Influence of phosphatidylcholine/cholesterol molar ratios in liposomes on cholesterol reactivity with cholesterol:oxygen oxidoreductase]

The reactivity of sonicated phosphatidylcholine-cholesterol liposomes with cholesterol : oxygene oxydoreductase, an enzyme which catalyses the oxidation of the 3 beta hydroxyl group of cholesterol to a ketone group, is compared with that of ternary system phosphatidylcholine-cholesterol-Thesit. Regardless to the phosphatidylcholines nature and the phosphatidylcholine/cholesterol molar ratio (R), the enzymatic oxidation rate of liposomal cholesterol is slower than when the reaction is developed in the present of Thesit, a surfactif agent which destroyes the lamellar particles. This is true whether Thesit is added during preparation of dispersions or during incubation with cholesterol oxydase. The enzymatic oxydation rate of cholesterol of ternary systems phosphatidylcholine-cholesterol-Thesit is independent of the (R) value and the phosphatidylcholine fatty acid unsaturation, whereas that of phosphatidylcholine-cholesterol dispersions depends on these two parameters. The reaction rate increases in the order: dipalmitoylphosphatidylcholine to yolk egg phosphatidylcholines, and dioleylphosphatidylcholine. The optimal conditions for cholesterol oxidation were found to be R = 0.5. This result is not affected by the phosphatidylcholines nature. In order to explain these data, various hypotheses are considered. In particular, the weak liposomal cholesterol reactivity with cholesterol oxidase could result from an inhibitory effect on the enzyme-substrate combination due to the polar phosphorylcholine groups.     

Trifluoroethanol and binding to model membranes stabilize a predicted turn in a peptide corresponding to the first extracellular loop of the angiotensin II AT(1A) receptor

Deuterium oxide solutions of a triple-helical polysaccharide schizophyllan, undergoing an order-disorder transition centered around 17 degrees C, were studied by the time-domain reflectometry (TDR) to obtain dielectric dispersions in the solution and frozen states. In the solution state, the dispersion below the transition temperature is resolved in three dispersions (relaxation times at 0 degrees C) ascribed to side chain glucose residue (1; 102 ns), structured water (s; 2.0 ns) and bulk water (h), respectively, from low to high frequencies. Bulk water is divided into slow water (h2; 0.04 ns) and free or pure water (h1; 0.02 ns). Above the transition temperature structured water almost disappears and is compensated by slow water. Structured water is similar to bound water for proteins but different from it because of this transition behavior. Another dispersion (l) seen at the lowest frequency is assigned to the rotation of side-chain glucose residue coupled with hydrated water. Parts of this dispersion and structured water are suggested to constitute bound water. In the frozen state were observed a major dispersion (h; 0.14 ns) and a minor one (m; 28 ns), which were ascribed to considerably mobile and less mobile waters. They are similar to but not exactly the same as that for unfreezable water in bovine serum albumin solutions argued by Miura et al. (Biopolymers, 1995, Vol. 36, p. 9). Water is molded into different structures by the triple helix.     

Differential scanning calorimetric study of a homologous series of fully hydrated saturated mixed-chain C(X):C(X + 6) phosphatidylcholines

The successive high-resolution differential scanning calorimetric (DSC) thermograms for aqueous dispersions of a homologous series of mixed-chain phosphatidylcholines, C(X):C(X + 6)PC, have been recorded and analyzed. In this series of saturated mixed-chain phosphatidylcholines, the total number of carbon atoms in the sn-1 acyl chain increases from 11 to 20, and the sn-2 acyl chain is always 6 methylene units longer than the sn-1 acyl chain. In the initial heating DSC thermograms, two prominent endothermic transitions are detected for all the samples prepared from the various C(X):C(X + 6)PCs except C(12):C(18)PC. In contrast, a single exothermic transition is observed on cooling for all the samples except C(13):C(19)PC. The temperature difference between the two endothermic transitions increases linearly as the acyl chain length of C(X):C(X + 6)PC becomes progressively longer. Interestingly, the main phase transition occurs before the subtransition for C(11):C(17)PC dispersions. Our DSC data further demonstrate that the thermodynamic parameters (Tm, delta H, and delta S) associated with the main phase transition for fully hydrated C(13):C(19)PC and other identical MW phosphatidylcholines are inversely related to the corresponding values of the chain-length inequivalence (delta C/CL) for these lipids. This linear relationship can be employed to map the Tm values for aqueous dispersions prepared from a large number of mixed-chain phosphatidylcholines whose values of delta C/CL are within the range of 0.1-0.4.     

Headgroup hydration and motional order of lipids in lamellar liquid crystalline and inverted hexagonal phases of unsaturated phosphatidylethanolamine--a time-resolved fluorescence study

By the use of frequency domain cross-correlation fluorometry, the fluorescence lifetime of the water soluble probe 8,1-anilinonapthalene sulfonic acid (ANS) in aqueous dispersions of dioleoylphosphatidylethanolamine (DOPE) and phosphatidylethanolamine transphosphatidylated from egg phosphatidylcholine (TPE) was measured. The orientational order parameter and rotational diffusion constant of the lipophilic probe 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) were also determined in TPE dispersions. In agreement with a previous study on DOPE (Cheng (1989) Biophys. J. 55, 1025-1031), abrupt changes in both the order packing and rotational diffusion constant were found at the lamellar liquid crystalline (L alpha) to inverted hexagonal (HII) phase transition of TPE. Owing to the subnanosecond resolution capability of this frequency domain fluorometric technique, the heterogeneous fluorescence decay of ANS was resolved into three distinct components with different decay lifetimes (tau's). They were 0 less than tau less than 0.5 ns, 2 less than tau less than 9 ns and tau greater than 15 ns. These lifetime regions were attributed to the partitioning of ANS into the bulk aqueous medium, the lipid/water interface and the lipid hydrocarbon region, respectively. These classifications of lifetime regions were further supported by the sensitivity of those lifetime components with the solvent isotopic shift of D2O. Similar to the changes of orientational order and rotational diffusion of lipophilic probe, the lifetime and intensity fraction of ANS associated with the lipid/water interfacial region declined abruptly at the L alpha-HII transition of both DOPE and TPE. This observation suggested that a dehydration of the lipid headgroup surface occurs at the L alpha-HII transition. This study provided evidence that both the lipid headgroup surface hydration and the lipid dynamics change drastically as a result of the macroscopic rearrangement of lipids at the L alpha-HII transition.     

Lipid chain length effect on the phase behaviour of PCs/PEG:2000-PEs mixtures. A spin label electron spin resonance and spectrophotometric study

Spin-label electron spin resonance (ESR) spectroscopy and spectrophotometry at fixed wavelength are used to study fully hydrated aqueous dispersions of phosphatidylcholines (PCs) with poly(ethylene glycol:2000)-phosphatidylethanolamines (PEG:2000-PEs). PEG:2000-PE is a micelle-forming polymer-lipid that is extensively used for increasing the lifetime of PC liposomes in the blood circulation through a steric stabilisation effect. The PC lipids and the PEG:2000-PE polymer-lipids have the same acyl chain length of either dimiristoyl (DM) or distearoyl (DS) chains. DMPC/PEG:2000-DMPE and DSPC/PEG:2000-DSPE mixtures were investigated over the entire range of relative compositions (0-100 mol%). In both dispersions, the low-temperature conventional spin label ESR spectra and the temperature dependence of the absorbance at 400 nm give an indication of the conversion from lamellae to micelles with increasing PEG:2000-PEs content. The physical state of the lipid assemblies, lamellar or micellar, is dependent not only on PEG:2000-PEs content, but also on the length of hydrocarbon chain of the lipid matrix. Micellisation is attained more readily in dispersions with longer hydrocarbon chains (i.e. in DSPC/PEG:2000-DSPE mixtures) than in those with shorter acyl chains (i.e. in DMPC/PEG:2000-DMPE mixtures). Saturation transfer ESR (ST-ESR) and absorbance measurements reflect the disaggregation of the bilayers and a reduction in the size of the lipid aggregates by PEG:2000-PEs at low content.     

Thermotropic phase behavior of model membranes composed of phosphatidylcholines containing iso-branched fatty acids. 1. Differential scanning calorimetric studies

The thermotropic phase behavior of aqueous dispersions of phosphatidylcholines containing one of a series of methyl iso-branched fatty acyl chains was studied by differential scanning calorimetry. These compounds exhibit a complex phase behavior on heating which includes two endothermic events, a gel/gel transition, involving a molecular packing rearrangement between two gel-state forms, and a gel/liquid-crystalline phase transition, involving the melting of the hydrocarbon chains. The gel to liquid-crystalline transition is a relatively fast, highly cooperative process which exhibits a lower transition temperature and enthalpy than do the chain-melting transitions of saturated straight-chain phosphatidylcholines of similar acyl chain length. In addition, the gel to liquid-crystalline phase transition temperature is relatively insensitive to the composition of the aqueous phase. In contrast, the gel/gel transition is a slow process of lower cooperativity than the gel/liquid-crystalline phase transition and is sensitive to the composition of the bulk aqueous phase. The gel/gel transitions of the methyl iso-branched phosphatidylcholines have very different thermodynamic properties and depend in a different way on hydrocarbon chain length than do either the "subtransitions" or the "pretransitions" observed with linear saturated phosphatidylcholines. The gel/gel and gel/liquid-crystalline transitions are apparently concomitant for the shorter chain iso-branched phosphatidylcholines but diverge on the temperature scale with increasing chain length, with a pronounced odd/even alternation of the characteristic temperatures of the gel/gel transition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thermotropic and mixing behavior of mixed-chain phosphatidylcholines with molecular weights identical with that of 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)     

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.     

Phase diagrams of pseudo-binary phospholipid systems. II. Selected calorimetric studies on the influence of branching on the mixing properties of phosphatidylcholines

The miscibility properties of branched phosphatidylcholines in mixtures of aqueous dispersions were studied by means of differential scanning calorimetry. The phase diagrams of four pseudo-binary systems from mixing type unbranched phosphatidylcholine/branched phosphatidylcholine/water (50 wt. % water) were investigated and discussed. The unbranched dipalmitoylphosphatidylcholine acts as a reference component of the mixtures. The phase diagrams of these four pseudo-binary phosphatidylcholine systems showed some connections between chain structure of the branched phosphatidylcholines and miscibility of the components. A change of the phase diagram type has been observed according to the branching and/or chain length differences of the phosphatidylcholines: complete miscibility and peritectic mixing behaviour. Generally we observed complete miscibility in the high-temperature phase (La-phase) and demixing in the low-temperature phases (gel phase). This is dependent on the branching and chain length differences of the mixing components.     

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)     

Correlation between internal motion and emission kinetics of tryptophan residues in proteins

Time-resolved fluorescence anisotropy measurements of tryptophan residues were carried out for 44 proteins. Internal rotational motion with a sub-nanosecond correlation time (0.9 +/- 0.6 ns at 10 degrees C) was seen in a large number of proteins, though its amplitude varied from protein to protein. It was found that tryptophan residues which were almost fixed within a protein had either a long (greater than 4 ns) or short (less than 2 ns) fluorescence lifetime, whereas a residue undergoing a large internal motion had an intermediate lifetime (1.5-3 ns). It is suggested that the emission kinetics of a tryptophan residue is coupled with its internal motion. In particular, an immobile tryptophan residue emitting at long wavelength was characterized by a long lifetime (greater than 4 ns). It appears that a tryptophan residue fixed in a polar region has little chance of being quenched by neighboring groups.     

Thermal phase transitions in deuterated lecithin bilayers.

Differential Thermal Analyses and Optical Density measurements show that perdeuteration of the fatty acid residues in phosphatidylcholines causes a 4–5°C decrease in the phase transition temperatures of bilayer dispersions prepared from these deuterated phospholipids. The implications of these findings on the use of deuterated phospholipids in membrane research will be briefly discussed.     

Temperature-induced phase transitions in proteins and lipids. Volume and heat capacity effects

The partial specific heat capacity and volume of globular proteins and dispersions of phosphatidylcholines in aqueous solutions have been determined over a broad temperature range using a precise scanning microcalorimeter and a vibrational densimeter. It is shown that the temperature-induced, gel-to-liquid crystalline phase transition in phosphatidylcholines proceeds without a noticeable change in heat capacity but with a significant increase in the specific volume, whereas heat denaturation in proteins takes place without a noticeable change in the volume but with a significant increase in heat capacity. This principal difference between temperature-induced conformational phase transitions in proteins and lipids demonstrates clearly that heat denaturation of proteins, in contrast to the gel-to-liquid crystalline phase transition in lipids, cannot be regarded as a process similar to melting. Consequently, the 'molten globule' does not appear to be a suitable model for a heat-denatured protein.     

Thermodynamics of interdigitated phases of phosphatidylcholine in glycerol.

Comparison of the electron spin resonance spectra of phosphatidylcholines spin-labeled in the sn-2 chain at a position close to the polar region and close to the methyl terminus indicate that symmetrical saturated diacyl phosphatidylcholines with odd and even chain lengths from 13 to 20 C-atoms (and probably also 12 C-atoms) have gel phases in which the chains are interdigitated when dispersed in glycerol. The chain-length dependences of the chain-melting transition enthalpies and entropies are similar for phosphatidylcholines dispersed in glycerol and in water, but the negative end contributions are smaller for phosphatidylcholines dispersed in glycerol than for those dispersed in water: d delta Ht/dCH2 = 1.48 (1.43) kcal.mol-1, d delta St/dCH2 = 3.9 (4.0) cal.mol-1K-1, and delta H o = -12.9 (-15.0) kcal.mol-1, delta S o = -29 (-40) cal.mol-1K-1, respectively, for dispersions in glycerol (water). These differences reflect the interfacial energetics in glycerol and in water, and the different structure of the interdigitated gel phase.     

Fluorescence lifetime distributions of diphenylhexatriene-labeled phosphatidylcholine as a tool for the study of phospholipid-cholesterol interactions.

Fluorescence lifetimes of 1-palmitoyl-2-diphenylhexatrienylpro-pionyl-phosphatidylc hol ine in vesicles of palmitoyloleoyl phosphatidylcholine (POPC) (1:300, mol/mol) in the liquid crystalline state were determined by multifrequency phase fluorometry. On the basis of statistic criteria (chi 2red) the measured phase angles and demodulation factors were equally well fitted to unimodal Lorentzian, Gaussian, or uniform lifetime distributions. No improvement in chi 2red could be observed if the experimental data were fitted to bimodal Lorentzian distributions or a double exponential decay. The unimodal Lorentzian lifetime distribution was characterized by a lifetime center of 6.87 ns and a full width at half maximum of 0.57 ns. Increasing amounts of cholesterol in the phospholipid vesicles (0-50 mol% relative to POPC) led to a slight increase of the lifetime center (7.58 ns at 50 mol% sterol) and reduced significantly the distributional width (0.14 ns at 50 mol% sterol). Lifetime distributions of POPC-cholesterol mixtures containing greater than 20 mol% sterol were within the resolution limit and could not be distinguished from monoexponential decays on the basis of chi 2red. Cholesterol stabilizes and rigidifies phospholipid bilayers in the fluid state. Considering its effect on lifetime distributions of fluorescent phospholipids it may also act as a membrane homogenizer.     

Chlorophyll organization in dark-grown and light-grown pine (Pinus brutia) and barley (Hordeum vulgare)

A study was conducted comparing the organization of chlorophyll during development of the photosynthetic apparatus in dark-grown and light-grown pine and barley. The rationale was that gymnosperms, but not angiosperms, have a capacity to synthesize chlorophyll in darkness. Seedlings of Pinus brutia were germinated and grown in darkness or under photoperiodic (day/night) conditions. The low-temperature (77 K) fluorescence spectra of newly-emerging dark-grown seedlings exhibited a single fluorescence band peaking at 678–679 nm, which decayed primarily with a ∼5.5 ns lifetime. Over the first few days of growth, the emission shifted to longer wavelengths and a subnanosecond lifetime component became prevalent. After several days of dark growth the emission spectrum and lifetime profile of the low temperature fluorescence came to resemble those of light-grown pine and barley. At room temperature, dark-grown pine showed little variable fluorescence, though addition of DCMU caused a substantial fluorescence rise. Illumination with moderate light for a few hours was sufficient to 'photoinduce' the appearance of normal variable fluorescence. At 77 K, DCMU-treated samples clearly showed a very long-lived (∼40 ns) fluorescence lifetime component in light-grown pine and barley. This component was undetectable in dark-grown pine. If, however, dark-grown samples were illuminated either before or after DCMU addition and then frozen to 77 K, the ∼40 ns lifetime component appeared at a fluorescence intensity similar to that in light-grown samples. These results are explained primarily in terms of photoactivation of the photosystem II (PSII) donor side. The temporary maintenance of PSII in an inactive, highly-quenched state is suggested to provide an available, yet protected precursor for active PSII.     

Calorimetric and spectroscopic studies of the polymorphic phase behavior of a homologous series of n-saturated 1,2-diacyl phosphatidylethanolamines.

The polymorphic phase behavior of a homologous series of n-saturated 1,2-diacyl phosphatidylethanolamines was investigated by differential scanning calorimetry, 31P-nuclear magnetic resonance, and Fourier transform infrared spectroscopy. Upon heating, aqueous dispersions of dried samples of the short- and medium-chain homologues (n < or = 17) exhibit single, highly energetic transitions from a dry, crystalline form to the fully hydrated, liquid-crystalline bilayer at temperatures higher than the lamellar gel-liquid-crystalline phase transition exhibited by fully hydrated samples. In contrast, the longer chain homologues (n > or = 18) first exhibit a transition from a dehydrated solid form to the hydrated L beta gel phase followed by the gel-liquid-crystalline phase transition normally observed with fully hydrated samples. The fully hydrated, aqueous dispersions of these lipids all exhibit reversible, fairly energetic gel-liquid-crystalline transitions at temperatures that are significantly higher than those of the corresponding phosphatidylcholines. In addition, at still higher temperatures, the longer chain members of this series (n > or = 16) exhibit weakly energetic transitions from the lamellar phase to an inverted nonlamellar phase. Upon appropriate incubation at low temperatures, aqueous dispersions of the shorter chain members of this homologous series (n < or = 16) form a highly ordered crystal-like phase that, upon heating, converts directly to the liquid-crystalline phase at the same temperature as do the aqueous dispersions of the dried lipid. The spectroscopic data indicate that unlike the n-saturated diacyl phosphatidylcholines, the stable crystal-like phases of this series of phosphatidylethanolamines describe an isostructural series in which the hydrocarbon chains are packed in an orthorhombic subcell and the headgroup and polar/apolar interfacial regions of the bilayer are effectively immobilized and substantially dehydrated. Our results suggest that many of the differences between the properties of these phosphatidylethanolamine bilayers and their phosphatidylcholine counterparts can be rationalized on the basis of stronger intermolecular interactions in the headgroup and interfacial regions of the phosphatidylethanolamine bilayers. These are probably the result of differences in the hydration and hydrogen bonding interactions involving the phosphorylethanolamine headgroup and moieties in the polar/apolar interfacial regions of phosphatidylethanolamine bilayers.     

Thermotropic phase behavior of model membranes composed of phosphatidylcholines containing iso-branched fatty acids. 2. Infrared and 31P NMR spectroscopic studies

The polymorphic phase behavior of aqueous dispersions of a number of representative phosphatidylcholines with methyl iso-branched fatty acyl chains was investigated by Fourier transform infrared (FT-IR) and phosphorus-31 nuclear magnetic resonance (31P NMR) spectroscopy. For the longer chain phosphatidylcholines, where two transitions are resolved on the temperature scale, the higher temperature event can unequivocally be assigned to the melting of the acyl chains (i.e., a gel/liquid-crystalline phase transition), whereas the lower temperature event is shown to involve a change in the packing mode of the methylene and carbonyl groups of the hydrocarbon chains in the gel state (i.e., a gel/gel transition). The infrared spectroscopic data suggest that the methyl iso-branched phosphatidylcholines assume a partially dehydrated, highly ordered state at low temperatures, resembling the Lc phase recently described for the long-chain n-saturated phosphatidylcholines. At higher temperatures, some branched-chain phosphatidylcholines appear to assume a fully hydrated, loosely packed gel phase similar to but not identical with the P beta, phase of their linear saturated analogues. Thus, the iso-branched phosphatidylcholine gel/gel transition corresponds, at least approximately, to a summation of the structural changes accompanying both the subtransition and the pretransition characteristic of the longer chain n-saturated phosphatidylcholines. The infrared spectroscopic data also show that, in the low-temperature gel state, there are significant differences between the odd- and even-numbered isoacylphosphatidylcholines with respect to their hydrocarbon chain packing modes as well as to their head group and interfacial hydration states.(ABSTRACT TRUNCATED AT 250 WORDS)     

Design, synthesis, and characterization of bis-phosphatidylcholine, a mechanistic probe of phosphatidylcholine transfer protein catalytic activity

The design, synthesis, and characterization of 1-(17,18-dithiatetratriacontandioyl)-bis(2-hexadecanoyl-sn-glycero -3- phosphocholine) is described. Bis-phosphatidylcholine is a dimeric phospholipid comprised of two glycerophosphocholine groups linked together by a disulfide bond at the distal ends of the sn-1 fatty acyl chains. Electron microscopy and [14C]glucose trapping studies indicate that hydrated dispersions of bis-phosphatidylcholine form closed, spherical structures which have diameters in the range of 125-500 nm. Sensitivity to phospholipase hydrolysis suggests that this bipolar lipid is organized in a membrane such that the two polar head groups of the molecular are oriented at the same surface of the membrane. Using conditions in which bovine liver phosphatidylcholine transfer protein transfers both unsaturated and saturated diacyl phosphatidylcholines between fluid phosphatidylcholine vesicles, no transfer of the bipolar phospholipid is observed. The lack of activity toward bis-phosphatidylcholine suggests that this molecule may be a useful tool for elucidating the role of membrane phosphatidylcholine in the catalytic mechanism of the phosphatidylcholine transfer protein.     

Phosphorescence study of chlorophyll d photophysics. Determination of the energy and lifetime of the photo-excited triplet state. Evidence of singlet oxygen photosensitization

Chlorophyll d (Chl d) is the major pigment in both photosystems (PSI and II) of the cyanobacterium Acaryochloris marina, whose pigment composition represents an interesting alternative in oxygenic photosynthesis. While abundant information is available relative to photophysical properties of Chl a , the understanding of Chl d photophysics is still incomplete. In this paper, we present for the first time a characterization of Chl d phosphorescence, which accompanies radiative deactivation of the photoexcited triplet state of this pigment. Reliable information was obtained on the energy and lifetime of the Chl d triplet state in frozen solutions at 77?K using diethyl ether and aqueous dispersions of Triton X100 as solvents. It is shown that triplet Chl d is effectively populated upon photoexcitation of pigment molecules and efficiently sensitizes singlet oxygen phosphorescence in aerobic solutions under ambient conditions. The data obtained are compared with the previous results of the phosphorescence studies of Chl a and Pheo a, and their possible biological implications are discussed.