Fluorine-19 nuclear magnetic resonance studies of lipid fatty acyl chain order and dynamics in Acholeplasma laidlawii B membranes. A direct comparison of the effects of cis and trans cyclopropane ring and double-bond substituents on orientational order

The hydrocarbon chain orientational order parameters of membranes of Acholeplasma laidlawii B, enriched with large quantities of fatty acids containing either a cis or a trans cyclopropane ring or a cis or trans double bond, plus small quantities of one of an isomeric series of monofluoropalmitic acids, were determined via fluorine-19 nuclear magnetic resonance spectroscopy over a range of temperatures spanning the corresponding gel to liquid-crystalline phase transitions (determined via differential scanning calorimetry). Membrane orientational order profiles in the liquid-crystalline state were generally similar, regardless of the particular fatty acid structure present, showing a region of relatively constant order preceding a region of progressively decreasing order toward the methyl terminus of the acyl chain. In the gel state, the order profiles in the presence of either a trans cyclopropane ring or trans double-bond substituent were similar and were characterized by a pronounced head to tail gradient of order at temperatures just below the lipid phase transition, while at temperatures far below the lipid phase transition this gradient was less pronounced, all chain positions showing a more uniformly high degree of orientational ordering. In the gel state, the order profiles in the presence of either a cis cyclopropane ring or a cis double-bond substituent were also similar but were highly unusual in that order first increased and only then subsequently decreased toward the acyl chain methyl terminus. In addition, the substituents in the cis configuration, whether a cyclopropane ring or a double bond, were overall more disordered in the gel state than the corresponding substituents in the trans configuration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Average structural and motional properties of a diunsaturated acyl chain in a lipid bilayer: effects of two cis-unsaturated double bonds

Isolinoleic acid (18:2 delta 6,9) deuterated at 10 different positions was esterified to form 1-palmitoyl-2-isolinoleoyl-sn-glycero-3-phosphocholine (PiLPC), and the average structural and motional properties of the diunsaturated chain, in aqueous dispersions of PiLPC, were examined by 2H NMR spectroscopy. For each sample, 2H spectra were acquired over a temperature range of 1-40 degrees C and the quadrupolar splittings interpreted in terms of carbon-deuterium bond order parameters, SCD. Furthermore, definition of the average orientation of the C8 methylene unit with respect to the bilayer normal [Baenziger, J. E., Smith, I. C. P., Hill, R. J., & Jarrell, H. C. (1988) J. Am. Chem. Soc. 110, 8229-8231] provided sufficient information to calculate both the average orientations and the molecular order parameters, Smol (which reflects the amplitudes of motion), for the C6-C7 and the C9-C10 double bonds. The results indicate that both the motional freedom (reflected in the order profile) and the average structure (reflected in the orientation of carbon segments with respect to the bilayer normal) are strongly affected by the presence of two cis-unsaturated double bonds. The data were interpreted in terms of two possible models whereby, in each case, the chain adopts a conformation consistent with the low-energy conformation of 1,4-pentadiene [Applegate, K. R., & Glomset, J. A. (1986) J. Lipid Res. 27, 658-680] but undergoes a two-site jump between the conformations. The jump motion arises mainly from rotations about the C7-C8 and the C8-C9 single bonds that disorder the C8 and the C9-C10 segments (Smol = 0.15 and 0.08, respectively) but leave the C6-C7 double bond relatively immobile (Smol = 0.55; all at 40 degrees C). It is suggested that acyl chains containing three or more double bonds could not undergo a similar jump motion and therefore would be highly ordered and not "fluid" as is generally thought.     

Effect of cholesterol on the bond ordering in hydrated unsaturated lipid bilayers

Molecular dynamics computer simulations of hydrated bilayers of unsaturated phosphatidylcholines in which double bonds are in the states: 18:0/18:1(n-9)cis (PC), 18:0/18:2(n-6)cis (PC), 18:0/18:3(n-3)cis (PC), 18:0/20:4(n-6)cis (PC), and 18:0/22:6(n-3)cis in the presence of cholesterol (40 mol%) and its absence have been performed. The simulation have been performed at 303 K and 1 atm, under the conditions corresponding to the experimentally observed liquid-crystalline state of the bilayer from phosphatidylcholine. The C-C and C-H bond order parameter profiles with respect to the bilayer normal and the C-C bond orientation distribution functions have been calculated. The widths of the functions and positions of their maxima have been determined. The dependence of these characteristics on the type of the bond, the degree of unsaturation of the chain, the presence of cholesterol in the bilayer, and the bond order parameters have been analyzed.     

Hydrocarbon chain packing and molecular motion in phospholipid bilayers formed from unsaturated lecithins. Synthesis and properties of sixteen positional isomers of 1,2-dioctadecenoyl-sn-glycero-3-phosphorylcholine.

Isomers of cis-octadecenoic acid, with the double bond in each position in the hydrocarbon chain, were used to synthesize the corresponding 1,2-diacyl-sn-glycero-3phosphorylcholines (lecithins). Differential thermal analysis of the lecithins, as a function of water content, permitted evaluation of the limiting transition temperature (Tc) of each isomer. Values of Tc plotted against double bond position fell on a smooth curve with a minimum at minus 22 degrees for the dioctadec-9'-enoyl compound. The presence of a "pretransition" endotherm in differential thermal analysis of 1,2-dioctadec-15'-and 1,2-dioctadec-16'-enoyl-sn-glycero-3-phosphorylcholine implies the existence of two beta crystalline forms. This was not observed with any of the other lecithins. Enthalpy and entropy data were then obtained from differential scanning calorimetry measurements. Values of delta H were lower (7.6 plus or minus 0.1 kcal mol- minus 1) when the center of unsaturation was near the middle of the hydrocarbon chain than they were (9.6 kcal mol- minus 1) when the center of unsaturation was close to either end of the chain. However, values of delta S showed no consistent variation with double bond position. Four positional isomers of 1-octadec-cis-enoyl-2-octadecanoyl-sn-glycero-3-phosphorylcholine were synthesized. With the double bond near the middle of the chain or close to the terminal group, the Tc values of the mixed acid lecithins were higher than those of the corresponding dioctadecenoyl lecithins. 13-C nuclear magnetic resonance relaxation measurements were used to obtain information about chain motion of selected 1,2-dioctadec-cis-enoyl-sn-glycero-3-phosphorylcholines at a temperature (52 degrees) above the Tc values. Spin-lattice relaxation times of the resolved resonances indicated that location of double bonds near the middle, as compared to either end, of the hydrocarbon chain favors enhanced molecular motion along the length of the chins and especially at the terminal methyl end. In the gel state, the minimum interaction potential energy of hydrocarbon chains in bilayers formed from dioctadecenoyl lipids appears to be minimized by localization of the double bond near the middle of the chains. It is suggested that in the case of homogeneous chains the double bond primarily affects the cooperativity of interactions and has very little steric effect on van der Waals' contacts. By contrast, in bilayers of mixed lecithins, with heterogeneous chains, the steric effect may become dominant, depending on double bond position. These differences in chain packing in the gel state are promulgated beyond the phase transition to the liquid crystalline state as an enhancement of chain motion as the temperature rises above Tc.     

Fluorine-19 nuclear magnetic resonance studies of lipid fatty acyl chain order and dynamics in Acholeplasma laidlawii B membranes. Gel-state disorder in the presence of methyl iso- and anteiso-branched-chain substituents

The hydrocarbon chain orientational order parameters of membranes of Acholeplasma laidlawii B enriched with large quantities of a linear saturated, a methyl iso-branched, or a methyl anteiso-branched fatty acid plus small quantities of various isomeric monofluoropalmitic acid probes were determined via fluorine-19 nuclear magnetic resonance spectroscopy (19F NMR) over a range of temperatures spanning the gel to liquid-crystalline phase transitions (determined by differential scanning calorimetry). Membrane orientational order profiles in the liquid-crystalline state were generally similar regardless of the particular fatty acyl structure, showing a region of relatively constant order preceding a region of progressive decline in order toward the methyl terminus of the acyl chain. In the gel state, the order profile of the linear saturated fatty acid enriched membranes was characteristically flat, with little head to tail gradation of order. In contrast, the methyl iso-branched and the methyl anteiso-branched enriched membranes exhibited a local disordering in the gel phase reflected in a very pronounced head to tail gradient of order, which remained at temperatures below the lipid phase transition. In addition, the methyl iso- and anteiso-branched fatty acid enriched membranes were overall more disordered than the membrane containing only linear saturated fatty acyl groups. Thus, at a constant value of reduced temperature below the lipid phase transition, overall order decreased in the progression 15:0 greater than 16:0i greater than 16:0ai, suggesting that these methyl-branched substituents lower the lipid phase transition by disrupting the gel phase lipid chain packing.(ABSTRACT TRUNCATED AT 250 WORDS)     

Raman spectroscopic study of polycrystalline mono- and polyunsaturated 1-eicosanoyl-d(39)-2-eicosenoyl-sn-glycero-3-phosphocholines: bilayer lipid clustering and acyl chain order and disorder characteristics

Polycrystalline lipid samples of a series of mono- and polyunsaturated, double bond positional isomers of 1-eicosanoyl-d(39)-2-eicosenoyl-sn-glycero-3-phosphocholines [C(20-d(39)):C(20:1 Delta(j))PC, with j = 5, 8, 11, or 13; C(20-d(39)):C(20:2 Delta(11,14))PC; and C(20-d(39)):C(20:3 Delta(11, 14,17))PC] were investigated using vibrational Raman spectroscopy to assess the acyl chain packing order-disorder characteristics and putative bilayer cluster formation of the isotopically differentiated acyl chains. Perdeuteration of specifically the saturated sn-1 acyl chains for these bilayer systems enables each chain's intra- and intermolecular conformational and organizational properties to be evaluated separately. Various saturated chain methylene CD(2) and carbon-carbon (C&bond;C) stretching mode peak height intensity ratios and line width parameters for the polycrystalline samples demonstrate a high degree of sn-1 chain order that is unaffected by either the double bond placement or number of unsaturated bonds within the sn-2 chain. In contrast, the unsaturated sn-2 chain spectral signatures reflect increasing acyl chain conformational disorder as either the cis double bond is generally repositioned toward the chain terminus or the number of double bonds increases from one to three. The lipid bilayer chain packing differences observed between the sn-1 and sn-2 chains of this series of monounsaturated and polyunsaturated 20 carbon chain lipids suggest the existence of laterally distributed microdomains predicated on the formation of highly ordered, saturated sn-1 chain clusters.     

A 2H-NMR analysis of dihydrosterculoyl-containing lipids in model membranes: structural effects of a cyclopropane ring

The molecular properties of lipid multilayers of 1-palmitoyl-2-[dideutero]dihydrosterculoyl-sn-glycero-3-phosphocholine (PDSPC) were investigated by means of 2H-NMR. The transition from the liquid-crystalline phase to a more highly ordered phase was found to take place between -10 degrees C and -15 degrees C. The temperature variation of the quadrupolar splittings of specifically dideuterated PDSPC molecules was analyzed in terms of 'segmental' and 'geometrical' order parameters: the lower half of the sn-2 chain (from the site of the cyclopropane ring to the terminal methyl group) was more conformationally disordered than the upper half. The apparently abnormal increase of the quadrupolar splitting of the pro-S deuteron at the C-2' position, with increasing temperature, was attributed to a change in the average orientation of that C-2H bond with respect to the axis of motion, resulting in an increase of the 'geometrical' order parameter, S gamma. The molecular order parameter matrix elements, Sij, of the cyclopropane ring were derived from the experimental SC-2H order parameters using similarity transformations. The matrix S was diagonalized and the molecular order parameter of the cyclopropane ring, Smol (or S*33), was determined by assuming axial symmetry for such matrices associated with molecules in a liquid-crystal medium. A value of Smol = 0.59 +/- 0.04 at 25 degrees C was thus calculated. This value represents a discontinuity in the positional dependence of the molecular order parameter for the sn-2 chain of PDSPC, indicating that the cyclopropane ring provides a rigid barrier separating the lipid bilayer into two regions: an ordered region from the bilayer surface to the site of the cyclopropane ring and a much more disordered region thereafter to the center of the bilayer.     

Interaction of pulmonary surfactant protein SP-A with DPPC/egg-PG bilayers

In the mixture of lipids and proteins which comprise pulmonary surfactant, the dominant protein by mass is surfactant protein A (SP-A), a hydrophilic glycoprotein. SP-A forms octadecamers that interact with phospholipid bilayer surfaces in the presence of calcium. Deuterium NMR was used to characterize the perturbation by SP-A, in the presence of 5 mM Ca²⁺, of dipalmitoyl phosphatidylcholine (DPPC) properties in DPPC/egg-PG (7:3) bilayers. Effects of SP-A were uniformly distributed over the observed DPPC population. SP-A reduced DPPC chain orientational order significantly in the gel phase but only slightly in the liquid-crystalline phase. Quadrupole echo decay times for DPPC chain deuterons were sensitive to SP-A in the liquid-crystalline mixture but not in the gel phase. SP-A reduced quadrupole splittings of DPPC choline β-deuterons but had little effect on choline α-deuteron splittings. The observed effects of SP-A on DPPC/egg-PG bilayer properties differ from those of the hydrophobic surfactant proteins SP-B and SP-C. This is consistent with the expectation that SP-A interacts primarily at bilayer surfaces.     

Simultaneous observation of order and dynamics at several defined positions in a single acyl chain using 2H NMR of single acyl chain perdeuterated phosphatidylcholines

Deuterium nuclear magnetic resonance (2H NMR) spectra from aqueous dispersions of phosphatidylcholines in which perdeuterated palmitic acid is esterified at the sn-1 position have several very useful features. The powder spectra show six well-resolved 90 degree edges which correspond to the six positions closest to the methyl end of the acyl chain. The spectral overlap inherent in the multiple powder pattern line shape of these dispersions can be removed by using a "dePaking" procedure [Bloom, M., Davis, J.H., & Mackay, A. (1981) Chem. Phys. Lett. 80, 198-202] which calculates the spectra that would result if the lipid bilayers were oriented in the magnetic field. This procedure produces six well-resolved doublets whose NMR properties can be observed without interference from the resonances of other labeled positions. The presence of a single double bond in the sn-2 chain increases the order of the saturated 16:0 sn-1 chain at every position in the bilayer compared with a saturated sn-2 chain at the same reduced temperature. Surprisingly, addition of five more double bonds to the sn-2 chain only slightly reduces the order of the 16:0 sn-1 chain at many positions in the bilayer compared with the single double bond. Calculating oriented spectra from a spin-lattice (T1) relaxation series of powder spectra allows one to obtain the T1 relaxation times of six positions on the acyl chain simultaneously. As an example of the utility of these molecules, we demonstrate that the dependence of the spin-lattice (T1) relaxation rate as a function of orientational order for two unsaturated phospholipids differs significantly from the corresponding fully saturated analogue. Interpreting this difference using current models of acyl chain dynamics suggests that the bilayers containing either of the two unsaturated phospholipids are significantly more deformable than bilayers made from the fully saturated phospholipid.     

Differential scanning calorimetry and (2)H nuclear magnetic resonance and Fourier transform infrared spectroscopy studies of the effects of transmembrane alpha-helical peptides on the organization of phosphatidylcholine bilayers

We have studied the effects of the incorporation of the alpha-helical transmembrane peptides Ac-K(2)-L(24)-K(2)-amide (L(24)) and Ac-K(2)-(L-A)(12)-K(2)-amide ((LA)(12)) on the thermotropic phase behavior of 1,2-dipalmitoyl-d(62)-sn-glycero-3-phosphocholine (DPPC-d(62)) and 1-palmitoyl-d(31)-2-oleoyl-sn-glycero-3-phosphocholine (POPC-d(31)) lipid bilayer model membranes by differential scanning calorimetry (DSC) and the conformational and orientational order of the phospholipid chains by Fourier transform infrared (FTIR) spectroscopy and (2)H nuclear magnetic resonance ((2)H-NMR) spectroscopy, respectively. Our DSC and FTIR spectroscopic studies indicate that the peptides L(24) and (LA)(12) both decrease the temperature and enthalpy of the gel/liquid-crystalline phase transition of DPPC-d(62) bilayers, with (LA)(12) having the greater effect in this regard. An examination of the frequencies of the CH(2) and CD(2) symmetric stretching bands of the infrared spectra of liquid-crystalline states of the peptide-free and peptide-containing DPPC-d(62) and POPC-d(31) samples, and a comparison with the orientational order as measured by (2)H-NMR spectroscopy as well as with the chain order as measured by electron spin resonance spectroscopy, lead us to conclude that the CH(2) (or CD(2)) stretching frequencies of lipid hydrocarbon chains are not a reliable measure of chain conformational order in lipid bilayers containing significant amounts of peptides or other lipophilic inclusions. In contrast, the results of our (2)H-NMR spectroscopic studies present a consistent picture in which both L(24) and (LA)(12) increased in a similar way the time-averaged orientational order of the lipid chains of their liquid-crystalline lipid bilayer hosts. The comparison of the effects L(24) and (LA)(12) on phosphatidylcholine bilayers indicates that the gel-to-liquid-crystalline phase transition appears to be more sensitive to small changes in transmembrane peptide surface topology than hydrocarbon carbon chain orientational order in the liquid-crystalline state.     

Influence of cis double bonds in the sn-2 acyl chain of phosphatidylethanolamine on the gel-to-liquid crystalline phase transition.

We have semisynthesized 19 species of mixed-chain phosphatidylethanolamines (PEs) in which the sn-1 acyl chain is derived from saturated fatty acids with varying chain lengths and the sn-2 acyl chain has different chain lengths but contains 0, 1, and 2 cis double bond(s). The gel-to-liquid crystalline phase transition temperatures (Tm) of lipid bilayers prepared from these 19 mixed-chain PEs were determined calorimetrically. When the Tm values are compared with those of saturated and monounsaturated counterparts, a common Tm profile is observed in the plot of Tm versus the number of cis double bonds. Specifically, a marked stepwise decrease in Tm is detected as the number of cis double bonds in the sn-2 acyl chain of the mixed-chain PE is successively increased from 0 to 1 and then to 2. The large Tm-lowering effect of the acyl chain unsaturation can be attributed to the increase in Gibbs free energy of the gel-state bilayer as a result of weaker lateral chain-chain interactions. In addition, we have applied molecular mechanics calculations to simulate the molecular structure of dienoic mixed-chain C(X):C(Y:2 delta n,n+3)PE in the gel-state bilayer, thus enabling the three independent structural parameters (N, delta C, and LS) to be calculated in terms of X, Y, and n, which are intrinsic quantities of C(X):C(Y:2 delta n,n+3)PE. When the Tm values and the corresponding N and delta C values of all dienoic mixed-chain PEs under study are first codified and then analyzed statistically by multiple regressions, the dependence of Tm on the structural parameters can be described quantitatively by a simple and general equation. The physical meaning and the usefulness of this simple and general equation are explained.     

Modulation of phospholipid acyl chain order by cholesterol. A solid-state 2H nuclear magnetic resonance study

The effect of cholesterol on the acyl chain order of three glycerophosphocholines with 14, 16, and 18 carbons per acyl chain, namely, di(14:0)PC, di(16:0)PC, and di(18:0)PC, above the gel to liquid-crystalline phase transition temperature was investigated by using 2H nuclear magnetic resonance spectroscopy. Average acyl chain lengths were calculated from the segmental order parameters (Smol) for the sn-1 and the sn-2 chains in the absence of cholesterol and at 3:1, 2:1, and 1:1 mole ratios of phospholipid-cholesterol. The three binary mixtures of cholesterol with phosphatidylcholines are in the liquid-ordered (lo) phase. For all the three phosphatidylcholine-cholesterol systems, the distance from the carbonyl groups to the terminal methyl groups is shorter than the length of the cholesterol molecule. A molecular model for the lo phase consistent with these observations has in a statistical sense a part of each cholesterol molecule in one monolayer extending into the other monolayer. This results in a packing arrangement akin to that in interdigitated systems. On the basis of the effect of cholesterol on phospholipid acyl chain orientational order, it is suggested that the liquid-disordered (ld) phase at low cholesterol concentrations corresponds to a packing mode in which the cholesterol molecule spans the entire transbilayer hydrophobic region. A molecular mechanism is proposed in which increasing the concentration of cholesterol has the effect of stretching the acyl chains of phospholipids by increasing the population of trans conformers up to a stage where the hydrophobic length is considerably longer than the cholesterol molecule. Beyond this concentration, the partially interdigitated phase forms.(ABSTRACT TRUNCATED AT 250 WORDS)     

A computer investigation of intramolecular bond ordering: unsaturated chains of natural lipids

Computer simulations (by the Monte Carlo method) of unperturbed linear hydrocarbon chains of 18-22 carbon atoms with methylene-interrupted cis-double bonds (18:0, 20:0, 22:0, 18:1delta11cis, 18:2delta9, 12cis, 18:3delta9, 12, 15cis, 20:3delta5, 8, 11cis, 20:4delta5, 8, 11, 14cis, 20:5delta5, 8, 11, 14, 17cis, 22:6delta4, 7, 10, 13, 16, 19cis), typical components of natural lipids, at a temperature of 298 K have been carried out. The conformations generated with continuous variation of all single C-C bond rotation angles within the (0, 360 degrees) range have been considered. The energy of nonbonded interactions and torsion and electrostatic terms have been taken into account. The intramolecular bond order parameters S(CC) and S(CH) about the axes along inertia tensor eigenvectors and bond orientation distributions rho(theta) with respect to the maximum molecule span axis (theta is the angle between the bond and the axis) have been calculated. The relation of the bond orientation distributions rho(theta) to the order parameters S are analyzed in terms of angles thetamax (a "geometric" factor, rho(thetamax) = max) and widths deltatheta of the distributions (factor of "fluctuations"). The results indicate that fluctuation factors depend on both the segment chemical structure and location in the chain; fluctuations increase from the centre of the chain towards the terminals, all things being equal. The two deltatheta values of C-H bonds flanking the cis-double bond are smaller than that obtained for adjacent CH2 groups by a factor of 1.5-2. Defining these properties is a necessary step to gaining a more complete understanding of polyunsaturated lipid hydrocarbon chains significance. The mean molecule magnitudes of ?S(CH)? decrease when unsaturation increases. The cis-double bond parameters S(CC) are found to be higher than those of adjacent single C-C bonds: the parameter S(CC) odd-even effect in the polyunsaturated molecules of such structure changes the "sign" between double bonds. The order parameter profiles -S(CH) of cis-18:1 and cis-18:2 obtained from the simulations (at the portion which corresponds to the double bonds location) are in qualitative agreement with experimental data on bilayers in the liquid-crystal phase. This has made possible the quantitative prognosis of the ordering properties of experimentally uninvestigated unsaturated lipids.     

NMR study of synthetic lecithin bilayers in the vicinity of the gel-liquid--crystal transition.

1H, 2H, and 31P NMR methods have been employed in the study of dimyristoyl lecithin bilayers hydrated with D2O in the gel (L beta'), intermediate (P beta') and liquid-crystalline (L alpha) phases. For D2O/lipid molar ratios, n, in the range 7 less than or equal to n less than or equal to 11 discontinuities are observed in the deuterium NMR splittings at both main and pretransitions. A partial phase diagram based on NMR and differential scanning calorimetry data is presented. 1H NMR dipolar splittings are observed for macroscopically oriented samples in all three phases. Changes in the 1H splittings are correlated with 2H and 31P data and interpreted to show that the chain tilt in the gel phase undergoes a discontinuous change on transition to the intermediate phase, which brings the chain axes closer to the bilayer normal. An estimate of chain tilt in the gel phase is made on the basis of NMR data and found to be approximately 23 degrees for a sample with n = 11 at 18 degrees C.     

Glycosphingolipid acyl chain orientational order in unsaturated phosphatidylcholine bilayers.

The glycosphingolipid, galactosyl ceramide (GalCer), was studied by 2H nuclear magnetic resonance (NMR) in fluid phospholipid bilayer membranes, with regard to arrangement of its acyl chain. For this purpose, species with perdeuterated 18-carbon fatty acid (18:0[d35]GalCer) or with perdeuterated 24-carbon fatty acid (24:0[d47] GalCer) were dispersed in bilayers of the 18-carbon phospholipid, 1-stearoyl-2-oleoyl-phosphatidylcholine (SOPC). For 18:0[d35] GalCer, smoothed profiles of the order parameter, SCD, were found to be very similar to one another over the range of glycolipid concentration, 5-40 mol%. In addition, they were very similar to orientational order parameter profiles well known from the literature on phospholipid and glycolipid acyl chains (which deals in general with membranes of homogeneous chain length in the range 14-18 carbons). Corresponding order parameter profiles for the long-chain species, 24:0[d47] GalCer, were also similar to one another for glycolipid concentrations between 5 and 40 mol%. Their shapes, however, were distinctly different from those of the shorter chain analogues. SCD profiles for the two species were quantitatively similar to a membrane depth of C15. SCD values at C16 and C17 were approximately 20 and 30%, respectively, higher for the long-chain glycosphingolipid than for its short-chain analogue in SOPC. Nitroxide spin labels attached rigidly to C16 of the long-chain glycolipid in SOPC gave electron paramagnetic resonance (EPR) order parameters that were twice as high as for a spin label at C16 on the shorter chain glycolipid. Comparison was made between spectra of 24:0[d47] GalCer in SOPC and fully hydrated bilayers of the pure 24:0[d47] GalCer, a system that is considered to be partially interdigitated in fluid and gel phases. The resultant 2H NMR order parameter profiles displayed similar features, indicating that related organizational properties exist in these fluid systems. Effective chain length of 24:0[d47] GalCer within the SOPC membrane was calculated using the method of Schindler and Seelig (1975. Biochemistry, 14:2283-2287). The result suggested that the long-chain fatty acid should protrude roughly one third of the host matrix chain length across the bilayer midplane. However, a treatment of the same order parameters making very few assumptions about chain conformation indicated a high degree of orientational flexibility for the "extra" length of the long chain fatty acid.(ABSTRACT TRUNCATED AT 400 WORDS)     

Studies of highly asymmetric mixed-chain diacyl phosphatidylcholines that form mixed-interdigitated gel phases: Fourier transform infrared and 2H NMR spectroscopic studies of hydrocarbon chain conformation and orientational order in the liquid-crystalline state.

Hydrocarbon chain conformational and orientational order in liquid-crystalline bilayers of the highly chain-asymmetric 1-O-eicosanoyl, 2-O-dodecanoyl and 1-O-decanoyl, 2-O-docosanoyl phosphatidylcholines were studied by Fourier transform infrared (FTIR) and deuterium nuclear magnetic resonance (2H-NMR) spectroscopy, respectively, and compared with appropriate symmetric-chain phosphatidylcholines at comparable reduced temperatures. FTIR spectroscopy indicates that these two asymmetric-chain phospholipids contain a slightly greater number of kink, a considerably larger number of double-gauche, but a somewhat smaller number of end-gauche conformers than does dipalmitoylphosphatidylcholine, a symmetric-chain phospholipid having the same total number of carbon atoms in its hydrocarbon chains. Moreover, the asymmetric-chain phospholipids also contain a larger total number of gauche conformers, suggesting that their hydrocarbon chains are more disordered overall than are those of dipalmitoylphosphatidylcholine. 2H-NMR studies of the specifically chain-perdeuterated analogs of these asymmetric-chain lipids reveal that the orientational order parameter profiles of their shorter and longer chains differ both qualitatively and quantitatively, regardless of whether they are esterified at the sn1- or sn2 positions of the glycerol molecule. The longer hydrocarbon chains exhibit unusual orientational order profiles in which the order gradient is steepest in the middle of the chain and relatively shallower in regions adjacent to the carboxyl and methyl termini, whereas the short hydrocarbon chains exhibit orientational order profiles typical of those commonly observed with conventional symmetric chain lipids. When compared at equivalent depths in the bilayer, the shorter hydrocarbon chains of the asymmetric-chain lipids are more orientationally disordered than are their longer chain counterparts. At comparable reduced temperatures, the shorter and longer chains of the asymmetric-chain lipids are more orientationally disordered than those of appropriate short and long symmetric-chain lipids, but the chain-averaged orientational order of the symmetric-chain lipid decreases more sharply with increases in temperature than does that of the comparable chain of the asymmetric-chain species. Moreover, the order plateau regions adjacent to the carboxyl groups of the longer chains of the asymmetric-chain phosphatidylcholines are shorter than those of symmetric-chain lipids of comparable hydrocarbon chain length. Overall, the data indicate that the conformational and orientational order in the liquid-crystalline states of these highly asymmetric-chain lipids differ significantly from those of comparable symmetric-chain lipids. Also, the unusual shape of the orientational order profile of the longer chains of the former is attributed to interaction between the methyl termini regions of the long chains with hydrocarbon chains in opposing monolayers.(ABSTRACT TRUNCATED AT 400 WORDS)     

Interaction modes of long-chain fatty acids in dipalmitoylphosphatidylcholine bilayer membrane: contrast to mode of inhalation anesthetics

The effects of long-chain fatty acids (four saturated and two unsaturated fatty acids, one derivative) on phase transitions of dipalmitoylphosphatidylcholine (DPPC) bilayer membranes were examined in the low concentration region, and the results were compared with those for an inhalation anesthetic. The effects of all fatty acids on the pre- and main-transition temperatures of the DPPC bilayer membrane appeared in the concentration range of μM order while that of the anesthetic appeared in the mM order. The appearance modes of these ligand actions were significantly different from one another. The three differential partition coefficients of the ligands between two phases of the DPPC bilayer membrane were evaluated by applying the thermodynamic equation to the variation of the phase-transition temperatures. The DPPC bilayer membranes showed the different receptivity for the ligands; the saturated fatty acids had an affinity for gel phase whereas unsaturated fatty acids and an anesthetic had an affinity for liquid-crystalline phase to the contrary. In particular, the receptivity for the ligands in the gel phase markedly changed depending on kinds of ligands. The interaction modes between the DPPC and fatty acid molecules in the gel phase were considered from the hexagonal lattice model. The disappearance compositions of the pretransition by the fatty acids coincided with the compositions at which the membrane is all covered by the units in each of which two fatty acids molecules are regularly distributed in the hexagonal lattice in a different way, and the distribution depended on the chain length and existence of a double bond for the fatty acids. The interpretation did not hold for the case of the anesthetic at all, which proved that a number of anesthetic molecules act the surface region of the bilayer membrane nonspecifically. The present study clearly implies that DPPC bilayer membranes have high ability to recognize kinds of ligand molecules and can discriminate among them with specific interaction by the membrane states.     

Molecular motions and dynamics of a diunsaturated acyl chain in a lipid bilayer: implications for the role of polyunsaturation in biological membranes.

The nature and dynamics of the motions of a diunsaturated fatty acyl chain in a lipid bilayer were examined using a comprehensive simulation program for 2H NMR line shapes developed by Wittebort et al. [Wittebort, R. J., Olejniczak, E. T., & Griffin, R. G. (1987) J. Chem. Phys. 36, 5411-5420]. A motional model in which the isolinoleoyl chain (18:2 delta 6,9) adopts two conformations consistent with the low energy structures proposed for 1,4-pentadiene [Applegate, K. R., & Glomset, J. A. (1986) J. Lipid Res. 27, 658-680], but undergoes a rapid jump between these states, is sufficient to account for the experimentally observed quadrupolar couplings, the 2H-2H and 1H-2H dipolar couplings, the longitudinal relaxation times, and the changes in the average conformation of the chain that occur with a variation in temperature. The jump motion originates via rotations about the C7-C8 and the C8-C9 carbon bonds and leads to the low order parameters assigned to the C8 methylene segment (0.18) and the C9-C10 double bond (0.11). In contrast, the C6-C7 double bond, which is not involved in the two-site jump, characterized by a relatively large order parameter (0.56). Fatty acyl chains containing three or more double bonds likely cannot undergo the same jump motion and consequently will be highly ordered structures. Correlation times for diffusion of the molecular long axis of the diunsaturated acyl chain about the bilayer normal (approximately 10(-10) s) and for the local jump motion (approximately 10(-10) s) were calculated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction of ceramides with phosphatidylcholine, sphingomyelin and sphingomyelin/cholesterol bilayers

Ceramides (Cers) may exert their biological activity through changes in membrane structure and organization. To understand this mechanism, the effect of Cer on the biophysical properties of phosphatidylcholine, sphingomyelin (SM) and SM/cholesterol bilayers was determined using fluorescence probe techniques. The Cers were bovine brain Cer and synthetic Cers that contained a single acyl chain species. The phospholipids were 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1,2-dipalmitoyl-sn-glyero-3-phosphocholine (DPPC) and bovine brain, egg yolk and bovine erythrocyte SM. The addition of Cer to POPC and DPPC bilayers that were in the liquid-crystalline phase resulted in a linear increase in acyl chain order and decrease in membrane polarity. The addition of Cer to DPPC and SM bilayers also resulted in a linear increase in the gel to liquid-crystalline phase transition temperature (T(M)). The magnitude of the change was dependent upon Cer lipid composition and was much higher in SM bilayers than DPPC bilayers. The addition of 33 mol% cholesterol essentially eliminated the thermal transition of SM and SM/Cer bilayers. However, there is still a linear increase in acyl chain order induced by the addition of Cer. The results are interpreted as the formation of DPPC/Cer and SM/Cer lipid complexes. SM/Cer lipid complexes have higher T(M)s than the corresponding SM because the addition of Cer reduces the repulsion between the bulky headgroup and allows closer packing of the acyl chains. The biophysical properties of a SM/Cer-rich bilayer are dependent upon the amount of cholesterol present. In a cholesterol-poor membrane, a sphingomyelinase could catalyze the isothermal conversion of a liquid-crystalline SM bilayer to a gel phase SM/Cer complex at physiological temperature.     

Combined influence of cholesterol and synthetic amphiphillic peptides upon bilayer thickness in model membranes.

Deuterium (2H) NMR was used to study bilayer hydrophobic thickness and mechanical properties when cholesterol and/or synthetic amphiphillic polypeptides were added to deuterated POPC lipid bilayer membranes in the liquid-crystalline (fluid) phase. Smoothed acyl chain orientational order profiles were used to calculate bilayer hydrophobic thickness. Addition of 30 mol% cholesterol to POPC at 25 degrees C increased the bilayer thickness from 2.58 to 2.99 nm. The peptides were chosen to span the bilayers with more or less mismatch between the hydrophobic peptide length and membrane hydrophobic thickness. The average thickness of the pure lipid bilayers was significantly perturbed upon addition of peptide only in cases of large mismatch, being increased (decreased) when the peptide hydrophobic length was greater (less) than that of the pure bilayer, consistent with the "mattress" model of protein lipid interactions (Mouritsen, O.G., and M. Bloom. 1984. Biophys. J. 46:141-153). The experimental results were also used to examine the combined influence of the polypeptides and cholesterol on the orientational order profile and thickness expansivity of the membranes. A detailed model for the spatial distribution of POPC and cholesterol molecules in the bilayers was proposed to reconcile the general features of these measurements with micromechanical measurements of area expansivity in closely related systems. Experiments to test the model were proposed.