Dglucopyranosyl diacylglycerol; DSC, differential scanning calorimetry; FTIR, Fourier transform infrared spectroscopy; HII, inverted hexagonal phase; L , lamellar liquid-crystalline phase; L , lamellar gel phase; Lc, lamellar crystalline phase; L/NL, lamellar/nonlamellar; NMR, nuclear magnetic resonance spectroscopy; PC, phosphatidylcholine; PE, phosphatidylethanolamine; QII, inverted cubic phase; XRD, X-ray diffraction. The fatty acyl chains of the lipids used in this study are described by the shorthand notation N :0, where N denotes the number of carbon atoms per acyl chain with the zero signifying the absence of double bonds. * Corresponding author. Fax: +1-780-4920095. E-mail address: [email protected] (R.N. McElhaney). 1 Present address: Department of Physics and Astronomy, Calvin College, 3201 Burton SE, Grand Rapids, MI 49546, USA. 2 Present address: CHESS/Department of Physics, Cornell University, 162 Clark Hall, Ithaca, NY 14853-2501, USA 0009-3084/01/$ see front matter © 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0009 -3084 (01 )00153 -0 D.A. Mannock et al. / Chemistry and Physics of Lipids 111 (2001) 139–161 140 of the corresponding diacyl phosphatidylethanolamines (PEs) and these glycolipids form inverted cubic phases at temperatures between the lamellar and HII phase regions. Our FTIR measurements indicate that in the L phase, the hydrocarbon chains form a hexagonally packed structure in which the headgroup and interfacial region are undergoing rapid motion, whereas the Lc 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-D-glucosyl-sn-glycerols. A comparison of the DSC data presented here with our earlier studies of other diacyl glycolipids shows that the rate of conversion from the L to the Lc phase in the -D-galactosyl glycerols is slightly faster than that seen in the -D-glucosyl glycerols and much faster than that seen in the corresponding -D-glucosyl glycerols. The similarities between the FTIR spectra and the first-order spacings for the lamellar phases in both the -D-glucosyl and galactosyl glycerols suggest that the headgroup orientations may be similar in both -anomers in all of their lamellar phases. Thus, the differences in their L /Lc conversion kinetics and the lamellar/nonlamellar phase properties of these lipids probably arise from subtly different hydration and H-bonding interactions in the headgroup and interfacial regions of these phases. In the latter case, such differences would be expected to alter the ability of the polar headgroup to counterbalance the volume of the hydrocarbon chains. This perspective is discussed in the context of the mechanism for the L /HII phase transition which we recently proposed, based on our X-ray diffraction measurements of a series of PEs. © 2001 Elsevier Science Ireland Ltd. All rights reserved.