Structure and properties of mixed-chain phosphatidylcholine bilayers

Phosphatidylcholines or C(X):C(Y)PC, quantitatively the most abundant lipids in animal cell membranes, are structurally composed of two parts: a headgroup and a diglyceride. The diglyceride moiety consists of the glycerol backbone and two acyl chains. It is the wide diversity of the acyl chains, or the large variations in X and Y in C(X):C(Y)PC, that makes the family of phosphatidylcholines an extremely complex mixture of different molecular species. Since most of the physical properties of phospholipids with the same headgroup depend strongly on the structures of the lipid acyl chains, the energy-minimized structure and steric energy of each diglyceride moiety of a series of 14 molecular species of phosphatidylcholines with molecular weights identical to that of dimyristoylphosphatidylcholine without the headgroup are determined in this communication by molecular mechanics (MM) calculations. Results of two types of trans-bilayer dimer for each of the 14 molecular species of phosphatidylcholines are also presented; specifically, the dimeric structures are constructed initially based on the partially interdigitated and mixed interdigitated packing motifs followed subsequently by the energy-minimized refinement with MM calculations. Finally, tetramers with various structures to model the lateral lipid-lipid interactions in a lipid bilayer are considered. Results of laborious MM calculations show that saturated diacyl C(X):C(Y)PC with delta C/CL values greater than 0.41 prefer topologically to assemble into tetramers of the mixed interdigitated motif, and those with delta C/CL values less than 0.41 prefer to assemble into tetramers with a repertoire of the partially interdigitated motif. Here, delta C/CL, a lipid asymmetry parameter, is defined as the normalized acyl chain length difference between the sn-1 and sn-2 acyl chains for a C(X):C(Y)PC molecule; an increase in delta C/CL value is an indication of increasing asymmetry between the two lipid acyl chains. These computational results are in complete accord with the calorimetric data presented previously from this laboratory (H-n. Lin, Z-q. Wang, and C. Huang. 1991. Biochim. Biophys. Acta. 1067:17-28).

Section: Introductionmentioning

confidence: 78%

Energy-minimized structures and packing states of a homologous series of mixed-chain phosphatidylcholines: a molecular mechanics study on the diglyceride moieties

Wang

Huo

et al. 1993

“…Teflon membranes with 0.5-micron pores were obtained from Alltech (Deerfield, IL). C(18):C(2)-PC synthesis C(18):C(2)-PC had been prepared previously by Tence et al (1981), Tokumura et al (1981Tokumura et al ( , 1985, Hirth and Barner (1982), Huang et al (1984), and Shah et al (1990). We prepared C(18):C(2)-PC by a variation of the reported methods (Gupta et al, 1977;Mason et al, 1981;Mangroo and Gerber, 1988;and Ali and Bittman, 1989) for the acylation of l-acyl-snglycero-3-phosphocholines (Iyso-PC).…”

Section: Methodsmentioning

confidence: 99%

“…1-stearoyl-2-acetyl-sn-glycerol-3phosphocholine (C(18):C(2)-PC) is also structurally very asymmetrical (i.e., an 18 carbon chain at the sn-i position and a 2 carbon chain at the sn-2 position). Earlier studies on C(18):C(2)-PC (Huang et al, 1984;Shah et al, 1990) show that C(18):C(2)-PC exhibits a complicated phase behavior in contrast to lysoPC (Mattai and Shipley, 1986;Hui and Huang, 1986). Based on the DSC data, suggested that, on heating, C(18):C(2)-PC converts from a crystalline phase --gel (I) -* gel (II) -* micellar phase.…”

Section: Introductionmentioning

confidence: 96%

“…Our previous work has been focused on studying the structural changes in hydrated mixed-chain phosphatidylcholines resulting from systematic reductions in the chain lengths of the fatty acids attached at the sn-i and sn-2 positions. Using differential scanning calorimetry (DSC) and x-ray diffraction (Mattai et al, 1987;Shah et al, 1990), it has been shown that the chain length asymmetry greatly influences the packing properties of the hydrocarbon chains in the bilayer gel phase, and depending upon the degree of asymmetry, the lipids adopt noninterdigitated structures (e.g., C(18):C(18)-PC), partially-interdigitated phases (e.g., C(18):C(16)-PC), mixed-interdigitated phases (e.g., C(18):C(10)-PC), or fully interdigitated phases (e.g., C(18):C(0)-PC) to compensate for the chain length asymmetry McIntosh et al, 1984;Hui et al, 1984;Mattai and Shipley, 1986;Mattai et al, 1987;Shah et al, 1990). However, above the main transition temperature highly asymmetric PCs (such as C(18):C(0)-PC and C(18):C(2)-PC), at high hydrations form a micellar phase (Mattai and Shipley, 1986;Huang et al 1984), whereas other PCs (e.g., C(18):C(18)--PC, C(18):C(16)-PC, C(18):C(10)-PC) exist in a lamellar liquid crystalline phase ).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

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

Shah¹,

Duclos²,

Shipley³

1994

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).

“…6 in Mattai et al, 1987). Furthermore, PCs in which the length of one of its constituent chains is approximately half the length of the other seem to pack in a "mixed interdigitated" monolayer structure, in this case with the two short chains from PC molecules on each side of the bilayer being aligned against each other and the hydrocarbon width essentially being determined by the long chain (Mattai et al, 1987;McIntosh et al, 1984;Hui et al, 1984;Shah et al, 1990). In contrast to the structural studies of mixed-chain PCs, relatively little is known about the structure and properties of SM containing hydrocarbon (sphingosine and N-acyl) chains of different length.…”

Section: + -(40°c;mentioning

confidence: 99%

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

Maulik

Shipley

1995

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. These structural parameters,together with calculated lipid thickness and molecular area considerations, suggest that the low temperature endotherm(T1) of hydrated C24:0-SM corresponds to a transition from a tilted, gel state (Gel I) with partially interdigitated chains to an untilted, or less tilted, gel state (Gel 11). At 600C (above T3), the usual liquid-crystalline La bilayer structure (d = 59.5-66.3A; dp p -46 A) is present at all hydrations. Comparison with the behavior of C18:0-SM indicates that the in equivalence of length of the sphingosine (C18) and lignoceryl (C24) chains results in a more complex gel phase polymorphism for the sphingosine (C18) and lignoceryl (C24) chains results in a more complex gel phase polymorphism for C24:0-SM.