Raman spectroscopic study of an interdigitated lipid bilayer. Dipalmitoylphosphatidylcholine dispersed in glycerol

Dipalmitoylphosphatidylcholine (DPPC) dispersed in perdeuterated glycerol was investigated in order to determine the effects on the Raman spectra of hydrocarbon chain interdigitation in gel-phase lipid bilayers. Interdigitated DPPC bilayers formed from glycerol dispersions in the gel phase showed a decrease in the peak height intensity $\text{I}{}_{2850}\text{/I}{}_{2880}$ ratio, for the symmetric and asymmetric methylene CH stretching modes, respectively, as compared to non-interdigitated DPPC/water gel-phase dispersions. The decrease in this spectral ratio is interpreted as an increase in chain-chain lateral interactions. Spectra recorded in the 700–740 cm^?1 CN stretching mode region, the 1000–1200 cm^?1 CC stretching mode region and the 1700–1800 cm^? CO stretching mode region were identical for both the interdigitated and non-interdigitated hydrocarbon chain systems. At low temperatures the Raman peak height intensity ratios $\text{I}{}_{2935}\text{/I}{}_{2880}$ were identical for the DPPC/glycerol and DPPC/water dispersions, indicating that this specific index for monitoring bilayer behavior is insensitive to acyl chain interdigitation. The increase, however, in the change of this index at the gel-liquid crystalline phase transition temperature for the DPPC/glycerol dispersions implies a larger entropy of transition in comparison to the non-interdigitated DPPC/water bilayer system.     

Haematoporphyrin changes the mechanical properties of lipid bilayer membranes.

The interactions between haematoporphyrin (HP) and bilayer lipid membranes (BLM) were studied. A weak effect of HP on BLM conductivity was observed at HP concentrations ranging between 10(-6) and 3 x 10(-5) mol/l. Modulus of elasticity in the direction normal to the membrane plane (E perpendicular) and dynamic viscosity coefficient (eta) were measured, both exhibiting HP-induced decrease by 22-31% in the dark. In this case, membrane potential Vm became negative and reached a value close to -50 mV. Under illumination by low-intensity (1 mW) He-Ne laser (lambda = 632 nm) the values of parameters E perpendicular and eta of the HP-modified membranes increased by 41-66%, and Vm decreased to -20 mV. Upon removing HP from the solution by perfusion, irreversible changes in mechanical properties of the HP-modified membranes induced by the laser light were observed. The reason could be the formation of stable complexes of HP with the lipid molecules. HP binds to membrane noncooperatively, with a binding constant K approximately 10(5) l/mol.     

Theory of lipid bilayer phase transitions as detected by fluorescent probes

Summary We present a quantitative theory that relates the fluorescence intensityvs. temperature (I vs. T) profile of a fluorescent-labeled two-component lipid bilayer to the phase diagram of the bilayer and the partition coefficientK of the fluorophore between fluid and solid phases of the bilayer. We show how the theory can be used to evaluateK from experimentalI vs. T profiles and the appropriate phase diagrams as well as to understand the different shapes ofI vs. T profiles obtained with particular fluorophores and phase diagrams. Using calculatedI vs. T graphs, we discuss the meaning of parameters, such as midpoint of the phase transition and onset and termination of a transition, which are often used to characterize phase transitions on the basis of fluorescence intensityvs. temperature profiles.     

Rupture and transformation of lipid bilayer membranes at thermal phase transitions

Nuclear magnetic resonance was used to study dimyristoylphosphatidylcholine vesicles. Loss of vesicle contents and transformation to more extended bilayer structures near the gel to liquid crystalline phase transition is related to potential cell membrane damage on lowering environmental temperatures.     

Initiation of anion-selective ionic channels in bilayer membranes at lipid phase transitions

A study was carried out to electric parameters of single ionic channels initiated at phase transition of bromidmetilate 1,2-distearoyl-rac-glycero-3-(O-beta-dimethylaminoethyl)-methylphosphonate, whose molecules under conditions given below are possibly charged. It has been shown that changes of transmembrane current appear at phase transition temperature. Comparison between ionic selectivity of channels initiated at Tph.t in the membranes of DSL and its phosphate analog suggests that the channel walls initiated at phospholipid phase transitions are covered with polar groups of molecules.     

A self-consistent chain model for the phase transitions in lipid bilayer membranes

We presented a mechanical model of a lipid bilayer membrane. The internal conformations of a polar head group and double hydrocarbon chains in a lipid molecule were described on the basis of the isomeric bond-rotation scheme. The thermodynamic properties of the lipid membranes were represented by a density matrix that described the rotational isomeric states of the head groups and chains. The parameters that determined the density matrix were obtained in the presence of the intermolecular interactions, which depend on the conformation of the molecules. The interchain interaction was given by the Kihara potential, which depends on the shape of the chains. The Coulomb interaction between the polar head groups and the lateral pressure were considered. The calculation was made for the three lipid molecules corresponding to DMPC, DPPC, and DSPC. The model agreed well with the following experimental results: the temperature, the latent heat of the gel-to-liquid crystalline phase transition, the temperature dependencies of (a) the intermolecular distance, (b) the number of gauche bonds in a hydrocarbon chain, (c) the order parameter for the bond orientation, (d) the volume of the membrane, (e) the thermal expansion coefficients, and (f) the birefringence.     

Photon correlation spectroscopy as a probe of planar lipid bilayer phase transitions

Photon correlation spectroscopy has been applied to study phase transitions of planar bilayer membranes. The membrane tension and one specific membrane viscosity are probed. Difficulties arising in the measurement of the temperature dependence of these properties are discussed and a servo-control system to overcome them is described. Typical data are presented for monoglyceride bilayers. Membranes incorporating cholesterol display effects below the lipid transition temperature which are interpreted in terms of separation within the membrane into cholesterol-rich fluid regions and regions of lipid in the gel phase. Some of the chlesterol-rich regions are apparently of macroscopic extent.     

Theory of lipid monolayer and bilayer phase transitions: Effect of headgroup interactions

Summary Headgroup and soft core interactions are added to a lipid monolayer-bilayer model and the surface pressure-area phase diagrams are calculated. The results show that quite small headgroup interactions can have biologically significant effects on the transition temperature and the phase diagram. In particular, the difference in transition temperatures of lecithins and phosphatidyl ethanolamines is easy to reproduce in the model. The phosphatidic acid systems seem to require weak transient hydrogen bonding which is also conjectured to play a role in most of the lipid systems. By a simple surface free energy argument it is shown that monolayers under a surface pressure of 50 dynes/cm should behave as bilayers, in agreement with experiment. Although the headgroup interactions are biologically very significant, in fundamental studies of the main phase transition in lipids they are secondary in importance to the hydrocarbon chain interactions (including the excluded volume interaction, the rotational isomerism, and the attractive van der Waals interaction).     

Conductivity and structural transitions in bilayer lipid membranes

5 structural transitions were found in bilayer lipid membranes (BLM) from egg lecithin (EL) within the temperature range 14-44 degrees C. In the transition zone BLM conductivity abruptly increases, in some cases current fluctuations of the order 150 pC of the channel type are initiated. The transition temperatures observed in BLM from EL coincide with those in biological membranes. The cause of this phenomenon is discussed, as well as possible use of these BLM in the region of structural transition as a model of cellular receptor to electromagnetic fields.     

"Blocking" by polyethyleneglycol of single lipid pores arising in unmodified bilayer lipid membranes during phase transitions

Changes in the ionic permeability of bilayer lipid membranes from dipalmitoyl phosphatidylcholine at temperatures of phase transition in LiCl (1 M) solution after the addition of polyethyleneglycols of different molecular masses have been studied. The transition of ionic membrane channels from the conducting state to a blocking nonconducting state using polymers makes it possible to calibrate lipid pores. It was shown that low-molecular-weight glycerol, polyethyleneglycols with molecular masses of 300 and 600 decrease the amplitude of fluctuations of the current through the membrane, the decrease being proportional to the size of the polymer molecule being incorporated. The addition of polyethyleneglycols with molecular masses of 1450, 2000, and 3350 decrease the current fluctuations to the basal noise level. This result is considered as a complete blockade of ion channel conductivity. In the presence of rather large polymers, such as polyethyleneglycols with molecular masses of 6000 and 20000, which are practically not incorporated into the pore, single current fluctuations occur again; however, their amplitudes are somewhat smaller than in the absence of polyethyleneglycol. It is assumed that the complete blockade of the conductivity of lipid ionic channels by polyethyleneglycols with molecular masses of 1450, 2000, and 3350 is due to the dehydration of the pore gap and the conversion of the hydrophilic pore to a hydrophobic pore.     

Alamethicin-induced changes in lipid bilayer morphology

We have found that alamethicin, in the absence of an electric field, modifies both the hydrophilic surface and hydrophobic core of lipid bilayers. As shown by freeze-fracture and X-ray diffraction experiments with multiwalled vesicles, alamethicin increases the fluid space between bilayers by as much as 50 nm, and at the same time perturbs the hydrocarbon regions of the bilayers. For suspensions of gel-state lipid treated with alamethicin, uniformly spaced rows of particles cover the fracture faces and corresponding linear arrays of stain-collecting depressions cover the hydrophilic surfaces. In the liquid-crystalline state, alamethicin induces an irregular granular texture on the fracture faces.     

A model for self-sustained potential oscillation of lipid bilayer membranes induced by the gel-liquid crystal phase transitions.

To clarify the mechanism of self-sustained oscillation of the electric potential between the two solutions divided by a lipid bilayer membrane, a microscopic model of the membrane system is presented. It is assumed, on the basis of the observed results (Yoshikawa, K., T. Omachi, T. Ishii, Y. Kuroda, and K. liyama. 1985. Biochem. Biophys. Res. Commun. 133:740-744; Ishii, T., Y. Kuroda, T. Omochi, and K. Yoshikawa. 1986. Langmuir. 2:319-321; Toko, K., N. Nagashima, S. liyama, K. Yamafuji, and T. Kunitake. Chem. Lett. 1986:1375-1378), that the gel-liquid crystal phase transition of the membrane drives the potential oscillation. It is studied, by using the model, how and under what condition the repetitive phase transition may occur and induce the potential oscillation. The transitions are driven by the repetitive adsorption and desorption of proton by the membrane surface, actions that are induced the periodic reversal of the direction of protonic current. The essential conditions for the periodic reversal are (a) at least one kind of cations such as Na+ or K+ are included in the system except for proton, and the variation of their permeability across the membrane due to the phase transition is noticeably larger than that of proton permeability; and (b) the phase transition has a hysteresis. When these conditions are fulfilled, the self-sustained potential oscillation may be brought about by adjusting temperature, pH, and the cation concentration in the solutions on both sides of the membrane. Application of electric current across the membrane also induces or modifies the potential oscillation. Periodic, quasiperiodic, and chaotic oscillations appear especially, depending on the value of frequency of the applied alternating current.     

Effect of phase transitions in hydrated 1,2-dipalmitoylphosphatidylethanolamine bilayers on the spin probe order parameter

The ‘main’ phase transition L_β→L_α of hydrated 1,2-dipalmitoylphosphatidylethanolamine (DPPE) bilayers in excess water affects the ESR order parameter S₃₃ of N-cetyl-N,N-dimethyl-N-tempoylammonium bromide (CAT-16), 5-doxylstearic acid (5-DSA) and 16-doxylstearic acid (16-DSA) spin probes. The ‘pretransition’ and ‘subtransition’ suggested to occur in hydrated DPPE by Chowdhry et al. [(1984) Biophys. J. 45, 901–904] and Silvius et al. [(1986) Biochemistry 25, 4249–4258], respectively, affect exclusively the S₃₃ of CAT-16, but not that of 5-DSA and 16-DSA spin probes. The subtransition occurs about 15 ± 1°C below the main transition.     

Thermotropic and barotropic phase transitions of dilauroylphosphatidylcholine bilayer

The bilayer phase transitions of dilauroylphosphatidylcholine (DLPC), containing two linear acyl chains with 12 carbon atoms, were observed by means of differential scanning calorimetry (DSC) under ambient pressure and light transmittance under high pressure. When the heating scan for the DLPC bilayer in 50 wt.% aqueous ethylene glycol (EG) solution began at -30 degrees C after cold storage, the DSC thermogram showed two endothermic peaks at 1.7 and 4.5 degrees C, which correspond to the transition from the lamellar crystalline (Lc) phase to the intermediate liquid crystalline (Lx) phase and the transition from the Lx phase to the liquid crystalline (L) phase, respectively. Extremely large enthalpy change (32.9 kJ mol(-1)) is characteristic of the Lc/Lx phase transition. The DSC thermogram for the heating scan beginning from -10 degrees C showed a single endothermic peak with 9.2 kJ mol(-1) at -0.4 degrees C, which was assigned as the so-called main transition between the metastable ripple gel (P'(beta)) and metastable Lalpha phases. The DLPC bilayer under high pressure underwent three kinds of transitions in EG solution, whereas only one transition was observed in water under high pressure. The middle-temperature transition in EG solution could be assigned to the main transition because of its consistency with the main transition in water. The lower-temperature transition is probably assigned as transition from the Lc phase to the P'(beta) phase. Since the slope (dT/dp) of the Lc/P'(beta) phase boundary is smaller than that for the main transition, the Lc/P'(beta) phase boundary and the main transition curves crossed each other at 40 MPa on the temperature-pressure phase diagram. The higher-temperature transition in EG solution refers to the transition from the Lx phase to the Lalpha phase. The Lx phase disappeared at about 180 MPa, and the direct transition from the P'(beta) phase to the Lalpha phase was observed at high pressures above 180 MPa.     

Phospholipid phase transitions in homogeneous nanometer scale bilayer discs

Nanoscale protein supported phospholipid bilayer discs, or Nanodiscs, were produced for the purpose of studying the phase transition behavior of the incorporated lipids. Nanodiscs and vesicles were prepared with two phospholipids, dipalmitoyl phosphatidylcholine and dimyristoyl phosphatidylcholine, and the phase transition of each was analyzed using laurdan fluorescence and differential scanning calorimetry. Laurdan is a fluorescent probe sensitive to the increase of hydration in the lipid bilayer that accompanies the gel to liquid crystalline phase transition. The emission intensity profile can be used to derive the generalized polarization, a measure of the relative amount of each phase present. Differential scanning calorimetry was used to further quantitate the phase transition of the phospholipids. Both methods revealed broader transitions for the lipids in Nanodiscs compared to those in vesicles. Also, the transition midpoint was shifted 3-4 degrees C higher for both lipids when incorporated into Nanodiscs. These findings are explained by a loss of cooperativity in the lipids of Nanodiscs which is attributable to the small size of the Nanodiscs as well as the interaction of boundary lipids with the protein encircling the discs. The broad transition of the Nanodisc lipid bilayer better mimics the phase behavior of cellular membranes than vesicles, making Nanodiscs a 'native-like' lipid environment in which to study membrane associated proteins.     

Phospholipids showing complex bilayer phase transitions. II. Four sulfur-containing phosphatidylcholines

The main phase transitions of aqueous dispersions of four synthetic phosphatidylcholines (PCs) containing sulfur atoms in thioester and thioether linkages have been studied by high sensitivity differential scanning calorimetry (DSC). The transition enthalpies ranged from 7.4 to 10.3 kcal mol-1, with values for tm, the temperature of maximal excess heat capacity, in the range 38.0 to 40.8 degrees C. The corresponding values for dipalmitoyl PC (DPPC) are 8.5 kcal mol-1 and 41.7 degrees C. Curve fitting required the sum of from one to four two-state components to give an accurate representation of the observed DSC curves. Comparison of the results given here with those reported by B.Z. Chowdhry, G. Lipka, J. Hajdu and J.M. Sturtevant, (1984) Biochemistry 23, 2044-2049, for PCs containing amide, ether or carbamoyl linkages in place of the usual ester bonds shows that small changes in organic structure can result in large changes in thermotropic behavior. The complexity in the cases showing more than a single two-state component is presumably due to a series of sequential cooperative transitions the character of which is at present unknown.