Metabolic changes of membrane lipid composition in Acholeplasma laidlawii by hydrocarbons, alcohols, and detergents: arguments for effects on lipid packing

The packing of lipids into different aggregates, such as spheres, rods, or bilayers, is dependent on the hydrophobic volume, the hydrocarbon-water interfacial area, and the hydrocarbon chain length of the participating molecules, according to the self-assembly theory [Israelachvili, J. N., Marcelja, S., & Horn, R. G. (1980) Q. Rev. Biophys. 13, 121-200]. The origin of the participating molecules should be of no importance with respect to their abilities to affect the above-mentioned parameters. In this investigation, Acholeplasma laidlawii, with a defined acyl chain composition of the membrane lipids, has been grown in the presence of three different classes of foreign molecules, known to partition into model and biological membranes. This results in an extensive metabolic alteration in the lipid polar head group composition, which is expressed as changes in the molar ratio between the lipids monoglucosyldiglyceride (MGDG) and diglucosyldiglyceride (DGDG), forming reversed hexagonal and lamellar phases in excess water, respectively. The formation of nonlamellar phases by A. laidlawii lipids depends critically upon the MGDG concentration [Lindblom, G., Brentel, I., Sj?lund, M., Wikander, G., & Wieslander, A. (1986) Biochemistry (preceding paper in this issue)]. The foreign molecules tested belong to the following groups: nonpolar organic solvents, alcohols, and detergents. Their effects on the gel to liquid crystalline phase transition temperature (Tm), on the order parameter of the acyl chains, and on the phase equilibria between lamellar and nonlamellar liquid crystalline phases in lipid-water model systems are known in several instances.(ABSTRACT TRUNCATED AT 250 WORDS)     

Langevin dynamics studies of unsaturated phospholipids in a membrane environment.

Computer simulations of three unsaturated phospholipids in a membrane environment have been carried out using Langevin dynamics and a mean-field based on the Marcelja model. The applicability of the mean-field to model unsaturated lipids was judged by comparison to available experimental NMR data. The results show that the mean-field methodology and the parameters developed for saturated lipids are applicable in simulations of unsaturated molecules, indicating that these simulations have good predictive capabilities. Single molecule simulations, each 100 ns in length, of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-elaidoyl-sn-glycero-3-phosphocholine (PEPC), and 1-palmitoyl-2-isolinoleoyl-sn-glycero-3-phosphocholine (PiLPC) reveal similarities between PEPC and DPPC. The presence of the trans double bond in PEPC has a minimum impact on the structural and dynamic properties of the molecule, which is probably the reason that isolated trans double bonds are rare in biological lipids. POPC exhibits different behavior, especially in the calculated average interchain distances, because of the cis double bond. The position of the two double bonds in PiLPC imparts special properties to the molecule.     

Polymyxin binding to charged lipid membranes. An example of cooperative lipid-protein interaction.

The binding of polymyxin-B to lipid bilayer vesicles of synthetic phosphatidic acid was studied using fluorescence, ESR spectroscopy and electron microscopy. 1,6-Diphenylhexatriene (which exhibits polarized fluorescence) and pyrene decanoic acid (which forms excimers) were used as fluorescence probes to study the lipid phase transition. The polymyxin binds strongly to negatively charged lipid layers. As a result of lipid/polymyxin chain-chain interactions, the transition temperature of the lipid. This can be explained in terms of a slight expansion of the crystalline lipid lattice (Lindeman's rule). Upon addition of polymyxin to phosphatidic acid vesicles two rather sharp phase transitions (width deltaT = 5 degrees C) are observed. The upper transition (at Tu) is that of the pure lipid and the lower transition (at T1) concerns the lipid bound to the peptide. The sharpness of these transitions strongly indicates that the bilayer is characterized by a heterogeneous lateral distribution of free and bound lipid regions, one in the crystalline and the other in the fluid state. Such a domain structure was directly observed by electron microscopy (freeze etching technique). In (1 : 1) mixtures of dipalmitoyl phosphatidic acid and egg lecithin, polymyxin induces the formation of domains of charged lipid within the fluid regions of egg lecithin. With both fluorescence methods the fraction of lipid bound to polymyxin-B as a function of the peptide concentration was determined. S-shaped binding curves were obtained. The same type of binding curve is obtained for the interaction of Ca2+ with phosphatidic acid lamellae, while the binding of polylysine to such membranes is characterized by a linear or Langmuir type binding curve. The S-shaped binding curve can be explained in terms of a cooperative lipid-ligand (Ca2+, polymyxin) interaction. A model is proposed which explains the association of polymyxin within the membrane plane in terms of elastic forces caused by the elastic distortion of the (liquid crystalline) lipid layer by this highly asymmetric peptide.     

Effects of anesthetic tetradecenols on phosphatidylcholine phase transitions. Implications for the mechanism of the bilayer pretransition.

The effects of cis- and trans-9,10-tetradecenols on the phase transitions of dimyristoyl-, dipalmitoyl-, and distearoyl-phosphatidylcholines were investigated using high sensitivity scanning calorimetry and Raman spectroscopy. Both alcohols lowered the gel to liquid crystalline phase transition temperatures for all three phosphatidylcholines, with cis-tetradecenol showing a considerably greater effect than trans-tetradecenol in each case. While both alcohols increased the temperature of the dimyristoylphosphatidylcholine pretransition, and decreased the temperature of the distearoylphosphatidylcholine pretransition, cis-tetradecenol lowered the temperature of the dipalmitoylphosphatidylcholine pretransition, while trans-tetradecenol dramatically raised the pretransition temperature. These results are interpreted in terms of the reduction in gel (L beta) phase chain tilt and changes in the ease of acyl chain trans-gauche isomerization which are introduced by the alcohols, and the consequent effects of these changes on the pretransition and the gel to liquid crystalline phase transition. The data clearly show that caution is necessary in applying information on lipid-anesthetic interactions obtained from model membranes to the problem of clinical anesthesia, since qualitatively different results may be obtained when lipids of differing acyl chain lengths are employed. Superficial interpretation of such data might lead to erroneous conclusions.     

Thermally induced aggregation of human transferrin receptor studied by light-scattering techniques.

The thermal stability of transferrin receptor isolated from human placenta in detergent-free solution has been investigated by static light-scattering and photon correlation spectroscopy. In detergent-free solution at 293.2 K, human transferrin receptor (hTfR) forms stable associates with a hydrodynamic radius of 16 nm. With increasing temperature the particles get more compact, above 340 K a phase transition takes, place and spontaneous aggregation of the receptor occurs. Under these conditions large clusters are formed that lead to fractal aggregates, coexisting with dendritic crystalline structures. The experimental findings are compatible with a model, which involves a reaction limited cluster-cluster aggregation mechanism in conjunction with a nucleation process. The molar enthalpy change associated with the phase transition was determined to be (1860 +/- 150) kJ/mol(-1) at a transition temperature of (341.3 +/- 0.2) K.     

Factors affecting the motion of the polar headgroup in phospholipid bilayers. A 31P NMR study of unsonicated phosphatidylcholine liposomes.

(1) The 129 MHZ and 36.4 MHZ 31 P NMR spectra of unsonicated liposomes consisting of phosphatidylcholines of varying chain length and unsaturation have been investigated. (2) In the liquid crystalline state the 31 P NMR liposome spectra are similar for both saturated and unsaturated phosphatidylcholines, demonstrating that the motion of the polar headgroup is not sensitive to the fatty acid composition in the disordered liquid crystalline state. (3) Below the hydrocarbon phase transition temperature there is a marked increase in the linewidth of the 31P NMR liposome spectra, indicating a reduction in the motion of the polar headgroup. (4) The addition of equimolar concentrations of cholesterol to phosphatidylcholine eliminates phase transition effects experienced by the polar headgroup. The motion of the polar headgroup is then very similar to that obtained in the liquid crystalline state for pure phosphatidylcholine bilayers. (5) In the liquid crystalline state the motion of the polar headgroup in the phosphate region is insensitive to changes in the available area per phosphatidy-choline molecule.     

Effects of cyclosporine A on biomembranes. Vibrational spectroscopic, calorimetric and hemolysis studies.

Cyclosporine A (CSA)-dipalmitoylphosphatidylcholine (DPPC) interactions were investigated using scanning calorimetry, infrared spectroscopy, and Raman spectroscopy. CSA reduced both the temperature and the maximum heat capacity of the lipid bilayer gel-to-liquid crystalline phase transition; the relationship between the shift in transition temperature and CSA concentration indicates that the peptide does not partition ideally between DPPC gel and liquid crystalline phases. This nonideality can be accounted for by excluded volume interactions between peptide molecules. CSA exhibited a similar but much more pronounced effect on the pretransition; at concentrations of 1 mol % CSA the amplitude of the pretransition was less than 20% of its value in the pure lipid. Raman spectroscopy confirmed that the effects of CSA on the phase transitions are not accompanied by major structural alterations in either the lipid headgroup or acyl chain regions at temperatures away from the phase changes. Both infrared and Raman spectroscopic results demonstrated that CSA in the lipid bilayer exists largely in a beta-turn conformation, as expected from single crystal x-ray data; the lipid phase transition does not induce structural alterations in CSA. Although the polypeptide significantly affects DPPC model membrane bilayers, CSA neither inhibited hypotonic hemolysis nor caused erythrocyte hemolysis, in contrast to many chemical agents that are believed to act through membrane-mediated pathways. Thus, agents, such as CSA, that perturb phospholipid phase transitions do not necessarily cause functional changes in cell membranes.     

Physical properties of glycosyl diacylglycerols. 1. Calorimetric studies of a homologous series of 1,2-di-O-acyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerols

The polymorphic phase behavior of aqueous dispersions of a homologous series of 1,2-di-O-acyl-3-O-(alpha-D-glucopyranosyl)-sn-glycerols was studied by differential scanning calorimetry. At fast heating rates unannealed samples of these lipids exhibit a strongly energetic transition, which has been identified as a lamellar gel/liquid crystalline (L beta/L alpha) phase transition (short- and medium-chain compounds) or a lamellar gel to inverted hexagonal (L beta/HII) phase transition (long-chain compounds) by X-ray diffraction studies (Sen et al., 1990). At still higher temperatures, some of the lipids that form lamellar liquid-crystalline phases exhibit an additional transition, which has been identified as a transition to an inverted nonbilayer phase by X-ray diffraction studies. The lamellar gel phase formed on initial cooling of these lipids is a metastable structure, which, when annealed under appropriate conditions, transforms to a more stable lamellar gel phase, which has been identified as a poorly hydrated crystal-like phase with tilted acyl chains by X-ray diffraction measurements (Sen et al., 1990). With the exception of the di-19:0 homologue, the crystalline phases of these lipids are stable to temperatures higher than those at which their L beta phases melt and, as a result, they convert directly to L alpha or HII phases on heating. Our results indicate that the length of the acyl chain affects both the kinetic and thermodynamic properties of the crystalline phases of these lipids as well as the type of nonbilayer phase that they form. Moreover, when compared with the beta-anomers, these alpha-D-glucosyl diacylglycerols are more prone to form ordered crystalline gel phases at low temperatures and are somewhat less prone to form nonbilayer phases at elevated temperatures. Thus the physical properties of glucolipids (and possibly all glycolipids) are very sensitive to the nature of the anomeric linkage between the sugar headgroup and the glycerol backbone of the lipid molecule. We suggest that this is, in part, due to a change in orientation of the glucopyranosyl ring relative to the bilayer surface, which in turn affects the way(s) in which the sugar headgroups interact with each other and with water.     

Deuterium magnetic resonance study of the gel and liquid crystalline phases of dipalmitoyl phosphatidylcholine.

Deuterium magnetic resonance is applied to the study of the liquid crystalline and gel phases, and of the phase transition, of a multilamellar dispersion of chain perdeuterated (d62)-dipalmitoyl phosphatidylcholine/H2O. Analysis of the deuterium spectra in terms of the moments of the spectra allows one to make quantitative statements concerning the distribution of quadrupolar splittings even in complicated situations, e.g., when using perdeuterated sampled or when there are mixed phases. This analysis indicates that d62-dipalmitoyl phosphatidylcholine in excess H2O undergoes a sharp phase transition (with a width of less than 1 degree C) at approximately 37 degrees C and that there appears to be hysteresis in the phase transition of approximately 1 degree C. In the lamellar liquid crystalline phase above 37 degrees C the spectra show a number of well-resolved features whose quadrupolar splittings can be followed as the temperature is varied. The gel phase near 20 degrees C possesses a very broad, almost featureless spectrum that does not seem to support a model of the gel phase wherein the hydrocarbon chains are fully extended in the all-trans conformation. At temperatures near 0 degrees C the spectra clearly indicate that a large fraction of the lipid molecules cease the rotation about their long axes, giving a spectrum more characteristic of a rigid or solid sample. These results give a picture of the gel phase as a phase characterized by considerable hydrocarbon chain disorder near 20 degrees C and becoming a more solid-like phase near 0 degrees C. The spin-lattice relaxation time, T1, has been measured at 20 degrees C in the gel phase, and at 37 and 45 degrees C in the liquid crystalline phase. The values of T1 obtained for each of the resolvable peaks in the spectrum at 37 degrees C are compared to the values (for each peak) of T2e, the decay time of the quadrupolar echo, obtained at the same temperature. These results are discussed in terms of a simple two-motion model.     

The phase behavior of hydrated cholesterol.

The thermotropic phase behavior of cholesterol monohydrate in water was investigated by differential scanning calorimetry, polarizing light microscopy, and x-ray diffraction. In contrast to anhydrous cholesterol which undergoes a polymorphic crystalline transition at 39 degrees C and a crystalline to liquid transition at 151 degrees C, the closed system of cholesterol monohydrate and water exhibited three reversible endothermic transitions at 86, 123, and 157 degrees C. At 86 degrees C, cholesterol monohydrate loses its water of hydration, forming the high temperature polymorph of anhydrous cholesterol. At least 24 hours were required for re-hydration of cholesterol and the rate of hydration was dependent on the polymorphic crystalline form of anhydrous cholesterol. At 123 degrees C, anhydrous crystalline cholesterol in the presence of excess water undergoes a sharp transition to a birefringent liquid crystalline phase of smectic texture. The x-ray diffraction pattern obtained from this phase contained two sharp low-angle reflections at 37.4 and 18.7 A and a diffuse wide-angle reflection centered at 5.7 A, indicating a layered smectic type of liquid crystalline structure with each layer being two cholesterol molecules thick. The liquid crystalline phase is stable over the temperature range of 123 to 157 degrees C before melting to a liquid dispersed in water. The observation of a smectic liquid crystalline phase for hydrated cholesterol correlates with its high surface activity and helps to explain its ability to exist in high concentrations in biological membranes.     

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

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

A calorimetry and deuterium NMR study of mixed model membranes of 1-palmitoyl-2-oleylphosphatidylcholine and saturated phosphatidylcholines

Binary phase diagrams have been constructed from differential scanning calorimetry (DSC) data for the systems 1-palmitoyl-2-oleylphosphatidylcholine (POPC)/dimyristoylphosphatidylcholine (DMPC), POPC/dipalmitoylphosphatidylcholine (DPPC) and POPC/distearoylphosphatidylcholine (DSPC). Mixtures of POPC with DMPC exhibit complete miscibility in the gel and liquid crystalline states. Mixtures of POPC with DPPC or with DSPC exhibit gel phase immiscibility over the composition range 0-75% DPPC (or DSPC). These results, when taken together with previous studies of mixtures of phosphatidylcholines, are consistent with the hypothesis that PCs whose order-disorder transition temperatures (Tm values) differ by less than 33 deg. C exhibit gel state miscibility. Those whose Tm values differ by more than 33 deg. C exhibit gel state immiscibility. 2H-NMR spectroscopy has been used to further study mixed model membranes composed of POPC and DPPC, in which either lipid has been labeled with deuterium in the 2-, 10- or 16-position of the palmitoyl chain(s) or in the N-methyls of the choline head group. POPC/DPPC mixtures in the liquid crystalline state are intermediate in order between pure POPC and DPPC at the same temperature. The POPC palmitoyl chain is always more disordered than the palmitoyl chains of DPPC in liquid crystalline POPC/DPPC mixtures. This is attributed to the fact that a POPC palmitoyl chain is constrained by direct bonding to have at least one oleyl chain among its nearest neighbors, while a DPPC palmitoyl chain must have at least one neighboring palmitoyl chain. When liquid crystalline POPC, DPPC and POPC/DPPC mixtures are compared at a reduced temperature (relative to the acyl chain order-disorder transition), POPC/DPPC mixtures are more disordered than predicted from the behavior of the pure components, in agreement with enthalpy data derived from DSC studies. Within the temperature range of the broad phase transition of 1:1 POPC/DPPC, a superposition of gel and liquid crystalline spectra is observed for 1:1 POPC/[2H]DPPC, while 1:1[2H]POPC/DPPC exhibits only a liquid crystalline spectrum. Thus, at temperatures within the phase transition region, the liquid crystalline phase is POPC-rich and the gel phase is DPPC-rich. Comparison of the liquid crystalline quadrupole splittings within the thermal phase transition range suggests that mixing of the residual liquid crystalline POPC and DPPC is highly non-ideal.     

A Raman spectroscopic investigation of the lipid state in acetylcholine receptor-rich membranes from Torpedo marmorata.

The lipid state in acetylcholine receptor (AcChR)-rich membranes purified from electric organ of Torpedo marmorata was studied in the temperature interval from 0 degrees C to 35 degrees C using the (C-H) stretching and (C-C) skeletal optical vibrations. The Raman spectra of AcChR-rich membranes, recorded immediately after preparation of the samples, indicate that the lipids are in a predominant triclinic crystalline lattice and do not undergo a phase transition when the temperature increases up to 35 degrees C. However, the polar groups of the lipids appear subject to temperature-induced variations. After extraction of 43-kd and other non-receptor proteins, spectra indicate an order-disorder phase transition of lipids at approximately 21 degrees C. This transition appears less cooperative than the transition of the membrane lipid extract. The role of the proteins in preservation of the crystalline state of lipids in AcChR-rich membranes is discussed.     

The thermal transitions and structural properties of 5alpha-cholestan-3beta-ol esters of aliphatic acids.

5 alpha-Cholestan-3 beta-ol esters of aliphatic acids undergo both enantiotropic and monotropic changes of state. Ten saturated and three unsaturated esters have been examined by differential scanning calorimetry and polarizing microscopy to determine transition temperatures, enthalpies, and entropies. The results are compared with an analogous series of cholesterol esters. All esters of even-numbered n-alkanoic acids from C2 to C20 melt from a crystalline state to an isotropic liquid. The crystalline state has been studied by X-ray powder diffraction. The C8 to C20 esters have progressively increasing crystalline melting transition temperatures from 76 to 99 degrees C and possess similar X-ray powder diffraction patterns, suggesting that these compounds form an isostructural series. Esters of C2, C4, and C6 acids exhibit polymorphism. Crystalline cholestanol oleate melts to an isotropic liquid, whereas cholestanol linoleate and linolenate fail to crystallize, even after several months at -20 degrees C. Esters of the even-numbered saturated acids from C4 to C14 form monotropic cholesteric liquid crystalline phases. Esters C10, C12, and C14 form smectic liquid crystalline phases. Cholestanol oleate, linoleate, and linolenate form both cholesteric and smectic mesophases. The lower smectic to cholesteric and cholesteric to isotropic transition temperatures of the cholestanol esters compared to the corresponding transition temperatures of the analogous cholesterol esters suggest that the delta 5 double bond in cholesterol increases the thermal stability of the mesophases of cholesterol esters.     

Structure and interactions of lipids in human plasma low density lipoproteins.

Temperature-dependent techniques (differential scanning calorimetry, polarizing microscopy, and x-ray scattering and diffraction techniques) were used to compare the properties of human plasma low density lipoproteins (LDL) with its extracted lipid classes. Three types of thermal transitions were characterized: (a) a reversible transition in intact LDL near body temperature associated with a liquid crystalline order-disorder phase change of cholesterol esters within the particles; (b) an irreversible high temperature transition (approximately 70-90 degrees) associated with LDL denaturation and release of cholesterol esters from the disrupted particles; and (c) low temperature transitions related to liquid crystalline and crystalline phase changes in these released esters. The temperature of the reversible transition in intact LDL varies among individual donors. Correlation analysis shows that the temperature of this transition negatively correlates with the amount of triglyceride relative to cholesterol ester in LDL. Studies on mixtures of cholesterol esters and triglycerides isolated from LDL show a similar effect, increasing amounts of triglycerides decreasing the temperature of the liquid leads to smectic liquid crystalline transition of the isolated esters. Thus, the amount of triglyceride in LDL influences the fluidity of the cholesterol esters in LDL. The enthalpy of the reversible transition in intact LDL is 0.69 cal/g of LDL cholesterol ester. This compares with 0.89 cal/g for the liquid leads to liquid crystalline transition of the cholesterol esters released from denatured LDL and 1.01 cal/g for the same transition in the extracted esters. Unlike the cholesterol esters released from denatured LDL, or isolated LDL esters, cholesterol ester in the intact LDL particle does not crystallize. These findings suggest that the behavior of cholesterol esters in intact LDL is constrained relative to their behavior when freed from the restrictions of the particle. These results together with experiments on partitioning of the individual lipid classes of LDL allow us to define the distribution and interaction of lipids in the intact LDL particle.     

Nematic polymers: Excluded-volume effects

We present a theoretical model which describes a cooperative helix–coil liquid-crystal phase transition. We show that this model predicts a first-order phase transition where certain types of chainlike macromolecules in solution make a transition from a nearly coiled to a nearly rigid conformation accompanied by a simultaneous development of long-range nematic-type liquid crystalline orientational order. From this model, the phase boundaries between nematic and isotropic phases are obtained as functions of concentration of macromolecules and of other physical parameters.     

The structure of human-plasma low-density lipoprotein B. An X-ray small-angle scattering study.

1. X-ray small-angle scattering of human plasma lipoprotein B of the low-density fraction (rho = 1.016--1.060 g.cm-3) has been recorded to high precision at different electron density contrasts. 2. The overall structure of the particles is characterized by a quasi-spherical shape and radial symmetry. A maximum diameter of 23 nm and a molecular weight of 2.4 X 10(6) have been determined. 3. The internal structure is described in terms of a model consisting of spherical layers with different electron densities indicating that the neutral lipids are arranged in the core of the molecule up to a radius of about 8 nm surrounded by a monolayer of free cholesterol, phospholipids and protein. The neural lipids are shown to be in an ordered, liquid crystalline state at 4 degrees C and to undergo a thermotropic transition into a disordered state at higher temperatures.     

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.     

The influence of charge on bilayer membranes. Calorimetric investigations of phosphatidic acid bilayers.

The pH-dependence of the phase transition of dimyristoyl phosphatidic acid and dihexadecyl phosphatidic acid has been investigated using differential scanning calorimetry. Varying the pH induces different degrees of ionization of the polar head group. The changes in transition temperature with pH as observed by calorimetry are in good agreement with those obtained by measuring the changes in light scattering, whereas the transition temperatures reported by the fluorescent probe N-phenylnaphthylamine do not always coincide with those determined from calorimetry [1]. The observed maximum of the transition temperature at pH 3.5 corresponds to a minimum in the transition enthalpy vs. pH diagram. At this pH a particular stable bilayer phase is formed. Full protonation of phosphatidic acids leads to suspensions of mycrocrystals. The transition enthalpy approaches the value of the melting enthalpy of crystalline anhydrous phosphatidic acid. The decrease in the transition enthalpy at high pH values is due to a change in the hydrocarbon chain interactions induced by the doubly charged head groups. The cooperativity of the transition varies with the degree of ionization of the head group, being lower for doubly charged phosphatidic acids.     

Two gel states of cerebrosides. Calorimetric and Raman spectroscopic evidence.

The enthalpy of the gel-to-liquid crystalline phase transition for kerasin (15.8 kcal/mol) is found to be markedly higher than that for phrenosin and unfractionated bovine brain cerebrosides (about 7 kcal/mol). Evidence for a higher degree of order in the hydrocarbon chains and a different configuration in the polar region of kerasin is supplied by Raman spectroscopic parameters for these gel phases. The high transition enthalpy for kerasin is ascribed to a lesser accommodation of gauche conformers in the hydrocarbon chains just below the transition temperature. The thermodynamic behavior of these cerebroside fractions, including hysteresis in kerasin gels, is compared to that previously reported for sphingomyelins.