PubMed · 8092464
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D Dollimore. 1994-06-15. Thermal analysis.. https://doi.org/10.1021/ac00084a002
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Phospholipids containing a saturated fatty acid in the primary position and an unsaturated fatty acid in the secondary position are a major structural part of biological membranes. The mixed-chain hydrophobic core of the membranes is the diacylglycerol part. To better understand the core properties of membranes we have studied the physical behavior of 1-stearoyl-2-linoleoyl-sn-glycerol (SLDG) by X-ray diffraction and differential scanning calorimetry (DSC) in the dry and hydrated states. Dry SLDG has four polymorphic phases: alpha (transition temperature, 11.6 degrees C; delta H = 7.5 kcal/mol); sub-alpha 1 (3.0 degrees C; 0.6 kcal/mol); sub-alpha 2(-1.0 degrees C; 0.5 kcal/mol); and beta' (16.1 degrees C; 15.4 kcal/mol). The alpha, sub-alpha 1, and sub-alpha 2 phases are metastable with a probable extended bilayer structure (d001 approximately 59.5 A). The chain packing of the alpha phase is hexagonal, while sub-alpha 1 and sub-alpha 2 have pseudohexagonal chain packing. The beta' phase has a tilted bilayer structure (46.9 A) with strong wide-angle diffractions, suggesting elements of orthorhombic perpendicular packing. Compared to saturated 1,2-diacylglycerols, SLDG packs much less efficiently, but, when compared to 1-stearoyl-2-oleoyl-sn-glycerol, it appears to pack somewhat more efficiently. Thus polyunsaturated linoleate chains appear to pack marginally more effectively with the saturated stearate chains than do monounsaturated chains. SLDG hydrates with 0.5 mol of H2O, which prevents the beta' phase from forming. Only one hydrated alpha phase (alpha w) and two hydrated sub-alpha (sub-alpha w1, sub-alpha w2) phases are formed. These phases are similar in structure to the nonhydrated alpha phases, but the bilayer period is increased by about 2 A (d001 approximately 61.5 A). This causes minor changes in polymorphism, including lower melting temperatures and enthalpy. A comparison of diacylglycerols to phosphatidylcholines with the same chains shows that the addition of a strong polar group (e.g., phosphocholine) to the free hydroxyl of the glycerol depresses chain melting and prevents the more efficient packing of the SLDG core of the bilayer.
Differential scanning calorimetry (DSC) and X-ray diffraction have been used to study the structure and properties of N-palmitoleoylgalactosylsphingosine (NPoGS; 16:1 galactocerebroside). DSC of fully hydrated NPoGS shows a complex pattern of three endothermic transitions at 35, 39 and 53 degrees C. Using a combination of thermal protocols (varying heating/cooling rates, incubation at different temperatures, etc.), the three ordered chain (gel) phases responsible for the transitions have been isolated; transition I (Tm = 35 degrees C; delta H(I) = 6.3 kcal/mol), transition II (Tm = 39 degrees C; delta HII = 8.6 kcal/mol), and transition III (Tm = 53 degrees C; delta HIII = 12.8 kcal/mol). The gel phases do not interconvert but rather form independently following cooling from the melted chain phase. X-ray diffraction data of the three isolated phases confirm that they all are bilayer structures with different bilayer periodicities (LI, 50.7 A; LII, 51.7 A; LIII = 49.2 A) and different chain packing modes. The LI, LII, and LIII bilayer phases each melt independently to the melted chain L alpha phase. Comparisons with other cerebrosides make it clear that alterations in chain length and chain unsaturation markedly affect the thermotropic behavior of cerebrosides and the metastable and stable phases they are able to form. As with phospholipids, introduction of cis-unsaturation into the N-acyl chain reduces both the chain melting temperature and enthalpy.
Differential scanning calorimetry has been used to investigate the effects of vitamin D3 on the physical properties of model membranes including pure phosphatidylcholines (PC's) of chain length from 14 to 18, pure dipalmitoylphosphatidylethanolamine (DPPE), and various mixtures of these lipids. The results demonstrate that the interactions of vitamin D3 with PC's are dependent on acyl chain lengths. It was found that vitamin D3 reduces the transition temperatures of PC's and PE's, broadening the transition and reducing the enthalpy, eventually abolishing the transition. The interaction of vitamin D3 with PC varies with chain length; the transition of DMPC is abolished by only 15 mol% vitamin D3 whereas 45 mol% vitamin D3 is required to abolish the transition of DSPC. These variations in vitamin D3 lipid interactions are further explored in various mixtures. In the mixture studies it is shown that vitamin D3 affects the mixing properties of the lipids in the mixtures. The results suggest that the presence of vitamin D3 in lipids can affect the lateral phase distributions of lipids, and thereby have important effects on membrane function.