Mixed Interdigitated Bilayer Research Articles

On the main phase transition temperatures of highly asymmetric mixed-chain phosphatidylcholines

The highly asymmetric mixed-chain phosphatidylcholines or C(X): C(Y) PC , which can self-assemble at T < T m into the mixed interdigitated gel-state bilayer in excess water, can be divided into two groups. Group I consists of C(X): C(Y) PC with X > Y, and Group II consists of C(X): C(Y) PC with X < Y. The main phase transition temperatures (T m ) of these C(X): C(Y) PC have been analyzed in terms of two structural parameters (δ and N f ). Specifically, these structural parameters are related to the packing geometry of the lipid's acyl chains as the lipid molecules are packed either in a mixed (δ) or a hypothetically partially (N f ) interdigitated packing motif at T < T m . Based on 28 and 29 T m values of Group I and II C(X): C(Y) PC , two general equations are derived, respectively, by multiple regression analyses. These equations correlate systematically the T m values of Group I and II phosphatidylcholines with their corresponding structural parameters. Using these two derived equations, the T m values of a total of 92 molecular species of C(X): C(Y) PC are generated, and these calculated T m values can be considered as the reliably predicted T m values for highly asymmetric C(X): C(Y) PC which have ΔC/CL values within the range of 0.42–0.66.

Studies of mixed-chain diacyl phosphatidylcholines with highly asymmetric acyl chains: a Fourier transform infrared spectroscopic study of interfacial hydration and hydrocarbon chain packing in the mixed interdigitated gel phase

The mixed interdigitated gel phases of unlabeled, specifically 13C = O-labeled, and specifically chain-perdeuterated samples of 1-O-eicosanoyl, 2-O-lauroyl phosphatidylcholine and 1-O-decanoyl, 2-O-docosanoyl phosphatidylcholine were studied by infrared spectroscopy. Our results suggest that at the liquid-crystalline/gel phase transition temperatures of these lipids, there is a greater redistribution in the populations of free and hydrogen-bonded ester carbonyl groups than is commonly observed with symmetric chain n-saturated diacyl phosphatidylcholines. The formation of the mixed interdigitated gel phase coincides with the appearance of a marked asymmetry in the contours of the C = O stretching band, a process which becomes more pronounced as the temperature is reduced. This asymmetry is ascribed to the emergence of a predominant lipid population consisting of free sn1- and hydrogen-bonded (hydrated) sn2-ester carbonyl groups. This suggests that the region of the mixed interdigitated bilayer polar/apolar interface near to the sn1-ester carbonyl group is less hydrated than is the case with the noninterdigitated gel-phase bilayers formed by normal symmetric chain phosphatidylcholines. In the methylene deformation region of the spectrum, the unlabeled lipids exhibit a pronounced splitting of the CH2 scissoring bands. This splitting is significantly attenuated when the short chains are perdeuterated and collapses completely upon perdeuteration of the long chains, irrespective of whether the long (or short) chains are esterified to the sn1 or sn2 positions of the glycerol backbone. These results are consistent with a global hydrocarbon chain packing motif in which the zigzag planes of the hydrocarbon chains are perpendicular to each other and the sites occupied by long chains are twice as numerous as those occupied by short chains. The experimental support for this chain-packing motif enabled more detailed considerations of the possible ways in which these lipid molecules are assembled in the mixed interdigitated gel phase. Generally, our results are compatible with a previously proposed model in which the mixed interdigitated gel phase is an assembly of repeat units which consists of two phosphatidylcholine molecules forming a triple-chain structure with the long chains traversing the bilayer and with the methyl termini of the shorter chains opposed at the bilayer center. Our data also suggest that the packing format which is most consistent with our results and previously published work is one in which the hydrocarbon chains of each repeat unit are parallel to each other with the repeat units themselves being perpendicularly packed.

Thermotropic and mixing behavior of mixed-chain phosphatidylcholines with molecular weights identical with L-.alpha.-dipalmitoylphosphatidylcholine

The thermotropic phase behavior of 10 mixed-chain phosphatidylcholines, in excess water, has been examined and compared with that of identical-chain C(16):C(16)PC by using high-resolution differential scanning calorimetry (DSC). The molecular weights (MW) of these 11 molecular species are the same, but their delta C/CL values, or the normalized chain length differences, vary considerably, ranging from 0.035 to 0.540. The thermodynamic parameters (Tm, delta H, and delta S) associated with the main phase transitions for these lipid dispersions exhibit biphasic V-shaped curves, when plotted against delta C/CL. Similar characteristic curves have been reported previously for aqueous dispersions of mixed-chain phosphatidylcholines with MW identical with that of C(17):C(17)PC [Lin et al. (1990) Biochemistry 29, 7063-7072]. The initial decrease in Tm (delta H or delta S) with increasing values of delta C/CL is attributed to the progressive increase in the magnitude of the chain-terminal perturbations on the conformational statistics of the adjacent hydrocarbon chains and hence the lateral chain-chain interactions of these mixed-chain phosphatidylcholines in the gel-state bilayer. At delta C/CL approximately equal to 0.42, the chain-end perturbation is presumably at its maximum; beyond this point, the highly asymmetric phosphatidylcholines are proposed to pack, at T less than Tm, into the mixed interdigitated bilayer. In this new packing mode, the methyl ends of the longer acyl chains are relocated at the interfaces between the hydrocarbon core of the bilayer and the aqueous medium. This disposition of the bulky chain ends releases a certain degree of chain-chain packing disorders, leading to an increase in Tm (delta H or delta S) with increasing delta C/CL.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamic motions of 1,6-diphenyl-1,3,5-hexatriene in interdigitated C(18):C(10)phosphatidylcholine bilayers

This study investigates the dynamic behavior of 1,6-diphenyl-1,3,5-hexatriene (DPH) in C(18):C(10)phosphatidylcholine [C(18):C(10)PC] bilayers. C(18):C(10)PC is an asymmetric mixed-chain phosphatidylcholine known to form mixed-interdigitated structures below the transition temperature and form partially interdigitated bilayers above the transition temperature. The rotation of DPH in C(18):C(10)PC has been described in terms of the thermal coefficient of rotation using the modified Y-plot method which takes into account the limiting anisotropy value. During the phase transition of C(18):C(10)PC, DPH exhibits a thermal coefficient b2M = 0.41 - 0.51 degrees C-1 which is similar to the b2M values obtained with noninterdigitated phosphatidylcholine bilayers. Differential polarized phase-modulation fluorometry has also been employed to study the dynamic behavior of DPH in C(18):C(10)PC in real time. The data show that DPH contains considerable motion in the highly ordered mixed interdigitated bilayers. The DPH motion steadily increases with an increase in temperature as shown by the rotational correlation time, and the wobbling diffusion constant. However, the limiting anisotropy, the order parameter, and the width of the lifetime distribution undergo an abrupt decrease, and a corresponding abrupt increase in the cone angle, at approximately 16 degrees C. This temperature range is near the onset temperature of the phase transition as determined by differential scanning calorimetry. The rotational parameters show strong hysteresis on heating and cooling. All the rotational parameters derived from DPH fluorescence in mixed interdigitated C(18):C(10)PC exhibit magnitudes similar to those obtained from non interdigitated gel phases of symmetric diacylphosphatidylcholines. These results, combined with those obtained with dehydroergosterol (Kao, Y. L., P. L.-G. Chong, and C. Huang. 1990. Biochemistry. 29:1315-1322), suggest that considerable rotational mobility of small molecules can be sustained in an intramolecularly highly ordered interdigitated lipid matrix, implying that the membrane maintains a fluid environment around membrane perturbants even when the lipid matrix is extensively interdigitated.

Mixing behavior of identical molecular weight phosphatidylcholines with various chain-length differences in two-component lamellae

It has recently been suggested that mixed-chain phosphatidylcholines with normalized chain length differences (deltaC/CL) in the range of 0.10-0.40 undergo spontaneous self-assembly in excess water at T less than Tm into the partially interdigitated bilayer and those with delta C/CL values in the range of 0.44-0.57 form, in excess water, mixed interdigitated bilayers at T less than Tm. The mixing behavior of binary mixtures of C(22):C(12)PC/C(17):C(17)PC, C(22):C(12)/C(15):C(19)PC, and C(15):C(19)PC/C(13):C(21)PC reported in this work is used to support this view. The values of delta C/CL for C(17):C(17)PC, C(15):C(19)PC, C(13):C(21)PC, and C(22):C(12)PC are 0.10, 0.15, 0.35, and 0.55, respectively. The binary mixture of C(15):C(19)PC/C(13):C(21)PC exhibits a lens-shaped phase diagram, indicating that these two identical molecular weight (MW) lipids with delta C/CL values less than 0.4 are completely miscible over the entire compositional range in both gel and liquid-crystalline phases. In contrast, the phase diagrams of C(22):C(12)PC/C(17):C(17)PC and C(22):C(12)PC/C(15):C(19)PC are eutectic, indicating immiscibility of the component lipids over a wide compositional range in the gel phase. This immiscibility of identical MW lipids in the bilayer plane can be attributed to the different packing properties of the component lipids in the bilayer at T less than Tm.

Differential scanning calorimetry study of mixed-chain phosphatidylcholines with a common molecular weight identical with diheptadecanoylphosphatidylcholine

To examine the thermotropic phase behavior of various mixed-chain phosphatidylcholines in excess water and to compare it with the known behavior of identical-chain phosphatidylcholines, we have carried out high-resolution differential scanning calorimetric (DSC) studies on aqueous dispersions of 10 different mixed-chain phosphatidylcholines. These lipids, C(16):C(18)PC, C(18):C(16)PC, C(15):C(19)PC, C(19):C(15)PC, C(14):C(20)PC, C(20):C(14)PC, C(13):C(21)PC, C(21):C(13)PC, C(12):C(22)PC, and C(22):C(12)PC, have a common molecular weight which is the same as that of C(17):C(17)PC, an identical-chain phosphatidylcholine with a molecular weight of 762.2. When the values of any of the thermodynamic parameters (Tm, delta H, and delta S) of the mixed-chain phosphatidylcholines and C(17):C(17)PC are plotted against the normalized chain-length difference (delta C/CL), a linear function with negative slope is obtained provided that the value of delta C/CL is within the range of 0.09-0.4. The linear relationship suggests that these mixed-chain phospholipids are packed in the gel-state bilayer similar to the bilayer structure of C(17):C(17)PC at T less than Tm; however, the negative slope suggests that the conformational statistics of the hydrocarbon chain and the lateral lipid-lipid interactions of these phosphatidylcholines in the gel-state bilayer are perturbed proportionally by a progressive increase in the chain-length inequivalence between the two acyl chains within each lipid molecule. When the value of delta C/CL for mixed-chain phosphatidylcholines reaches the range of 0.44-0.55, the thermotropic phase behavior deviates markedly from that of less asymmetric phosphatidylcholines, suggesting that these highly asymmetric lipids are packed into mixed interdigitated bilayers at T less than Tm. The heating and cooling pathways of aqueous dispersions prepared from the 10 mixed-chain phospholipids are also discussed.

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.

Time-resolved fluorometric and differential scanning calorimetric investigation of dehydroergosterol in 1-stearoyl-2-capryl phosphatidylcholine bilayers

Thermal and dynamic properties of dehydroergosterol (DHE) in 1-stearoyl-2-capryl-sn-glycero-3-phosphocholine [C(18):C(10)PC] have been studied by differential scanning calorimetry (DSC) and multifrequency phase-modulation fluorometry. C(18):C(10)PC is an asymmetric mixed-chain phosphatidylcholine known to form highly ordered mixed interdigitated bilayers below the maximal transition temperature, Tm, and partially interdigitated bilayers above Tm. This lipid system is thus unique in assessing the interactions between sterols and interdigitated lipid bilayers. DHE is a fluorescent analogue of cholesterol shown in previous studies to behave like cholesterol in noninterdigitated symmetric diacylphosphatidylcholines. DSC data show that DHE exhibits similar characteristics to cholesterol [Chong & Choate (1989) Biophys. J. 55, 551-556] in C(18):C(10)PC bilayers. DHE abolishes the phase transition of C(18):C(10)PC at 27 mol % compared to 25 mol % for cholesterol and decreases Tm, the onset temperature (To), and the completion temperature (Tc), at a similar rate to cholesterol at about -0.25 degrees C per mole percent DHE. Fluorescence data show that the rotational motion of DHE can be described by a hindered anisotropic model. In the gel state of C(18):C(10)PC, the rotational correlation of DHE decreases monotonically with increasing DHE content up to 24 mol %, suggesting that DHE causes a disordering/spacing effect on the packing of mixed interdigitated C(18):C(10)PC bilayers. The rotational correlation time undergoes an abrupt increase from 24 to 27 mol % DHE. Abrupt changes in the DSC parameters were also observed in the neighborhood of 27 mol %, suggesting that major reorganization takes place around this concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Mixed-chain phospholipids and interdigitated bilayer systems

The lipid bilayer is a fundamental structural component for all biological membranes. The function of lipid bilayers may be considered not only to serve as a permeability barrier preventing the free flows of ions and polar molecules between the cell interior and its external environment, but also to modulate the activity of bilayer-spanning proteins or glycoproteins in biological membranes. Based on the location of the terminal methyl groups of the two hydrocarbon chains in phospholipids or glycosphingolipids, two broad categories of lipid bilayers have been recognized: (1) the noninterdigitated and (2) the interdigitated bilayers. Depending on the chain-length difference between the two hydrocarbon chains, three different types of packing models for interdigitated bilayer systems have been identified. Among the three types, the mixed interdigitated bilayer is perhaps unique, since the hydrocarbon chains in the bilayer core are observed to be interdigitated in both the gel and the liquid-crystalline states. In this communication, various experimental data supporting the chain packing characteristics of the mixed interdigitated bilayer for a large number of mixed-chain phospholipid species are considered. In addition, two types of phase diagrams for binary phospholipid mixtures obtained with mixed-chain phospholipids are also presented. These studies may be of great importance in understanding the functional control of bilayer-spanning proteins in biological membranes, and for providing basic information explaining the dynamic regulation of membrane activities in general.

Mixtures of semisynthetic species of cerebroside sulfate with dipalmitoyl phosphatidylcholine. Thermotropic phase behavior and permeability

The phase behavior of mixtures of dipalmitoyl phosphatidylcholine (DPPC) with semisynthetic species of cerebroside sulfate (CBS) containing palmitic acid (C16:0-CBS) or lignoceric acid (C24:0-CBS) in 0.1 M KCl was studied using differential scanning calorimetry. DPPC and C16:0-CBS were miscible in all proportions in the gel phase above 10 mol% CBS and in the liquid-crystalline phase. However, C24:0-CBS was less miscible with DPPC over a wide concentration range in the gel phase. At high CBS concentrations it was probably also not entirely miscible with DPPC in the liquid-crystalline phase. Small amounts of both species of CBS lowered the transition temperature and enthalpy of DPPC, suggesting that they are more soluble in the liquid-crystalline phase of DPPC than the gel phase. The transition temperature at higher CBS concentrations was also less than expected, especially after cycling through the phase transition in the case of C24:0-CBS, suggesting that mixing with DPPC inhibited the intermolecular hydrogen bonding interactions and dehydration of CBS. In C24:0-CBS-DPPC mixtures several populations were present over a wide compositional range, including two solid-solid solutions of fixed composition. At high C24:0-CBS concentrations some C24:0-CBS also phase separated out of the mixture. Structural considerations suggested that the C24:0-CBS which is mixed with DPPC must be interdigitated into the DPPC bilayer. Other populations that are present may have a different structural organization. A fatty acid spin label in these mixtures was a little less ordered than in either lipid by itself. The permeability of these lipids, as well as the two asymmetric species 1-stearoyl-2-caproyl phosphatidylcholine and 1-stearoyl-2-myristoyl phosphatidylcholine (18:10PC and 18:14PC), to a water-soluble spin label tempocholine chloride was also measured. The studies with 18:10PC and 18:14PC indicated that both triple-chain mixed interdigitated bilayers and double-chain partially interdigitated bilayers can trap water-soluble substances and have low permeability. Both species of CBS could also entrap the spin label and had low permeability at 4 degrees C. However, they rapidly lost the entrapped compound when they transformed into their stable dehydrated phases or into the liquid-crystalline phase. Mixing with DPPC prevented both of these losses. These studies supported the conclusion that a significant amount of the CBS was mixed with the DPPC and that this mixing prevented the dehydration changes which CBS undergoes by itself.(ABSTRACT TRUNCATED AT 400 WORDS)

The ordering and dynamics of the terminal methyl group on the lignoceric acid chain in cerebroside sulfate

Cerebroside sulfate containing lignoceric acid deuterated at the terminal methyl group was prepared by a semi-synthetic method in order to study the ordering and motion of the fatty acyl chain by deuterium NMR spectroscopy. The lipid was hydrated with either 2 M KCl or 2 M LiCl. The NMR results are consistent with the formation of an interdigitated bilayer in the gel phase as suggested by spin label and X-ray diffraction studies, although they do not delineate whether the interdigitation is of the partial or mixed triple-chain type. In the low-temperature stable phase formed by this lipid on slow cooling from the liquid-crystalline phase, which has been shown by X-ray diffraction to be partially interdigitated, the dominant molecular motion is reorientation of the lipid about its along axis. The motion had a correlation time of the order of the deuterium quadrupole coupling constant (10 −6 s). The rate of axial diffusion is greater than that found for symmetric phospho- or glycosphingo-lipids in their crystalline subgel phases, but less than that of a lipid which forms a non-interdigitated gel phase bilayer such as DPPE, or a lipid which forms a fully interdigitated gel phase bilayer like DHPC. The rate of motion appears to be greater in a low temperature, more hydrated metastable phase formed by this lipid after cooling rapidly from the liquid crystalline phase. This phase is thought to be a mixed interdigitated bilayer based on spin label studies. The disorder observed at the C-terminal methyl group in the L α phase, which is probably partially interdigitated according to X-ray diffraction studies, is greater than that of a non-interdigitated bilayer formed by a symmetric lipid. In the CBS/Li + system at high temperature, there is evidence for a liquid-crystalline to liquid-crystalline phase transition, in which the average orientation of the terminal methyl group changes.

Structural aspects of pressure effects on infrared spectra of mixed-chain phosphatidylcholine assemblies in deuterium oxide

The barotropic behavior of D2O dispersions of 1-stearoyl-2-caproyl-sn-glycero-3-phosphocholine, C(18):C(10)PC, a highly asymmetric phospholipid in which the length of the fully extended acyl chain at the sn-1 position of the glycerol backbone is twice as long as that at the sn-2 position, has been investigated by high-pressure Fourier transform infrared spectroscopy. This asymmetric phosphatidylcholine bilayer at room temperature displays a pressure-induced phase transition corresponding to the liquid-crystalline----gel phase transition at 1.4 kbar. A conformational ordering of the lipid acyl chains is observed to take place abruptly at the transition pressure of 1.4 kbar. However, the lamellar lipid molecules and their acyl chains remain to be orientationally disordered in the gel phase until the applied pressure reaches 5.5 kbar. In the gel phase of fully hydrated C(18):C(10)PC, the asymmetric lipid molecules assemble into mixed interdigitated bilayers with perpendicular orientation of the zigzag planes among neighboring acyl chains. The role of excess water played in the interchain structure and the behavior of excess water and bound water under high pressure are also discussed.

Binary mixtures of saturated and unsaturated mixed-chain phosphatidylcholine. A differential scanning calorimetry study.

High-resolution differential scanning calorimetry (DSC) has been used to study the aqueous dispersions of mixed-chain phosphatidylcholines prepared from colyophilized mixtures of C(18):C(11:1 delta 10) PC/C(18):C(10)PC and C(18):C(11:1 delta 10) PC/C(18):C(11)PC of various molar ratios. These mixed-chain phospholipids are characterized by a marked disparity in their acyl-chain lengths; however, the sn-1 acyl chain in the fully extended conformation is about twice as long as the sn-2 acyl chain. Their thermotropic behavior was determined, and the phase diagrams of these two mixtures were constructed from the calorimetric data. Results indicate that C(18):C(11:1 delta 10)PC/C(18):C(10)PC and C(18):C-(11:1 delta 10)PC/C(18):C(11)PC are miscible in all proportions with a near-ideal behavior of mixing in the gel and liquid-crystalline phases. Equimolar mixtures of diC(14)PC/C(18):C(11:1 delta 10)PC, diC(14)PC/C(18):C(10)PC, and diC(14)PC/C(18):C(11)PC have also been studied by DSC. These phosphatidylcholines in the 1:1 mixture differ in Tm by less than 11 degrees C; however, they exhibit gel-phase immiscibility in the plane of the bilayer. Taken together, these studies suggest that C(18):C(11)PC and C(18):C(11:1 delta 10)PC are packed similarly to C(18):C(10)PC in excess water as mixed interdigitated bilayers, at T less than Tm, which transform into partially interdigitated bilayers when heated above Tm.

Interdigitated lipid bilayers of long acyl chain species of cerebroside sulfate. A fatty acid spin label study

The metastable phase behavior of semi-synthetic species of cerebroside sulfate (CBS), with hydroxy and non-hydroxy fatty acids from 16 to 26 carbons in length, was compared in Li + and K + using differential scanning calorimetry. The structure of the metastable and various stable phases formed in the presence of these two cations was investigated using a fatty acid spin label, 16-doxylstearate. A number of stable phases with successively higher phase transition temperatures and enthalpies occur in the presence of K + (see the preceding paper). Li + prevents formation of the most stable phases with the highest transition temperatures and enthalpies for all species of CBS. However, it does not prevent a transition from the metastable phase to the first stable phase of the longer chain C24 and C26 species. Furthermore, it allows C24:0h-CBS to undergo a similar transition, in contrast to a high K + concentration, which prevents it. The spin label has anisotropic motion in the metastable gel phase formed by all species of CBS on cooling from the liquid crystalline phase. The spectra resemble those in gel phase phospholipids. The spin label is partially insoluble in the most stable phases formed by all the lipids, including the unsaturated C24:1 species, preventing further elucidation of their structure using the technique. However, the spin label is soluble in the first stable phase formed on cooling by the longer chain C24:0 and C26:0-CBS in Li + and K + and by C24:0h-CBS in Li +, and is motionally restricted in this phase. The motional restriction is similar to that observed in the mixed interdigitated bilayers of asymmetric species of phosphatidylcholine and fully interdigitated bilayers formed by symmetric phospholipids. It strongly suggests that the highly asymmetric long chain species of CBS form a mixed interdigitated bilayer in their first stable gel phases while the metastable phase of these and the shorter chain lipids may be partially interdigitated. The metastable phase of C24:1-CBS is more disordered suggesting that it may not be interdigitated at all. Thus the results suggest that (i) the hydroxy fatty acid inhibits but does not prevent formation of a mixed interdigitated bilayer by long chain species of CBS, (ii) an increase in non-hydroxy fatty acid chain length from 24 to 26 carbons promotes it, and (iii) a cis double bond probably prevents any form of interdigitation. These results may be relevant to the physiological and pathological roles of these structural modifications of CBS.

Binary mixtures of asymmetric phosphatidylcholines with one acyl chain twice as long as the other.

High-resolution differential scanning calorimetry and 31P NMR spectroscopy have been used to study aqueous phosphatidylcholine (PC) dispersions prepared from colyophilized mixtures of C(10):C(22)PC/C(22):C(12)PC of various molar ratios. These two lipid species are highly asymmetric but have a common structural feature; namely, one acyl chain in the fully extended conformation is about twice as long as the other. Our experimental results support two conclusions: (1) These two component lipids are miscible in all proportions in both gel and liquid-crystalline states. This type of system behaves as a nearly ideal mixture. Its calorimetric parameters are those expected on the basis of the mole fraction weighted average of the corresponding parameters for the pure components. (2) The component lipids appear to self-assemble, at T less than Tm, into a mixed interdigitated bilayer in excess water. In a mixed interdigitated bilayer, the short acyl chain of one asymmetric phosphatidylcholine on one side of the bilayer leaflet is apposed with the short acyl chain of another lipid molecule on the other side of the bilayer leaflet, while the longer acyl chain from each of the two leaflets crosses the entire hydrocarbon width of the bilayer. The fundamental packing unit, as well as the dynamic unit describing the axial rotator motion about the bilayer normal for this mixed interdigitated bilayer, is thus a dimer, whereas the packing unit assigned for the noninterdigitated bilayer such as C(16):C(16)PC lamellae is a monomer.

Scanning calorimetric study of fully hydrated asymmetric phosphatidylcholines with one acyl chain twice as long as the other

The asymmetric C(18):C(10)PC molecules are known by X-ray diffraction to self-assemble, in excess water, into a lamellar structure known as the mixed interdigitated bilayer at T less than Tm. In this structure, the long C(18)-acyl chain is interdigitated fully across the entire hydrocarbon width of the bilayer, while the shorter C(10)-acyl chain, which is about half as long as the C(18)-acyl chain, packs end to end with a C(10)-acyl chain of another lipid molecule in the opposing bilayer leaflet. We have synthesized the following asymmetric phosphatidylcholines (PC's): C(16):C(9)PC, C(16):C(10)PC, C(18):C(10)PC, C(18):C(11)PC, C(20):C(11)PC, C(20):C(12)PC, C(22):C(12)PC, C(22):C(13)PC, C(8):C(18)PC, and C(10):C(22)PC. These 10 asymmetric phosphatidylcholines have a common characteristic; i.e., the length of the longer extended acyl chain is about twice as long as that of the shorter acyl chain. On the basis of the known lamellar structure of C(18):C(10)PC, we anticipate that these asymmetric phosphatidylcholines will also form mixed interdigitated bilayers. We have employed high-resolution differential scanning calorimetry (DSC) to investigate the thermotropic behavior of liposomes prepared from these asymmetric phosphatidylcholines. If our anticipation is correct, one would find that the thermodynamic data (Tm, delta H, or delta S) associated with the main thermal phase transitions of these asymmetric phosphatidylcholine dispersions will fit into a continuous curve as they are plotted as a function of the hydrocarbon width of the putative mixed interdigitated bilayer. Experimental data presented in this paper indeed bear this out. For comparison, a DSC study of multilamellar dispersions prepared from a series of saturated symmetric phosphatidylcholines has also been carried out.(ABSTRACT TRUNCATED AT 250 WORDS)

Calorimetric and fatty acid spin label study of subgel and interdigitated gel phases formed by asymmetric phosphatidylcholines

Several saturated asymmetric and symmetric phosphatidylcholines were studied by ESR spectroscopy and differential scanning calorimetry in order to determine the behavior of a fatty acid spin labeled near its terminal methyl, 16-doxylstearate, in the mixed interdigitated gel phase and the L c subgel phase and other properties of these lipids. This spin label was motionally restricted in the mixed interdigitated gel phases of 18:10PC and 18:12PC. The motional restriction was similar to that reported earlier for fully interdigitated phases. This spin label was motionally restricted almost to the same degree in 10:18PC suggesting that this asymmetric lipid may also form a mixed interdigitated bilayer. In contrast the spin label had more motion in the gel phase of 18:14PC than in symmetric forms of PC, consistent with conclusions from X-ray diffraction studies that this less asymmetric lipid does not form a mixed interdigitated phase. The spin label was partially frozen out of the L c subgel phases of symmetric forms of PC and 18:14PC formed by storage at low temperature. The phase behavior of the other asymmetric lipids also depended on the sample history. Storage at low temperature caused 10:18PC and 18:12PC to go into ordered phases. The enthalpy of the transition of these ordered phases to the liquid-crystalline phase was 2–2.4-times greater than that of the transition of the gel phase formed on cooling back from the liquid-crystalline phase. The temperature of this high enthalpy transition was 0.8 K below that of the lower enthalpy get to liquid-crystalline phase transition for 18:12PC, but 4.6 K higher for 10:18PC. The spin label was frozen out of these ordered phases, as it was out of the L c subgel phases, suggesting that 18:12PC and 10:18PC may also form an L c phase. 18:10PC was not observed to form an ordered phase although storage of the sample at low temperatures did affect the temperature of its transition from the liquid-crystalline phase back to the gel phase upon cycling through its phase transition.

Acyl chain interdigitation in saturated mixed-chain phosphatidylcholine bilayer dispersions.

The molecular packing of various fully hydrated mixed-chain phosphatidylcholines was studied by X-ray diffraction and electron microscopy. All of the mixed-chain phosphatidylcholines under study were shown to adopt a lamellar or bilayer form in aqueous media. The bilayer thickness of these mixed-chain phosphatidylcholines was determined from the lamellar repeat distance in the small-anglé X-ray diffraction region by controlled swelling experiments. At T greater than Tm, the bilayer thickness of C(18):C(12)PC and C(18):C-(10)PC is found to be comparable to that of C(14):C(14)PC. In contrast, the bilayer thickness of these highly asymmetric phosphatidylcholines is considerably less than that of the symmetric C(14):C(14)PC at temperatures below Tm. Moreover, the wide-angle X-ray diffraction patterns taken at T less than Tm consist of at least two sharp reflections at 4.2 and 4.6 A. These X-ray diffraction data suggest that these highly asymmetric mixed-chain phospholipids, in excess water, form mixed interdigitated bilayers in the gel state and that the acyl chain packing in the gel-state bilayer is not hexagonal. The freeze-fracture planes of these mixed-chain phosphatidylcholines are discontinuous at T less than Tm, supporting the conclusion drawn from X-ray diffraction data that these highly asymmetric phosphatidylcholines form interdigitated bilayers at temperatures below Tm. The molecular packing of fully hydrated C(18):C(14)PCs in bilayers is distinctively different from that of C(18):C(10)PCs or C(18):C(10)PCs.(ABSTRACT TRUNCATED AT 250 WORDS)