Physical behavior of lipase substrates.
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Carboxylic Ester Hydrolases↗
Biomedical subjects
D M Small
Publications and source records attributed to D M Small.
Behavior of tetrahydrolipstatin in biological model membranes and emulsions.
Tetrahydrolipstatin (orlistat) (S)-1-[(2S,3S)-3-hexyl-4-oxooxetan-2-yl]methyl]dodecyl N-formyl-L-leucinate, a potent inhibitor of pancreatic lipase, is hydrophobic, amphipathic, and water-insoluble. It binds irreversibly to pancreatic lipases and inhibits fat absorption. The focus of this investigation is on the distribution of orlistat in emulsified fat and vesicular membranes such as might be present in the intestine during fat absorption. The models used were unilamellar vesicles and microemulsion particles. [13C]orlistat was synthesized containing 99% 13C in the leucine carbonyl. Spectrawere collected on a Bruker DMX 500 Spectrometer. The chemical shift of the [13C]leucinate carbon was recorded in solvents with increasing hydrogen bonding capacity. The chemical shift moved downfield as H-bonding increased. [13C]orlistat was incorporated into triolein in the presence or absence of water, into sonocated unilamellar egg yolk phosphotidylcholine (EYPC) vesicles, and into microemulsions approximately 300 A in diameter containing triolein and phospholipid in roughly equal molar proportions. [13C] orlistat was soluble in triolein and had a chemical shift at 20 degrees C of 171.46 ppm. When a small amount of water was added, the chemical shift moved down field to 171.69 ppm. When [13C]orlistat was incorporated into EYPC unilamellar vesicles, the chemical shift increased to approximately 172.0 ppm at 25 degrees C, indicating an orientation of [13C]leucinate in orlistat closer to the aqueous interface of vesicles, i.e., more surface oriented. In all systems there was a modest downfield increase in chemical shift as the temperature was raised from 5 degrees to 46 degrees C. When small amounts of [13C]orlistat (1% relative to the emulsion mass) were incorporated into microemulsions, the chemical shift was identical to that in the unilamellar vesicles indicating a surface-like orientation of [13C]orlistat. However, when 3% was incorporated, two peaks appeared, one related to the surface at about 172 ppm, and one related to the core at about 171.65 ppm. Thus, orlistat first partitions into the surface and then when the surface is saturated, it moves into the more hydrophobic core. The fact that the two pools can be resolved using 13C NMR spectroscopy indicates a modestly slow exchange between the core and surface pools. Thus, the potent lipase inhibitor orlistat is ideally situated in the surface layer of emulsion particles and membranes for interaction with enzymes that superficially bind to such surfaces.
Emulsions↗
Calorimetry of apolipoprotein-A1 binding to phosphatidylcholine-triolein-cholesterol emulsions.
The thermotropic properties of triolein-rich, low-cholesterol dipalmitoyl phosphatidylcholine (DPPC) emulsion particles with well-defined chemical compositions (approximately 88% triolein, 1% cholesterol, 11% diacyl phosphatidylcholine) and particle size distributions (mean diameter, approximately 1000-1100 A) were studied in the absence and presence of apolipoprotein-A1 by a combination of differential scanning and titration calorimetry. The results are compared to egg yolk PC emulsions of similar composition and size. Isothermal titration calorimetry at 30 degrees C was used to saturate the emulsion surface with apo-A1 and rapidly quantitate the binding constants (affinity Ka = 11.1 +/- 3.5 x 10(6) M-1 and capacity N = 1.0 +/- 0.09 apo-A1 per 1000 DPPC) and heats of binding (enthalpy H = -940 +/- 35 kcal mol-1 apo-A1 or -0.92 +/- 0.12 kcal mol-1 DPPC). The entropy of association is -3070 cal deg-1 mol-1 protein or -3 cal deg-1 mol-1 DPPC. Without protein on the surface, the differential scanning calorimetry heating curve of the emulsion showed three endothermic transitions at 24.3 degrees C, 33.0 degrees C, and 40.0 degrees C with a combined enthalpy of 1.53 +/- 0.2 kcal mol-1 DPPC. With apo-A1 on the surface, the heating curve showed the three transitions more clearly, in particular, the second transition became more prominent by significant increases in both the calorimetric and Van't Hoff enthalpies. The combined enthalpy was 2.70 +/- 0.12 kcal mol-1 DPPC and remained constant upon repeated heating and cooling. Indicating that the newly formed DPPC emulsion-Apo-A1 complex is thermally reversible during calorimetry. Thus there is an increase in delta H of 1.17 kcal mol-1 DPPC after apo-A1 is bound, which is roughly balanced by the heat released during binding (-0.92 kcal) of apo-A1. The melting entropy increase, +3.8 cal deg-1 mol-1 DPPC of the three transitions after apo-A1 binds, also roughly balances the entropy (-3 cal deg-1 mol-1 DPPC) of association of apo-A1. These changes indicate that apo-A1 increases the amount of ordered gel-like phase on the surface of DPPC emulsions when added at 30 degrees C. From the stoichiometry of the emulsions we calculate that the mean area of DPPC at the triolein/DPPC interface is 54.5 A2 at 41 degrees C and 54.2 A2 at 30 degrees C. The binding of apo-A1 at 30 degrees C to the emulsion reduces the surface area per DPPC molecule from 54.2 A2 to 50.8 A2. At 30 degrees apo-A1 binds with high affinity and low capacity to the surface of DPPC emulsions and increases the packing density of the lipid domain to which it binds. Apo-A1 was also titrated onto DPPC emulsions at 45 degrees C. This temperature is above the gel liquid crystal transition. No heat was released or adsorbed. Furthermore, egg yolk phosphatidylcholine emulsions of nearly identical composition were also titrated at 30 degrees C with apo-A1 and were euthermic. Association constants were previously measured using a classical centrifugation assay and were used to calculate the entropy of apo-A1 binding (+28 cal deg-1 mol-1 apo-A1). This value indicates that apo-A1 binding to a fluid surface like egg yolk phosphatidylcholine or probably DPPC at 45 degrees C is hydrophobic and is consistent with hydrocarbon lipid or protein moities coming together and excluding water. Thus the binding of apo-A1 to partly crystalline surfaces is entropically negative and increases the order of the already partly ordered phases, whereas binding to liquid surfaces is mainly an entropically driven hydrophobic process.
1,2-Dipalmitoylphosphatidylcholine↗
Switch blades, straight performers and distortion by enzymes.
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Animals↗
Monoacylglycerols alter the lipid composition and molecular mobility of phosphatidylcholine bilayers: 13C NMR evidence of dynamic lipid remodeling.
The physical effects of monoacylglycerols (MAG) in small unilamellar vesicles composed of phosphatidyl-choline (PC), triolein, cholesterol, and varying amounts of monopalmitin and monoolein were studied by 13C-NMR. The signal to noise ratio of the carbonyls of PC and triolein were enhanced by the addition of 1,2-di-[1-13C]palmitoylphosphatidylcholine and tri-[1-13C]oleoylglycerol. The linewidths of the carbonyl-13C, choline methyl, olefinic carbon, and terminal methyl resonances were measured digitally from vesicles with 0 to 42 mol % of MAG. Significant increases in the linewidth of carbonyl (P < 0.05), olefinic and terminal methyl carbons (P < 0.01) of vesicles containing 42 mol % monopalmitin indicated that these groups experienced restricted molecular mobility at high monopalmitin concentrations. However, more striking was the apparent displacement of triolein from the surface environment of PC bilayers to an oil-like environment in systems containing only 8 mol % monopalmitin. Displacement of triolein from the surface by monoolein occurred only above 15 mol %. Thus, saturated and monounsaturated monoacylglycerols, natural products of lipoprotein metabolism, dynamically alter both the lipid composition and molecular mobility of lipoprotein surfaces in distinct ways.
Chemical Phenomena↗
Differences in LDL subspecies involve alterations in lipid composition and conformational changes in apolipoprotein B.
In order to investigate causes of variability in low density lipoprotein (LDL) particle size, we have assessed LDL composition in plasma from 66 subjects, each with a single LDL band, by 2-16% gradient gel electrophoresis, with a total of eight discrete sizes (LDL-1 to LDL-8). Lipoprotein concentrations were analyzed by standard methods; specific proteins were assessed by immunoassay and electrophoresis. Results showed decreased anhydrous molecular weight with size (2.67 +/- 0.07 x 10(6) to 1.78 +/- 0.19 x 10(6)), along with decreased relative content for cholesteryl ester (41.5% to 24.3%), free cholesterol (10.1% to 4.6%), and phospholipid (23.7% to 18.9%), and increased triglyceride (4.1% to 21.0%) and protein (20.5% to 31.2%) content. As LDL size decreased, the ratio of surface cholesterol to phospholipid decreased from 0.53 to 0.29, and the fraction of surface area covered by lipid decreased from 0.74 to 0.47. Moreover, core volume decreased with size from 24.2 A3 x 10(5) to 15.9 A3 x 10(5), and the ratio of surface-to-core lipids fell from 0.59 to 0.46. Based on surface pressures of 30 mN/m, the area covered by surface lipid was calculated to range from 6.45 A2 x 10(4) in the largest LDL, to 3.10 A2 x 10(4) in the smallest. Computer modeling indicates that alterations in the tertiary structure of apoB-100 are required to account for surface changes. The estimated core surface area requiring coverage by apoB increased with decreasing particle size from 2.26 A2 x 10(4) to 3.46 A2 x 10(4). To accommodate coverage of increasing relative surface area associated with decreasing size, apoB thickness at the interface was calculated to decrease from approximately 25 A to 16 A. Such conformational changes in apoB may alter exposed epitopes, possibly causing changes in LDL receptor binding affinity and resistance to oxidation.
Adult↗
Physical behavior of the hydrophobic core of membranes: properties of 1-stearoyl-2-linoleoyl-sn-glycerol.
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.
Calorimetry, Differential Scanning↗
Murine mammary-derived cells secrete the N-terminal 41% of human apolipoprotein B on high density lipoprotein-sized lipoproteins containing a triacylglycerol-rich core.
The cDNA encoding the N-terminal 41% of human apolipoprotein B (apoB), apoB-41, was transfected into nonhepatic, nonintestinal, mammary-derived mouse cells (C127) to generate stably transfected cells expressing human apoB-41 (C127B-41). As determined by centrifugation, apoB-41 is secreted exclusively on lipoproteins (LPs) having a peak density of 1.13 g/ml. Electron microscopy of apoB-41-containing LPs purified by immunoaffinity chromatography showed round particles about 12 nm in diameter. No discoidal particles were observed. Characterization of apoB-41-associated lipids after labeling C127B-41 cells with [3H]oleate and immunoprecipitating the secreted LPs with antibodies to apoB showed that 3H-labeled triacylglycerols were a major lipid class and accounted for about 54% of the total labeled lipids. Cholesterol esters and phospholipids accounted for about 6% and 22%, respectively. Incubation of cells with 0.4 mM oleate resulted in an increased incorporation of the added oleate into lipids associated with secreted apoB-41, along with a 2- to 3-fold increased secretion of apoB-41. The newly formed LPs appear to be transported through the Golgi complex, as brefeldin A (1 microgram/ml) and monensin (1 microM) greatly reduced (> 90%) the secretion of labeled apoB-41 and the amount of triacylglycerol and phospholipid associated with it. Microsomal triacylglycerol transfer protein (MTP) was not detected in these cells. Taken together, the data presented demonstrate that apoB-41 can direct the assembly and secretion of LPs that contain a triacylglycerol-rich core in nonhepatic cells that apparently lack MTP. These cells, therefore, represent an important model for studying LP assembly and may offer some advantages over cultured hepatic or intestinal cells that express their endogenous apoB gene.
Animals↗
Mucin-vesicle interactions in model bile: evidence for vesicle aggregation and fusion before cholesterol crystal formation.
Nucleation of cholesterol monohydrate crystals from bile is a critical step in the formation of cholesterol gallstones. Measurement of nucleation in model bile system and the characteristics of the initial nucleus have proven elusive. In this study we have used three separate physical chemical techniques to examine vesicle aggregation and fusion, including dynamic light scattering (DLS), transmission electron microscopy (TEM), and fluorescent biochemical assays. These assays enabled us to quantify the effect of biliary proteins, such as gallbladder mucin, on vesicle fusion and aggregation. In the absence of mucin, fusion is a relatively slow process occurring over 24 hours, whereas physiological concentrations of mucin are able to accelerate almost complete fusion of vesicles within 6 hours. Vesicle fusion and aggregation as characterized by TEM result in the formation of aggregates of multilamellar vesicles and giant fusion bodies associated with a background of mucin. These mucin-vesicle aggregate bodies may represent true nuclei and precede cholesterol monohydrate crystal nucleation. In future studies, these vesicle fusion assays can be used to quantitatively examine the effect of putative pro- and anti-nucleating proteins on the earliest steps of cholesterol crystal nucleation.
Animals↗
Crystal structure of a mixed chain diacylglycerol, 1-stearoyl-3-oleyl-glycerol.
Diacylglycerols composed of one saturated and one unsaturated chain make up the hydrophobic core of many biological membranes. We report here the first crystalline structure of such a mixed chain diacylglycerol. The mixed chain diacylglycerol, 1-stearoyl-3-oleyl-glycerol (1,3-SODG) was produced by solution isomerization of 1-stearoyl-2-oleyl-sn-glycerol. 1,3-SODG was isolated by flash chromatography and crystallized by slow evaporation in ethyl acetate at 4 degrees C. The melting point was 42.5 degrees C and the enthalpy was 18.0 kcal/mol. The crystal structure was determined to a final R factor of 0.127. Four molecules are present in the monoclinic unit cell: space group Cc, a = 9.362(2), b = 5.495(2), c = 77.92(3)A, beta = 91.46(2), V = 4007(4)A3, Z = 4, D = 1.032 g/cm3. The molecule forms an extended V-shaped conformation with the oleate and stearate chains coming off the two ends of the glycerol with an angle between their planes of 94 degrees. The two chains pack separately in individual layers and do not interact. The hydrogen bonds between the free hydroxyl group on the glycerol-2 position and the carbonyl oxygen on the oleyl chain of an adjacent molecule are 2.78 A in length and stabilize the glycerol layers. The stearoyl chain is roughly straight and packed in a triclinic parallel subcell. Both portions of the oleyl chain also pack in triclinic parallel packing. The torsion angle sequence along the double bond extending from the oleyl carbons C7 to C13 is tscsst (173 degrees, -152 degrees, -17 degrees, -157 degrees, -163 degrees, -178 degrees). This sequence is different from other monounsaturates. The torsion angle sequence around the glycerol region shows that the glycerol conformation is quite similar to the A conformer of racemic alpha monolaurin, and to 1,3-di-11-bromoundecanoyl glycerol, but completely unlike 1,2-diacyl-sn-glycerols. Thus the glycerol conformation appears to be driven by the hydrogen bond formation, which in turn determines whether chains interact or are segregated. In 1,3-diacylglycerols the two acyl chains point in different directions and are segregated. In 1,2-diacylglycerols the acyl chains lie side by side and must interact. When the two chains are quite different, then serious problems in packing occur, giving rise to disordered crystal packing. Probably as a result of the disordered chain packing in 1-stearoyl-2-oleyl-sn-glycerol (Di, L. and D.M. Small. 1993. J. Lipid Res. 34: 1611-1623) we were not successful in growing adequate crystals suitable for crystallographic structure determination of this 1,2 mixed chain diacylglycerol.
Diglycerides↗
Mechanical properties of model atherosclerotic lesion lipid pools.
Structural analysis of atherosclerotic coronary arteries has suggested that stress concentrations are associated with plaque rupture and that these stress concentrations are critically dependent on the geometry and mechanical properties of the fibrous cap and lipid pool. Recent clinical trials of lipid-lowering therapy have shown a significant reduction in cardiac events associated with plaque rupture perhaps because of the changing composition of subintimal lipid pools. To test the hypothesis that changes in lipid composition can change the mechanical properties of lipid pools, we measured the dynamic shear moduli of combinations of cholesterol monohydrate crystals, phospholipids, and triglycerides similar to those found in atherosclerotic lesions. Increasing the cholesterol monohydrate concentration from 0% to 50% increased the real component of the dynamic shear modulus (storage modulus or stiffness) by 4.5 times at a frequency of 1 Hz (P < .001). All specimens demonstrated an increase in stiffness with increasing frequencies of stress ranging from 0.1 to 3 Hz. We conclude that the stiffness of model atherosclerotic plaque lipid pools is related to the concentration of cholesterol monohydrate crystals. Because the relative concentration of cholesterol monohydrate increases during early regression of experimental atherosclerosis, the resultant stiffening of the lipid pool may reduce stresses in plaque caps. However, the magnitude of the contribution of changing lipid stiffness to the reduction of cardiac events seen in clinical studies is unclear.
Arteriosclerosis↗
Effects of side chain length on ionization behavior and transbilayer transport of unconjugated dihydroxy bile acids: a comparison of nor-chenodeoxycholic acid and chenodeoxycholic acid.
13C-NMR spectroscopy was used to examine the effect of side chain length on the ionization properties and transmembrane transport rate of 3 alpha,7 alpha-dihydroxy bile acids. When solubilized in taurocholate micelles, [23-13C]nor-chenodeoxycholic acid (nor-CDCA) had a pKa of 6.1, similar to that of CDCA (pKa 6.2), its C24 homologue. In unilamellar phosphatidylcholine vesicles, the pKa of nor-CDCA was 7.0, whereas that of CDCA was 6.6. Lineshape analysis indicated that the rate of ionization of nor-CDCA as a micellar solute or as a vesicle component was very slow (0.4 x 10(5) sec-1) compared to that of acetic acid in water (8.7 x 10(5) sec-1). Lineshape analysis of spectra of the protonated form of nor-CDCA at acidic bulk pH indicated that the transbilayer transport rate of nor-CDCA (580 sec-1) was six times faster than that of CDCA (100 sec-1). It is proposed that the shorter side chain of the nor-CDCA molecule causes it to reside more deeply inside the vesicle bilayer than CDCA, explaining its weaker ionization and more rapid flip-flop rate. These in vitro experiments imply that, in vivo, a given C23 nor-dihydroxy bile acid will ionize less readily when present in membranes, and it will also flip-flop faster than its C24 homologue.
Biological Transport↗
Formation of cholesterol monohydrate crystals in macrophage-derived foam cells.
In a previous study using the J774 macrophage foam cells, we quantitated the accumulation of unesterified (free) cholesterol derived from cholesteryl ester hydrolysis in lysosomes, after phagocytic uptake of cholesteryl ester droplets. In the present study, we examined whether the accumulation of free cholesterol in lysosomes leads to the formation of cholesterol monohydrate crystals by analyzing the lipid composition of low density lysosome fractions isolated from cholesteryl ester-loaded macrophages after a 24-h incubation. Phase diagrams of the constituent lipids in the lipid-filled lysosomes predicted the formation of cholesterol monohydrate crystals. The formation of cholesterol monohydrate crystals was observed in cholesteryl ester-loaded macrophages after a 48-h incubation by polarizing light microscopy. The crystals had a density of 1.04 g/ml and the morphology of cholesterol monohydrate crystals with an acute edge angle of about 80 degrees. The crystals appeared as needles as well as plates and melted only when heated to greater than 85 degrees C. The physical properties of these crystals are characteristic of cholesterol monohydrate. In our studies, crystal formation was observed even when cells had active acyl-CoA:cholesterol acyltransferase or when cholesterol efflux was stimulated. Electron microscopy and acid phosphatase cytochemistry of lysosomes in cholesteryl ester-loaded cells confirmed that cholesterol crystal formation occurred within lipid-loaded lysosomes. Time-lapse video microscopic studies revealed that most of the cells containing cholesterol monohydrate crystals not only remain viable but also have the capacity to translocate single crystals within cells. The data demonstrate that lysosomal accumulation of free cholesterol in macrophages after phagocytic uptake and hydrolysis of cholesteryl ester droplets leads to the formation of cholesterol monohydrate crystals within lipid-filled lysosomes. Such a process may lead to deposition of free cholesterol and cholesterol monohydrate crystals in macrophage foam cells during the progression of atherosclerosis.
Animals↗
Bovine gallbladder mucin accelerates cholesterol monohydrate crystal growth in model bile.
BACKGROUND: Gallbladder mucin accelerates cholesterol crystal nucleation, an early step in the pathogenesis of gallstones. To examine the role of gallbladder mucin in postnucleation gallstone maturation, the influence of mucin on cholesterol monohydrate crystal growth was studied in a novel model system. METHODS: Cholesterol crystals of a uniform size were incubated in model biles at 37 degrees C with varying cholesterol saturation indices. Crystal size was quantitated by measuring the width and length of individual crystals under polarizing light microscopy and calculating average crystal area. RESULTS: Crystal growth was dependent on the degree of cholesterol supersaturation of bile. Bovine gallbladder mucin (0.5-8 mg/mL) accelerated crystal growth in supersaturated model bile in a concentration- and time-dependent fashion compared with control incubations with bovine serum albumin or model bile alone (P < 0.05). Cholesterol crystal growth was accompanied by a progressive decrease in cholesterol saturation and an increase in total cholesterol crystal mass. Crystal growth was also accompanied by a decrease in total crystal number, suggesting net transfer of cholesterol to larger crystals. CONCLUSIONS: The acceleration of cholesterol crystal growth by gallbladder mucin may be of pathophysiological importance in the postnucleation maturation of cholesterol gallstones.
Animals↗
Physical behavior of the mixed chain diacylglycerol, 1-stearoyl-2-oleoyl-sn-glycerol: difficulties in chain packing produced marked polymorphism.
Diacylglycerols (DAG) play an important role in metabolism, signal transduction and protein kinase activation. Naturally occurring DAGs usually contain a saturated chain in the 1-position and an unsaturated chain in the 2-position. We have investigated the physical behavior of 1-stearoyl-2-oleoyl-sn-glycerol (sn-SODG) both in the dry and hydrated states by means of differential scanning calorimetry, X-ray diffraction, and NMR. In the dry state the saturated stearate and unsaturated oleate chains have difficulties in packing. As a result marked polymorphism occurs as the chains try to find a suitable packing. Eight phases were found in the dry state: alpha (transition temperature = 16.4 degrees C; delta H = 6.8 kcal/mol); beta 4 (20.7 degrees C; 13.8); beta 3 (21.5 degrees C; 13.8); beta 2 (22.2 degrees C; 14.4); beta 1 (23.1 degrees C; 12.3); beta' (25.7 degrees C; 11.9); gamma 1 (-2.9 degrees C; 0.5), and gamma 2 (-5.9 degrees C; 1.2), all of relatively low stability compared to 1,2 distearoyl-sn-glycerol (beta', 77.2 degrees C; 30.6). gamma 1 and gamma 2 are metastable low temperature phases. beta 1-beta 4 are bilayers (d001 = 3.4, 43.4, 44.7, 46.1 A, respectively) with elements of triclinic parallel chain packing, while beta' is a bilayer (d001 = 47.1 A) with orthorhombic perpendicular chain packing. The metastable alpha phase has hexagonal chain packing and an unusual eight-layer structure (d001 = 174 A). Hydrated sn-SODG contains about one-half of a water molecule per diacylglycerol. Three phases can be distinguished gamma w, alpha w (15.1 degrees C; 6.7) and beta w (19.9 degrees C, 14.3). Both alpha w and beta w are bilayers but alpha w has hexagonal chain packing and beta w is predominantly triclinic parallel packing. Thus, when saturated and unsaturated chains must pack side by side, complex chain conformation, disorder, and instability result giving rise to marked polymorphism. Hydration appears to partly stabilize the interactions.
Calorimetry, Differential Scanning↗
1992 Aspen Bile Acid/Cholesterol/Lipoprotein Conference. Report of a conference.
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Animals↗
Single crystal structure of a mixed-chain triacylglycerol: 1,2-dipalmitoyl-3-acetyl-sn-glycerol.
The mixed chain triacylglycerol 1,2-dipalmitoyl-3-acetyl-sn-glycerol was synthesized and its crystal structure was determined to a final reliability factor (R) of 0.11. Two molecules are present in the monoclinic unit cell: space group P2(1); a = 5.375(1), b = 8.286(2), c = 42.96(1) A; beta = 93.30(2) degrees, V = 1910 A3, rho = 1.065 g/cm3, and mu = 5.7 cm-1. The structure is a trilayer: a bilayer of palmitate chains packed in the beta mode (T parallel) and an interdigitated monolayer of acetates. The glycerol backbone and acetate extend roughly linearly from the sn-1 chain. The sn-2 chain bends around the C-2 carbon to lie next to the sn-1 chain. Analysis of the torsion angles indicate that the glycerol conformation of 1,2-dipalmitoyl-3-acetyl-sn-glycerol is markedly different from single acid triacylglycerols and from 1,2-diacyl-sn-glycerols but very similar to 1,2-dimyristoyl-sn-glycero-3-phosphocholine.
Crystallography↗
Do dietary fatty acids affect the biophysics of platelet-membrane function in thrombosis?
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Animals↗