Effect of chain unsaturation on the structure and thermotropic properties of galactocerebrosides.

Differential scanning calorimetry (DSC) and x-ray diffraction have been used to study the effect of increasing chain-unsaturation on the structure and properties of the hydrated cerebrosides N-stearoyl, -oleoyl, and -linoleoyl galactosylsphingosine (NSGS, NOGS, and NLnGS, respectively). DSC of hydrated (70 wt% water) NSGS shows an endothermic transition at 85 degrees C (delta H = 18.0 kcal/mol NSGS) and a broad exothermic transition at 40-60 degrees C, the latter being dependent upon the previous cooling rate. X-Ray diffraction patterns recorded at 21, 61, and 86 degrees C provide evidence for interconversions between metastable and stable crystalline NSGS bilayer phases. The properties of the unsaturated-chain cerebrosides are more complex. Hydrated NOGS shows a single endothermic transition at 44.8 degrees C (delta H = 11.5 kcal/mol NOGS). However, incubation of NOGS at 49 degrees C for 24 h results in a second transition at 55.5 degrees C. By cycling NOGS between 0 and 49 degrees C complete conversion into this higher melting phase (delta H = 12.1 kcal/mol NOGS) is achieved. X-ray diffraction confirms a bilayer phase at all temperatures and delineates the conversions between a crystalline phase at 21 degrees C (bilayer period d = 56.5A), a second crystalline phase at 47 degrees C (d = 69.9A), and a liquid crystalline phase at 59 degrees C (d = 52.0A). The more unsaturated NLnGS shows two transitions, a sharp transition at 28 degrees C (delta H = 8.0 kcal/mol NLGS) and a broad, low-enthalpy transition at 42 degrees C (delta H = 0.4 kcal/mol NLGS). Again, incubation between the two transitions leads to a single transition at 44 degrees C (delta H = 9.3 kcal/mol NLGS). X-ray diffraction demonstrates conversions between two crystalline bilayer phases (d = 55.2A and d = 68.4A), and a liquid crystalline bilayer phase (d = 51.8A). Thus, increased unsaturation in the amide-linked fatty acyl chain of cerebrosides results in decreased chain-melting temperatures (NSGS greater than NOGS greater than NLnGS) and has marked effects on their structural properties.     

Structure and metastability of N-lignocerylgalactosylsphingosine (cerebroside) bilayers

Differential scanning calorimetry (DSC) and X-ray diffraction have been used to study hydrated N-lignocerylgalactosylsphingosine (NLGS) bilayers. DSC of fully hydrated NLGS shows an endothermic transition at 69-70 degrees C, immediately followed by an exothermic transition at 72-73 degrees C; further heating shows a high-temperature (Tc = 82 degrees C), high-enthalpy (delta H = 15.3 kcal/mol NLGS) transition. Heating to 75 degrees C, cooling to 20 degrees C and subsequent reheating shows no transitions at 69-73 degrees C; only the high-temperature (82 degrees C), high-enthalpy (15.3 kcal/mol) transition. Two exothermic transitions are observed on cooling; for the upper transition its temperature (about 65 degrees C) and enthalpy (about 6 kcal/mol NLGS) are essentially independent of cooling rate, whereas the lower transition exhibits marked changes in both temperature (30----60 degrees C) and enthalpy (2.2----9.5 kcal/mol NLGS) as the cooling rate decreases from 40 to 0.625 Cdeg/min. On reheating, the enthalpy of the 69-70 degrees C transition is dependent on the previous cooling rate. The DSC data provide clear evidence of conversions between metastable and stable forms. X-ray diffraction data recorded at 26, 75 and 93 degrees C show clearly that NLGS bilayer phases are present at all temperatures. The X-ray diffraction pattern at 75 degrees C shows a bilayer periodicity d = 65.4 A, and a number of sharp reflections in the wide-angle region indicative of a crystalline chain packing mode. This stable bilayer form converts to a liquid-crystal bilayer phase; at 93 degrees C, the bilayer periodicity d = 59.1 A, and a diffuse reflection at 1/4.6 A-1 is observed. The diffraction pattern at 22 degrees C represents a combination of the stable and metastable low-temperature bilayer forms. NLGS exhibits a complex pattern of thermotropic changes related to conversions between metastable (gel), stable (crystalline) and liquid-crystalline bilayer phases. The structure and thermotropic properties of NLGS are compared with those of hydrated N-palmitoylgalactosylsphingosine reported previously (Ruocco, M.J., Atkinson, D., Small, D.M., Skarjune, R.P., Oldfield, E. and Shipley, G.G. (1981) Biochemistry 20, 5957-5966).     

Tris buffer causes acyl chain interdigitation in phosphatidylglycerol

The structure of the gel phase and the properties of the acyl chain disordering transition of dipalmitoyl phosphatidylglycerol (DPPG) have been studied using differential scanning calorimetry, differential scanning dilatometry, and X-ray diffraction. In the presence of small, monovalent cations, DPPG at 22 degrees C exists in a lamellar phase in which the hydrocarbon chains are tilted from the perpendicular to the bilayer surface. Around 34 degrees C, there is a small pretransition (delta H less than 1 kcal/mol) followed by the main transition at 40.4 degrees C (delta H = 8.3 kcal/mol; delta V = 0.0381 ml/g). If DPPG is suspended in Tris-HCl buffer in the absence of other monovalent cations, X-ray diffraction data show that at 22 degrees C, the gel phase consists of interdigitated acyl chains perpendicular to the plane of the bilayer. No pretransition is observed and the main transition occurs at 41.3 degrees C with delta H = 9.1 kcal/mol and delta V = 0.0514 ml/g. If sufficient Na+ or K+ ions are added to the Tris-buffered DPPG, the phase behavior reverts to what is observed in the absence of Tris. Analysis of the energetics of the main transition shows that the increase in van der Waals interaction energy resulting from the larger delta V in Tris can be compensated by the favorable energetics of removing terminal methyl groups from the bilayer surface. The amount of disordering, i.e. formation of gauche rotamers, is likely to be the same in Tris as it is in buffers without amphiphilic cations.     

Structure and properties of mixed-chain phosphatidylcholine bilayers

The structural and thermotropic properties of the hydrated mixed-chain phosphatidylcholines (PCs), C(8):C(18)-PC and C(10):C(18)-PC, have been studied by X-ray diffraction and differential scanning calorimetry. For fully hydrated C(8):C(18)-PC, the reversible chain melting transition is observed at 9.9 degrees C (delta H = 7.3 kcal/mol). X-ray diffraction at 0 degrees C (below the chain melting transition) shows a small bilayer repeat distance, d = 51.0 A, and a sharp, symmetric wide-angle reflection at 4.1 A, characteristic of a mixed interdigitated bilayer gel phase [see McIntosh, T. J., Simon, S. A., Ellington, J. C., Jr., & Porter, N. A. (1984) Biochemistry 23, 4038-4044; Hui, S. W., Mason, J. T., & Huang, C. (1984) Biochemistry 23, 5570-5577]. At 30 degrees C (above the chain melting transition), a diffuse band is observed at 4.5 A characteristic of an L alpha phase but with an increased bilayer periodicity, d = 61 A. Both the calculated lipid bilayer thickness (d1) and that determined directly from electron density profiles (dp-p) show unusual increases as a consequence of chain melting. In contrast, fully hydrated C(10):C(18)-PC shows an asymmetric endothermic transition at 11.8 degrees C. Below the chain melting transition, two lamellar phases are present, corresponding to coexisting interdigitated (d = 52.3 A) and noninterdigitated (d = 62.5 A) bilayer gel phases. The relative amounts of these phases depend upon the low-temperature incubation and/or hydration conditions, suggesting conversions, albeit kinetically complex, between metastable, and stable phases. The different behavior of C(8):C(18)-PC and C(10):C(18)-PC, as well as their positional isomers, is rationalized in terms of the molecular conformation of PC.     

X-ray diffraction and calorimetric study of N-lignoceryl sphingomyelin membranes.

Differential scanning calorimetry and x-ray diffraction have been used to investigate hydrated multibilayers of N-lignoceryl sphingomyelin (C24:0-SM) in the hydration range 0-75 wt % H2O. Anhydrous C24:0-SM exhibits a single endothermic transition at 81.3 degrees C (delta H = 3.6 kcal/mol). At low hydration (12.1 wt % H2O), three different endothermic transitions are observed: low-temperature transition (T1) at 39.4 degrees C (transition enthalpy (delta H1) = 2.8 kcal/mol), intermediate-temperature transition (T2) at 45.5 degrees C, and high-temperature transition (T3) at 51.3 degrees C (combined transition enthalpy (delta H2 + 3) = 5.03 kcal/mol). On increasing hydration, all three transition temperatures of C24:0-SM decrease slightly to reach limiting values of 36.7 degrees C (T1), 44.4 degrees C (T2), and 48.4 degrees C (T3) at approximately 20 wt % H2O. At 22 degrees C (below T1), x-ray diffraction of C24:0-SM at different hydration levels shows two wide-angle reflections, a sharp one at 1/4.2 A-1 and a more diffuse one at 1/4.0 A-1 together with lamellar reflections corresponding to bilayer periodicities increasing from d = 65.4 A to a limiting value of 71.1 A. Electron density profiles show a constant bilayer thickness dp-p approximately 50 A. In contrast, at 40 degrees C (between T1 and T2) a single sharp wide-angle reflection at approximately 1/4.2 A-1 is observed. The lamellar reflections correspond to a larger bilayer periodicity (increasing from d = 69.3-80.2 A) and there is some increase in dp-p (52-56 A) with hydration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Structure and thermotropic properties of hydrated 1-eicosyl-2-dodecyl-rac-glycero-3-phosphocholine and 1-dodecyl-2-eicosyl-rac-glycero-3-phosphocholine bilayer membranes

The ether-linked phosphatidylcholines 1-eicosyl-2-dodecyl-rac-glycero-3-phosphocholine (EDPC) and 1-dodecyl-2-eicosyl-rac-glycero-3-phosphocholine (DEPC) have been investigated by differential scanning calorimetry (DSC) and X-ray diffraction. DSC of hydrated EDPC shows a single endothermic transition at 34.8 degrees C (delta H = 11.2 kcal/mol) after storage at -4 degrees C while DEPC shows three endothermic transitions at 7.7 and approximately 9.0 degrees C (combined delta H approximately 0.4 kcal/mol) and at 25.2 degrees C (delta H = 4.7 kcal/mol). Both the single transition of EDPC and the two higher temperature transitions of DEPC are reversible, while the approximately 7.7 degrees C transition of DEPC increases in enthalpy on low-temperature incubation. At 23 degrees C, X-ray diffraction of hydrated EDPC shows a sharp reflection at 4.2 A together with lamellar reflections corresponding to a bilayer periodicity, d = 56.2 A. Electron density profiles derived from swelling experiments show a phosphate-phosphate intrabilayer distance, dp-p, of 36 A at all hydrations. This, together with calculated lipid thickness and molecular area considerations, suggests an interdigitated, three chains per head group, bilayer gel phase, L beta*, with no hydrocarbon chain tilt. This is structurally analogous to the bilayer gel phase of hydrated 18:0/10:0 ester PC [McIntosh, T. J., Simon, S. A., Ellington, J. C., Jr., & Porter, N. A. (1984) Biochemistry 23, 4038]. In contrast, DEPC at -4 degrees C shows an L beta' bilayer gel phase with tilted hydrocarbon chains (d = 61.1 A). However, this transforms above 9 degrees C to an interdigitated, triple-chain, L beta* bilayer gel phase (identical with that of EDPC) with d = 56.6 A and a phosphate-phosphate distance of 36 A. Above their respective chain melting transitions, Tm, EDPC and DEPC exhibit liquid-crystalline L alpha bilayer phases with d = 64.5 and 65.0 A at 55 and 45 degrees C, respectively. The ability of both EDPC and DEPC to form triple-chain interdigitated gel-state bilayers suggests that the conformational inequivalence at the sn-1 and sn-2 positions is less pronounced in the ether-linked PCs compared to the ester-linked PCs, where only one of the positional isomers, e.g., 18:0/10:0 PC but not 10:0/18:0 PC, forms the triple-chain structure (J. Mattai, unpublished results). Thus, a different conformation around the glycerol is predicted for ether-linked PC compared to ester-linked PC.     

Gel phase polymorphism in ether-linked dihexadecylphosphatidylcholine bilayers

The structure and properties of the ether-linked 1,2-dihexadecylphosphatidylcholine (DHPC) have been examined as a function of hydration. By differential scanning calorimetry, DHPC exhibits an endothermic (chain melting) transition with the transition temperature (limiting value, 44.2 degrees C) and enthalpy (limiting value, delta H = 8.0 kcal/mol) being hydration dependent. For hydration values greater than 30 wt % water, DHPC exhibits a pretransition at approximately 36 degrees C (delta H = 1.1 kcal/mol) and a subtransition at approximately 5 degrees C (delta H = 0.2 kcal/mol). By X-ray diffraction, at 22 degrees C DHPC exhibits a normal bilayer gel structure with the bilayer periodicity increasing from 58.0 to 62.5 A over the hydration range 9.5-25.4% water. At 30-32% water, two coexisting gel phases are observed with d = 63-64 A and d = 44-45 A; at higher hydration, only the latter phase is present, reaching a limiting d = 47.0 A at 37.5% water. Two different gel phases clearly exist at low and high hydrations. Electron density profiles at low hydration (9.5-25.4%) show a bilayer thickness dp-p = 46 A, whereas at greater than 32% water the bilayer thickness is markedly reduced, dp-p = 30 A. These and other structural parameters indicate a hydration-dependent gel----gel structural transition between a normal bilayer (two chains per polar group) and the chain-interdigitated bilayer (four chains per polar group) arrangement described previously for DHPC [Ruocco, M. J., Siminovitch, D. J., & Griffin, R. G. (1985) Biochemistry 24, 2406-2411].(ABSTRACT TRUNCATED AT 250 WORDS)     

Temperature and compositional dependence of the structure of hydrated dimyristoyl lecithin.

Differential scanning calorimetry and x-ray diffraction techniques have been used to investigate the structure and phase behavior of hydrated dimyristoyl lecithin (DML) in the hydration range 7.5 to 60 weight % water and the temperature range -10 to +60 degrees C. Four different calorimetric transitions have been observed: T1, a low enthalpy transition (deltaH approximately equal to 1 kcal/mol of DML) at 0 degrees C between lamellar phases (L leads to Lbeta); T2, the low enthalpy "pretransition" at water contents greater than 20 weight % corresponding to the transition Lbeta leads to Pbeta; T3, the hydrocarbon chain order-disorder transition (deltaH = 6 to 7 kcal/mol of DML) representing the transition of the more ordered low temperature phases (Lbeta, Pbeta, or crystal C, depending on the water content) to the lamellar Lalpha phase; T4, a transition occurring at 25--27 degrees C at low water contents representing the transition from the lamellar Lbeta phase to a hydrated crystalline phase C. The structures of the Lbeta, Pbeta, C, and Lalpha phases have been examined as a function of temperature and water content. The Lbeta structure has a lamellar bilayer organization with the hydrocarbon chains fully extended and tilted with respect to the normal to the bilayer plane, but packed in a distorted quasihexagonal lattice. The Pbeta structure consists of lipid bilayer lamellae distorted by a periodic "ripple" in the plane of the lamellae; the hydrocarbon chains are tilted but appear to be packed in a regular hexagonal lattice. The diffraction pattern from the crystalline phase C indexes according to an orthorhombic cell with a = 53.8 A, b = 9.33 A, c = 8.82 A. In the lamellae bilayer Lalpha strucure, the hydrocarbon chains adopt a liquid-like conformation. Analysis of the hydration characteristics and bilayer parameters (lipid thickness, surface area/molecule) of synthetic lecithins permits an evaluation of the generalized hydration and structural behavior of this class of lipids.     

Properties of ganglioside GM1 in phosphatidylcholine bilayer membranes.

Gangliosides have been shown to function as cell surface receptors, as well as participating in cell growth, differentiation, and transformation. In spite of their multiple biological functions, relatively little is known about their structure and physical properties in membrane systems. The thermotropic and structural properties of ganglioside GM1 alone and in a binary system with 1,2-dipalmitoyl phosphatidylcholine (DPPC) have been investigated by differential scanning calorimetry (DSC) and x-ray diffraction. By DSC hydrated GM1 undergoes a broad endothermic transition TM = 26 degrees C (delta H = 1.7 kcal/mol GM1). X-ray diffraction below (-2 degrees C) and above (51 degrees C) this transition indicates a micellar structure with changes occurring only in the wide angle region of the diffraction pattern (relatively sharp reflection at 1/4.12 A-1 at -2 degrees C; more diffuse reflection at 1/4.41 A-1 at 51 degrees C). In hydrated binary mixtures with DPPC, incorporation of GM1 (0-30 mol%; zone 1) decreases the enthalpy of the DPPC pretransition at low molar compositions while increasing the TM of both the pre- and main transitions (limiting values, 39 and 44 degrees C, respectively). X-ray diffraction studies indicate the presence of a single bilayer gel phase in zone 1 that can undergo chain melting to an L alpha bilayer phase. A detailed hydration study of GM1 (5.7 mol %)/DPPC indicated a conversion of the DPPC bilayer gel phase to an infinite swelling system in zone 1 due to the presence of the negatively charged sialic acid moiety of GM1. At 30-61 mol % GM1 (zone 2), two calorimetric transitions are observed at 44 and 47 degrees C, suggesting the presence of two phases. The lower transition reflects the bilayer gel --> L alpha transition (zone 1), whereas the upper transition appears to be a consequence of the formation of a nonbilayer, micellar or hexagonal phase, although the structure of this phase has not been defined by x-ray diffraction. At > 61 mol % GM1 (zone 3) the calorimetric and phase behavior is dominated by the micelle-forming properties of GM1; the presence of mixed GM1/DPPC micellar phases is predicted.     

Interactions of N-stearoyl sphingomyelin with cholesterol and dipalmitoylphosphatidylcholine in bilayer membranes.

Differential scanning calorimetry and x-ray diffraction have been utilized to investigate the interaction of N-stearoylsphingomyelin (C18:0-SM) with cholesterol and dipalmitoylphosphatidylcholine (DPPC). Fully hydrated C18:0-SM forms bilayers that undergo a chain-melting (gel -->liquid-crystalline) transition at 45 degrees C, delta H = 6.7 kcal/mol. Addition of cholesterol results in a progressive decrease in the enthalpy of the transition at 45 degrees C and the appearance of a broad transition centered at 46.3 degrees C; this latter transition progressively broadens and is not detectable at cholesterol contents of >40 mol%. X-ray diffraction and electron density profiles indicate that bilayers of C18:0-SM/cholesterol (50 mol%) are essentially identical at 22 degrees C and 58 degrees C in terms of bilayer periodicity (d = 63-64 A), bilayer thickness (d rho-p = 46-47 A), and lateral molecular packing (wide-angle reflection, 1/4.8 A-(1)). These data show that cholesterol inserts into C18:0-SM bilayers, progressively removing the chain-melting transition and altering the bilayer structural characteristics. In contrast, DPPC has relatively minor effects on the structure and thermotropic properties of C18:0-SM. DPPC and C18:0-SM exhibit complete miscibility in both the gel and liquid-crystalline bilayer phases, but the pre-transition exhibited by DPPC is eliminated at >30 mol% C18:0-SM. The bilayer periodicity in both the gel and liquid-crystalline phases decreases significantly at high DPPC contents, probably reflecting differences in hydration and/or chain tilt (gel phase) of C18:0-SM and DPPC.     

Sphingomyelin--lecithin bilayers and their interaction with cholesterol.

Utilizing X-ray diffraction and differential scanning calorimetry (DSC), we have studied (1) the structure and thermotropic properties of hydrated N-palmitoylsphingomyelin, (2) the interaction of N-palmitoylsphingomyelin with dimyristoyllecithin, and (3) the interaction of cholesterol with N-palmitoylsphingomyelin and dimyristoyllecithin, both individually and in a 50:50 (mol/mol) mixture. N-Palmitoylsphingomyelin forms bilayers which undergo a thermotropic order--disorder (gel--liquid crystalline) transition at 40.5 degrees C (delta H = 5.8 kcal/mol). The bilayer repeat distance is 66.8 A at 10 degrees C and 61.6 A at 50 degrees C. N-Palmitoylsphingomyelin exhibits miscibility with dimyristoylecithin in both the gel and liquid-crystalline phases, and no lateral phase separation occurs. Scanning calorimetry indicates that interaction with cholesterol is similar for both N-palmitoylsphingomyelin and dimyristoyllecithin and that in a 50:50 (mol/mol) mixture cholesterol shows no preferential affinity for either phospholipid.     

Structure and thermotropic properties of 1-stearoyl-2-acetyl-phosphatidylcholine bilayer membranes.

The structural and thermotropic properties of 1-stearoyl-2-acetyl-phosphatidylcholine (C(18):C(2)-PC) were studied as a function of hydration. A combination of differential scanning calorimetry and x-ray diffraction techniques have been used to investigate the phase behavior of C(18):C(2)-PC. At low hydration (e.g., 20% H2O), the differential scanning calorimetry heating curve shows a single reversible endothermic transition at 44.6 degrees C with transition enthalpy delta H = 6.4 kcal/mol. The x-ray diffraction pattern at -8 degrees C shows a lamellar structure with a small bilayer periodicity d = 46.3 A and two wide angle reflections at 4.3 and 3.95 A, characteristic of a tilted chain, L beta' bilayer gel structure. Above the main transition temperature, a liquid crystalline L alpha phase is observed with d = 53.3 A. Electron density profiles at 20% hydration suggest that C(18):C(2)-PC forms a fully interdigitated bilayer at -8 degrees C and a noninterdigitated, liquid crystalline phase above its transition temperature (T > Tm). Between 30 and 50% hydration, on heating C(18):C(2)-PC converts from a highly ordered, fully interdigitated gel phase (L beta') to a less ordered, interdigitated gel phase (L beta), which on further heating converts to a noninterdigitated liquid crystalline L alpha phase. However, the fully hydrated (> 60% H2O) C(18):C(2)-PC, after incubation at 0 degrees C, displays three endothermic transitions at 8.9 degrees C (transition I, delta H = 1.6 kcal/mol), 18.0 degrees C (transition II), and 20.1 degrees C (transition III, delta HII+III = 4.8 kcal/mol). X-ray diffraction at -8 degrees C again showed a lamellar gel phase (L beta') with a small periodicity d = 52.3 A. At 14 degrees C a less ordered, lamellar gel phase (L beta) is observed with d = 60.5 A. However, above the transition III, a broad, diffuse reflection is observed at approximately 39 A, consistent with the presence of a micellar phase. The following scheme is proposed for structural changes of fully hydrated C(18):C(2)-PC, occurring with temperature: L beta' (interdigitated)-->L beta (interdigitated)-->L alpha(noninterdigitated)-->Micelles. Thus, at low temperature C(18):C(2)-PC forms a bilayer gel phase (L beta') at all hydrations, whereas above the main transition temperature it forms a bilayer liquid crystalline phase L alpha at low hydrations and a micellar phase at high hydrations (> 60 wt% water).     

Interactions of divalent cations with phosphatidylserine bilayer membranes

The interaction of divalent cations with a homologous series of diacylphosphatidylserines (diacyl-PS) has been studied by differential scanning calorimetry and X-ray diffraction. Hydrated di-C14-PS (DMPS) exhibits a gel leads to liquid-crystal bilayer transition at 39 degrees C (delta H = 7.2 kcal/mol of DMPS). With increasing MgCl2 concentration, progressive conversion to a phase exhibiting a high melting (98 degrees C), high enthalpy (delta H congruent to 11.0 kcal/mol of DMPS) transition is observed. Similar behavior is observed for DMPS with increasing CaCl2 concentration. In this case, the high-temperature transition of the Ca2+-DMPS complex occurs at approximately 155 degrees C and is immediately followed by an exothermic transition probably associated with PS decomposition. For di-C12-, di-C14-, di-C16- (DPPS), and di-C18-PS, the transition temperatures of the Ca2+-PS complexes are in the range 151-155 degrees C; only di-C10-PS exhibits a significantly lower value, 142 degrees C. A different pattern of behavior is exhibited by DPPS in the presence of Sr2+ or Ba2+, with transitions in the range 70-80 degrees C being observed. X-ray diffraction of the Ca2+-PS complexes at 20 degrees C provides evidence of structural homology. All Ca2+-PS complexes exhibit bilayer structures, the bilayer periodicity increasing linearly from 35.0 A for di-C10-PS to 52.5 A for di-C18-PS. Wide-angle X-ray diffraction data indicate that hydrocarbon chain "crystallization" occurs on Ca2+-PS complex formation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Crystallization of phosphatidylserine bilayers induced by lithium

Utilizing differential scanning calorimetry and x-ray diffraction, 1,2-dimyristoyl-L-glycero-3-phospho-L-serine (DMPS) was shown to form hydrated bilayer membrane structures exhibiting a gel leads to liquid crystalline transition at 39 degrees C (delta H = 7.2 kcal/mol). Addition of up to molar concentrations of the alkali halides NaCl, KCl, Rl Cl, and CsCl produced relatively minor changes in DMPS bilayer structure or stability. For example, in the presence of 0.5 M NaCl, the transition temperature (Tc = 42 degrees C) and transition enthalpy (delta H = 7.0 kcal/mol) show only minor changes. In marked contrast, addition of LiCl results in "'crystallization" of the DMPS bilayer membrane structure. At 0.5 M LiCl, the crystalline DMPS exhibits a bilayer gel leads to liquid crystal transition at 89 degrees C accompanied by a high enthalpy change, delta H = 16.0 kcal/mol. Thus, Li+ induces an isothermal crystallization of DMPS bilayers, the hydrocarbon chains adopting a more ordered packing mode than the "hexagonal" arrangement of the gel state. In view of the widespread use of lithium in the treatment of manic-depressive illness, we also raise the possibility that interaction of Li+ with anionic membrane phospholipids could play a role in its pharmacological action.     

Thermodynamics of glucagon aggregation.

Heats of dilution of concentrated glucagon solutions have been measured calorimetrically at 10 and 25 degrees C in 0.2 M potassium phosphate buffer of pH 10.6. Analysis of the data in terms of a monomer-trimer equilibrium gives the following thermodynamic parameters for the association reaction at 25 degrees C: delta G degrees = 7.34 kcal/mol of trimer, delta H degrees = -31.2 kcal/mol, deltaS degrees = -80 cal/(K mol), deltaCp = 430 cal/(K mol). The sensitivity of heat of dilution data to the association constant and stoichiometry of the reaction is discussed.     

Synthesis, structure, and thermal properties of 1,2-dipalmitoylgalloylglycerol (DPGG), a novel self-adhering lipid

A novel diacyl glycerol-based lipid with a polyphenolic head group has been synthesized and characterized. X-ray diffraction experiments show that this lipid, 1,2-dipalmitoylgalloylglycerol (DPGG), hydrates to form gel phase bilayers at 20 degrees C with extremely narrow interbilayer fluid separations, indicating that apposing DPGG bilayers strongly adhere to each other. Differential scanning calorimetry shows that fully hydrated DPGG exhibits a pretransition exotherm (3.7 kcal/mol) at 52 degrees C and a high enthalpy (11.3 kcal/mol) main endothermic transition at 69 degrees C. These thermal properties are similar to those of galactosylceramides with similar hydrocarbon chain compositions. The adhesive and thermal properties of DPGG are likely due to both intermolecular hydrogen-bonding and hydrophobic interactions between the aromatic rings on the gallic acids.     

A calorimetric study of the thermotropic behavior of pure sphingomyelin diastereomers

The phase-transition properties of sphingomyelins were investigated in detail with totally synthetic, chemically and stereochemically pure (2S,3R)-(N-stearoylsphingosyl)-1-phosphocholine (D-erythro-C18-SPM) (1) and the corresponding 2S,3S isomer (L-threo-C18-SPM) (2). Heating scans of an unsonicated dispersion of 1 right after hydration showed a main transition (I) at 44.7 degrees C (delta H = 6.8 kcal/mol). Upon incubation at 20-25 degrees C a second transition (II) appeared at 36.0 degrees C (delta H = 5.7 kcal/mol). The two gel phases were designated as G alpha and G beta phases, respectively. The G beta phase was also metastable and relaxed to a third gel phase (G gamma) upon incubation below 10 degrees C. Conversion of the G gamma phase to the liquid-crystalline phase occurred via two new endotherms at 33.4 degrees C (2.6 kcal/mol) (III) and 43.6 degrees C (8.0 kcal/mol) (IV) as well as a main transition at 44.7 degrees C (9.5 kcal/mol). Possible interpretations have been proposed to account for the observed phase transitions. The L-threo isomer 2 showed similar thermotropic behavior to dipalmitoylphosphatidylcholine (DPPC): a "main transition" at 44.2 degrees C (6.0 kcal/mol), a "pretransition" at 43.1 degrees C (1.8 kcal/mol), and upon incubation at 7 degrees C for 2 weeks, a very broad "subtransition" at ca. 35 degrees C. The results are substantially different from previous studies of sphingomyelins using mixtures of stereoisomers. Mixing of 1 with 2, 1 with DPPC, and 2 with DPPC removed the metastability of the gel phase and resulted in a single transition.     

The kinetics of formation and structure of the low-temperature phase of 1-stearoyl-lysophosphatidylcholine

A combination of differential scanning calorimetry (DSC) and X-ray diffraction have been used to study the kinetics of formation and the structure of the low-temperature phase of 1-stearoyl-lysophosphatidylcholine (18:0-lysoPC). For water contents greater than 40 weight %, DSC shows a sharp endothermic transition at 27 degrees C (delta H = 6.75 kcal/mol) corresponding to a low-temperature phase----micelle transition. This sharp transition is not reversible, but is regenerated in a time and temperature-dependent manner. For example, with incubation at 0 degrees C the maximum transition enthalpy (delta H = 6.75 kcal/mol) is generated in about 45 min after an initial slow nucleation process of approx. 20 min. The kinetics of formation of the low-temperature phase is accelerated at lower temperatures and may be related to the disruption of 18:0-lysoPC micelles by ice crystal formation. X-ray diffraction patterns of 18:0-lysoPC recorded at 10 degrees C over the hydration range 20-80% are characteristic of a lamellar gel phase with tilted hydrocarbon chains with the bilayer repeat distance increasing from 47.6 A at 20% hydration to a maximum of 59.4 A at 39% hydration. At this maximum hydration, approx. 19 molecules of water are bound per molecule of 18:0-lysoPC. Electron density profiles show a phosphate-phosphate distance of 30 A, indicating an interdigitated lamellar gel phase for 18:0-lysoPC at all hydration values. The angle of chain tilt is calculated to be between 20 and 30 degrees. For water contents greater than 40%, this interdigitated lamellar phase converts to the micellar phase at 27 degrees C in a kinetically fast process, while the reverse (micelle----interdigitated bilayer) transition is a kinetically slower process (see also Wu, W. and Huang, C. (1983) Biochemistry 22, 5068-5073).     

Phase behaviour of a diglyceride prodrug: spontaneous formation of unilamellar vesicles

A prodrug (Fig. 1(IV)) is synthesized consisting of the beta-blocker bupranolol which is covalently linked to 1, 3-dipalmitoyl-2-succinyl-glycerol. The resulting lipid-like prodrug is amphipathic and surface active. It disperses readily in H2O above 30 degrees C forming a smectic lamellar phase. This prodrug bears one positive charge at neutral pH and hence the swelling behaviour of dispersions in H2O is similar to that of charged phospholipids: the dispersions show continuous swelling with increasing water content and consequently in the excess H2O region of the phase diagram the thermodynamically most stable structure is the unilamellar vesicle. This includes oligomeric vesicles which may be defined as unilamellar vesicles containing smaller, also unilamellar vesicles entrapped in their internal aqueous compartment. The prodrug dispersions in H2O are polydisperse with vesicle sizes ranging from 0.1 micron to several micron. Sonication of these dispersions produce small unilamellar vesicles of an average size and size distribution similar to sonicated egg phosphatidylcholine dispersions. Unsonicated dispersions of the prodrug in H2O undergo reversibly sharp order-disorder transitions at 32 degrees C with an enthalpy change of delta H = 10 kcal/mol. In sonicated aqueous dispersions this phase transition is asymmetric and significantly broadened indicating that the cooperativity is markedly reduced. The peak temperature and enthalpy change of this broad transition are reduced compared to the transition observed with unsonicated dispersions. The temperature dependence of the electron spin resonance (ESR) hyperfine splitting and order parameter also reflects the order-disorder transition. From ESR spin labeling it is concluded that in sonicated dispersions the prodrug molecule is more mobile and its anisotropy of motion is reduced compared to unsonicated dispersions. This result indicates that the molecular packing in the highly curved bilayers of small unilamellar prodrug vesicles is significantly perturbed compared to bilayers of unsonicated dispersions.     

Scanning calorimetry reveals a new phase transition in L-alpha-dipalmitoylphosphatidylcholine.

We report a new phase transition in fully hydrated dispersions of dipalmitoylphosphatidylcholine (DPPC). This new transition, called the sub-subtransition, exhibits a transition enthalpy of 0.25 kcal/mol with a Tm at 6.8 degrees C. Unlike the subtransition, no extended low temperature incubation is required to observe the sub-subtransition. This new sub-subgel (SGII) phase may be a precursor to the subgel (SGI) phase, and this discovery is discussed in relation to the current knowledge regarding the polymorphic gel phases of both ester- and ether-linked lipids with identical acyl chains.