Phase transition behavior of single phosphatidylcholine bilayers on a solid spherical support studied by DSC, NMR and FT-IR

For the first time, the chain melting transition from the gel phase to the liquid crystalline phase of a single DPPC bilayer on a solid, spherical support (silica beads) is observed by differential scanning calorimetry (DSC). This transition is remarkably cooperative, exhibits a transition temperature T(m) which is 2 degrees C lower than usually found for DPPC multilamellar vesicles and its excess enthalpy is about 25% less than in DPPC multilayers. 31P- and 2H-NMR data as well as FT-IR data provide evidence that despite the highly asymmetric characteristic of the model system, the whole single bilayer undergoes the transition at T(m), i.e., there is no decoupling of the two monolayer leaflets at the main phase transition. Furthermore, our results show that the formation of the ripple (P(beta')) phase is inhibited in single bilayers on a solid support. This result confirms a conclusion which we reached previously on the basis of neutron scattering data obtained on planar supported bilayers. The most likely reason for this inhibition as well as for the above mentioned thermodynamic differences between multilamellar vesicles and supported membranes is a significantly higher lateral stress in the latter. Moreover, the exchange of lipids between two populations of spherical supported vesicles (DMPC and chain perdeuterated DMPC) is studied by DSC. It is shown that this exchange process is symmetric and its half-time is a factor of 3-4 higher than observed for small sonicated DMPC vesicles.

Macroscopic orientation effects in broadline NMR-spectra of model membranes at high magnetic field strength: A method preventing such effects

The partial orientation of multilamellar vesicles (MLV) in high magnetic fields has been studied and a method to prevent such effects is herewith proposed. The orientation effect was measured with (2)H-, (31)P-NMR and electron microscopy on MLVs of dipalmitoyl phosphatidylcholine with 30 mol% cholesterol. We present the first freeze-etch electron microscopy data obtained from MLV samples that were frozen directly in the NMR magnet at a field strength of 9.4 Tesla. These experiments clearly show that the MLVs adopt an ellipsoidal (but not a cylindrical) shape in the magnetic field. Best fit (31)P-NMR lineshape calculations assuming an ellipsoidal distribution of molecular director axes to the experimentally obtained spectra provide a quantitative measure of the average semiaxis ratio of the ellipsoidal MLVs and its change with temperature. The application of so-called spherical supported vesicles (SSV) is found to prevent any partial orientation effects so that undistorted NMR powder pattern of the bilayer can be measured independently of magnetic field strength and temperature.The usefulness of SSVs is further demonstrated by a direct comparison of spectral data such as (31)P-and (2)H-NMR lineshapes and relaxation times as well as (2)H-NMR dePaked spectra obtained for both model systems. These experiments show that spectral data obtained from partially oriented MLVs are not unambiguous to interpret, in particular, if an external parameter such as temperature is varied.

Changes of the physical properties of the liquid-ordered phase with temperature in binary mixtures of DPPC with cholesterol: A H-NMR, FT-IR, DSC, and neutron scattering study

The structure of the so-called liquid-ordered (lo) phase of binary mixtures of DPPC-d(62) with cholesterol was studied between 20-50 mol% cholesterol using (2)H-NMR, FT-IR, DSC, and neutron specular reflection. Different model systems such as multilamellar vesicles, spherical supported vesicles, and oriented multilayers were used. We observed significant changes of the lo phase structure in the physiological relevant temperature region between 30-45 degrees C. (2)H-NMR in combination with lineshape simulations provides evidence for a drastic effect of cholesterol on the shape of multilamellar vesicles due to magnetic field orientation. Moreover, the data indicates a significant change of the extent of this partial orientation for DPPC-d(62) multilamellar vesicles containing 25 mol% cholesterol between 36-42 degrees C. The semiaxes ratio of the (due to magnetic field orientation) ellipsoidal multilamellar vesicles changes over this temperature range by approximately 25%. (2)H-NMR and FT-IR measurements indicate changes of the average orientational order at the bilayer center in the same temperature range and a significant increase of the number of end-gauche conformers while the majority of the methylene groups remain in an all-trans conformation. Additionally, specular reflection of neutrons shows a concomitant reduction of the bilayer thickness by 4 A. Based on a model of the arrangement of DPPC and cholesterol in the lo phase, a molecular mechanism is proposed in which increasing the temperature between 30 and 45 degrees C has the effect of driving cholesterol from the bilayer center towards the head group region.

Derivation of a closed form analytical expression for fluorescence recovery after photo bleaching in the case of continuous bleaching during read out

Measurements of very slow diffusive processes in membranes, like the diffusion of integral membrane proteins, by fluorescence recovery after photo bleaching (FRAP) are hampered by bleaching of the probe during the read out of the fluorescence recovery. In the limit of long observation time (very slow diffusion as in the case of large membrane proteins), this bleaching may cause errors to the recovery function and thus provides error-prone diffusion coefficients. In this work we present a new approach to a two-dimensional closed form analytical solution of the reaction-diffusion equation, based on the addition of a dissipative term to the conventional diffusion equation. The calculation was done assuming (i) a Gaussian laser beam profile for bleaching the spot and (ii) that the fluorescence intensity profile emerging from the spot can be approximated by a two-dimensional Gaussian. The detection scheme derived from the analytical solution allows for diffusion measurements without the constraint of observation bleaching. Recovery curves of experimental FRAP data obtained under non-negligible read-out bleaching for native membranes (rabbit endoplasmic reticulum) on a planar solid support showed excellent agreement with the analytical solution and allowed the calculation of the lipid diffusion coefficient.

Modulation of cytochrome C coupling to anionic lipid monolayers by a change of the phase state: a combined neutron and infrared reflection study

The effect of monolayer domain formation on the electrostatic coupling of cytochrome c from the subphase to a monolayer at the air/water interface was studied using a combination of neutron reflection (NR) and infrared reflection absorption spectroscopy (IRRAS) techniques. The monolayers consisted of a binary mixture of the zwitterionic phosphatidylcholine and the anionic phosphatidylglycerol. For a monolayer of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylglycerol (DMPG, 30 mol%), which exhibits a non-ideal mixing of the two lipid components, we observed a significantly higher protein coupling to the liquid-condensed phase compared to the liquid-expanded state. In contrast, this higher protein binding was not observed when the two lipids had identical chain lengths (nearly ideal mixing). Similarly, for an equimolar mixture of DPPC and DMPG, we did not observe significant differences in the protein binding for the two phase states. The results strongly suggest that the domain formation in a condensed monolayer under non-ideal lipid mixing conditions is crucial for the cytochrome c binding strength. Furthermore, this study demonstrates the significant advantages of gathering information on protein-monolayer coupling by the combined use of a dedicated IRRAS set-up with the NR technique.

Collective membrane motions of high and low amplitude, studied by dynamic light scattering and micro-interferometry

Undulations of lipid bilayers were experimentally studied for the two limiting cases of high and weak lateral tension using two well established model systems: freely suspended planar lipid bilayers, so-called black lipid membranes (BLM) for high-tension studies and large unilamellar vesicles (LUV) for measurements at weak tension. This variation in tension results in changes of undulation amplitudes from several hundred nm (LUV) down to 1 nm (BLM), thus requiring different physical methods for their detection. We have employed microinterferometric techniques (RICM) for studying the regime of weak tension and dynamic light scattering (DLS) for that of high tension. The dedicated DLS set-up allowed the measurements of undulations over a wide wave vector range of 250 < q/cm-1 < 35,000 cm-1. This enabled the observation of collective membrane modes in two regimes, the oscillating one at low q and the overdamped regime at high q. The transition between both regimes at the bifurcation point is rather abrupt and depends on the lateral tension of the bilayer, as is demonstrated by comparing the dispersion curves of pure lipid and of lipid-cholestrol BLMs over the same q-range. The DLS measurements allowed a critical test of a hydrodynamic theory of the dispersion behaviour of membrane collective modes under tension. The DLS measurements are compared with RICM results of undulatory excitations of giant vesicles weakly adhering to substrates in the 10(-6)-2.5 x 10(-7) m wavelength regime and at low frequencies (0.1-25 Hz). Experimental evidence for the strong decrease in the relaxation rate by the hydrodynamic coupling of the membrane with the wall is established.

Formation of domains of cationic or anionic lipids in binary lipid mixtures increases the electrostatic coupling strength of water-soluble proteins to supported bilayers

The electrostatic binding strength of water-soluble proteins having either an excess positive (cytochrome c) or negative (beta-lactoglobulin) electric charge to oppositely charged supported planar bilayers (SPBs) was studied as a function of the bilayer phase state (fluid or gel phase) by IR-ATR spectroscopy. The bilayer consisted of mixtures of zwitterionic DEPC with either cationic DMTAP or anionic DMPG. We observed drastic differences in the binding strength of both proteins for the two bilayer phase states, with the gel phase exhibiting a higher binding strength than the fluid phase, under conditions where the two lipid components had different hydrophobic chain lengths resulting in a nonideal mixing behavior. In addition, for beta-lactoglobulin we observed a strong binding to a gel phase SPB comprising DEPC/DMTAP, while raising the temperature of the SPB above the chain melting transition temperature of the mixture resulted in a complete unbinding of the protein. In contrast, for DMPC/DMTAP having the same cationic charge content but no hydrophobic chain mismatch, no phase-dependent coupling strength of the protein to the SPB was observed. Our results suggest that the formation of charge-enriched domains by partial demixing of the bilayer lipids at the transition to the gel state is crucial for modulation of the protein binding strength to the SPB, while the intrinsic charge of the solid support surface is of minor importance.

The effect of S-layer protein adsorption and crystallization on the collective motion of a planar lipid bilayer studied by dynamic light scattering

A dedicated dynamic light scattering (DLS) setup was employed to study the undulations of freely suspended planar lipid bilayers, the so-called black lipid membranes (BLM), over a previously inaccessible spread of frequencies (relaxation times ranging from 10(-2) to 10(-6) s) and wavevectors (250 cm(-1) < q < 38,000 cm(-1)). For a BLM consisting of 1,2-dielaidoyl-sn-3-glycero-phosphocholine (DEPC) doped with two different proportions of the cationic lipid analog dioctadecyl-dimethylammonium bromide (DODAB) we observed an increase of the lateral tension of the membrane with the DODAB concentration. The experimentally determined dispersion behavior of the transverse shear mode was in excellent agreement with the theoretical predictions of a first-order hydrodynamic theory. The symmetric adsorption of the crystalline bacterial cell surface layer (S-layer) proteins from Bacillus coagulans E38-66 to a weakly cationic BLM (1.5 mol % DODAB) causes a drastic reduction of the membrane tension well beyond the previous DODAB-induced tension increase. The likely reason for this behavior is an increase of molecular order along the lipid chains by the protein and/or partial protein penetration into the lipid headgroup region. S-layer protein adsorption to a highly cationic BLM (14 mol % DODAB) shows after 7 h incubation time an even stronger decrease of the membrane tension by a factor of five, but additionally a significant increase of the (previously negligible) surface viscosity, again in excellent agreement with the hydrodynamic theory. Further incubation (24 h) shows a drastic increase of the membrane bending energy by three orders of magnitude as a result of a large-scale, two-dimensional recrystallization of the S-layer proteins at both sides of the BLM. The results demonstrate the potential of the method for the assessment of the different stages of protein adsorption and recrystallization at a membrane surface by measurements of the collective membrane modes and their analysis in terms of a hydrodynamic theory.

Effect of cationic lipids in the formation of asymmetries in supported bilayers

We have studied the formation of a supported bilayer containing both cationic and zwitterionic lipids by fusion of small unilamellar vesicles (SUV) onto the solid surface at low salt conditions using a combination of attenuated total reflection infrared (ATR-IR) and deuterium NMR spectroscopy with microcalorimetry. The data suggest that a significant cationic lipid asymmetry between the outer (distal) and the inner (proximal) monolayer of a supported bilayer results under conditions of prolonged incubation times of the solid support with the SUV coating solution. For a SUV composition of DPPC/DHDAB (4:1) we observed an enrichment of the cationic component in the proximal monolayer of up to 200% compared to the distal monolayer after 12 h incubation. It is suggested that the electrostatic potential arising from the solid surface is the driving force for the creation of this asymmetry by means of directed flip-flop between the monolayers and/or by temporary fusion between SUV from the bulk with the supported bilayer.

Collective membrane motions in the mesoscopic range and their modulation by the binding of a monomolecular protein layer of streptavidin studied by dynamic light scattering

Using a dedicated dynamic light scattering setup, we have studied the angstrom-scale amplitude undulations of freely suspended planar lipid bilayers, so-called black lipid membranes (BLM's), over a previously not accessible spread of frequencies (relaxation times ranging from 10(-2) to 10(-6) s) and wave vectors (250 cm(-1)<q<35 000 cm(-1)). This allowed a critical test of a simple hydrodynamic theory of collective membrane modes, and the results obtained for a synthetic lecithin BLM are found to be in excellent agreement with the theoretical predictions. In particular, the transition of the transverse shear mode of a BLM between an oscillatory or propagating regime and an overdamped regime by passing through a bifurcation point was clearly observed. It is shown that the collective motions in the time- and wave-vector regime covered are dominated by the membrane tension, while membrane curvature does not contribute. The binding of the protein streptavidin to the BLM via membrane anchored specific binders (receptors) causes a drastic change in frequency and amplitude of the collective motions, resulting in a drastic increase of the membrane tension by a factor of 3. This effect is probably caused by a steric hindrance of the transverse shear motions of the lipid by the tightly bound proteins.

Direct detection of domains in phospholipid bilayers by grazing incidence diffraction of neutrons and atomic force microscopy

The geometry of domains in phospholipid bilayers of binary (1:1) mixtures of synthetic lecithins with a difference in chain length of four methylene groups has been studied by two independent, direct and complementary methods. Grazing incidence diffraction of neutrons provided gel domain sizes of less than 10 nm in both the gel and the coexistence phase of the mixture, while no domains were detected for the fluid phase. For the coexistence region, the neutron data suggest that domains grow in number rather than in size with decreasing temperature. Atomic force microscopy was used to study gel phase size and shape of the domains. The domains imaged by atomic force microscopy exhibit a rather irregular shape with an average size of 10 nm, thus confirming the neutron results for this phase. The good agreement between atomic force microscopy and neutron results, despite the completely different nature of their observables, has potential for the future development of refined models for the interpretation of neutron data from heterogeneous membranes in terms of regularly spaced and spatially extended scatterers.

Differences in the physical properties of lipid monolayers and bilayers on a spherical solid support

A monolayer of 1,2-dipalmitoyl-d62-glycero-3-phosphocholine (DPPC-d62) coated onto silanized silica beads (spherical supported monolayer: SSM) is studied by 2H-NMR and DSC. The results are compared with those obtained from a single bilayer on the same solid support (spherical supported vesicles: SSV) and from multilamellar vesicles (MLV). The phase transition temperature (Tm) of the SSMs is significantly higher than that of the bilayer systems and the extent of this difference depends on the lipid density in the monolayer that is determined during its preparation. 2H-NMR reveals a gel and fluid phase coexistence in the SSM transition region. A comparison of the 2H-NMR line shapes suggests the presence of highly curved structures for the fluid phase of the SSM samples. From a comparison of SSM and SSV transverse relaxation in the fluid phase we can conclude that the lateral diffusion coefficient D1 in supported monolayers is similar to that in bilayers.

The thermodynamic control of protein binding to lipid bilayers for protein chromatography

A new concept of charge-selective bioseparation with certain advantages over the established ion-exchange technique is reported. The procedure is based on the temperature-controlled creation and dispersion of domain structures in single phospholipid bilayers by means of the phase transition between the gel phase and the fluid phase of the bilayer. The bilayers are presented on a solid support of silica gel. Rather than altering the ionic strength of the buffer, protein elution is accomplished by a change of the chromatography column temperature. The application of temperature gradients improves the protein selectivity of phase transition chromatography.

The effect of increasing membrane curvature on the phase transition and mixing behavior of a dimyristoyl-sn-glycero-3-phosphatidylcholine/ distearoyl-sn-glycero-3-phosphatidylcholine lipid mixture as studied by Fourier transform infrared spectroscopy and differential scanning calorimetry

The phase transition behavior of a lipid bilayer of dimyristoyl-sn-glycero-3-phosphatidylcholine/distearoyl-sn-glycero-3- phosphatidylcholine (DMPC-d54/DSPC) (1:1) on a solid support with varying curvatures was investigated with differential scanning calorimetry, infrared spectroscopy, and model calculations. With increasing curvature the temperatures of the liquidus and solidus points are shifted to lower values by up to 7 degrees C and 15 degrees C, and the mixing of the two lipid species in the two phase region is altered. With increasing curvature the DSPC dominates the gel phase, whereas the DMPC-d54 is expelled to the fluid phase. Whereas the planar system shows a nearly simultaneous phase transition of DSPC and DMPC-d54, the spherical system with the highest curvature exhibits an almost complete separation of the phase transitions of the two lipids. Model calculations suggest that the shift of the liquidus point can be understood as a reduction of the lateral pressure in the bilayer with increasing curvature. The shift of the solidus line is interpreted as a result of the increased demixing of the two components in the two-phase region with increasing curvature due to lowering of the lateral pressure.

Time-resolved infrared ATR measurements of liposome transport kinetics in human keratinocyte cultures and skin reveals a dependence on liposome size and phase state

A novel in vitro method for studying the permeation kinetics of superficially applied liposomes or vesicles through layers of human skin or keratinocytes on a solid support is presented, employing attenuated total reflection infrared spectroscopy. The method is applied to investigate transport kinetics of unilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) through cultured human keratinocyte layers and through human skin. We find a strong resemblance of the qualitative features of the permeation kinetics of small unilamellar DMPC vesicles for skin and keratinocytes. Detailed studies of the vesicles transport through keratinocyte layers show that DMPC vesicles with an average diameter of 55 nm can readily permeate through the layer at 37 degrees C with a diffusion constant of D = (4.0 +/- 0.8) x 10(-15) m2/second, whereas larger vesicles of twice that diameter do not permeate at all. In contrast, liposomes containing a chemical permeation enhancer permeate through the layer significantly faster [D = (7.0 +/- 0.5) x 10(-15) m2/second] than the small DMPC vesicles despite their five-times-larger diameter. Moreover, the transport of the DMPC vesicles depends drastically on their phase state. No permeation was observed for small DMPC vesicles at a temperature of 10 degrees C when the lipid is in the crystalline phase state. Our results indicate that keratinocyte culture layers can pose a significant permeation barrier for vesicles. The permeation mechanism can be explained by diffusion of the vesicles through small pores and gaps in the layer, presumably driven by transdermal osmotic gradients.

Hydration dependence of chain dynamics and local diffusion in L-alpha-dipalmitoylphosphtidylcholine multilayers studied by incoherent quasi-elastic neutron scattering

Incoherent quasi-elastic neutron scattering is applied to study the local diffusion and chain dynamics of L-alpha-diplamiotylphosphatidylcholine molecules in oriented model membranes. Different motions are distinguished by changing the hydration of the multilayers as well as by measuring below and above the gel-to-liquid crystalline phase transition. The time range of the utilized time-of-flight spectrometer permits to observe two types of motion to be observed more closely: chain defect motions and the local diffusion of the whole molecule in its solvation cage. Oriented lipid membranes are a useful system for the observation of chain defects, as they can be macroscopically oriented, in contrast to most polymers. As a representative model for a chain defect a kink is chosen and the corresponding scattering functions are derived. The kink motion can explain the entire dynamics seen in the gel phase, and the lifetime of such a defect was found to be 10-15 ps, in good agreement with theoretical predictions. On the other hand the dynamics in the liquid crystalline phase cannot be explained even by a superposition of several kinks and thus requires the consideration of an additional motion: the local diffusion of the molecule in its solvation cage. The size of the solvation cage is increasing with multilayer hydration and reduced temperature. Particularly interesting in view of recent discussions about the origin of the short-range repulsive forces between membranes is the experimental finding of an out-of-plane motion with an amplitude of 1-1.5 A, which cannot be explained by the undulation of the whole membrane.

Lipid transfer between small unilamellar vesicles and single bilayers on a solid support: self-assembly of supported bilayers with asymmetric lipid distribution

The transfer of lipids between small unilamellar vesicles of either dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidylglycerol (DMPG), or dioctadecyl diammonium bromide (DODAB) and a single bilayer on a solid support of chain perdeuterated DMPC-d54 has been studied by time-resolved ATR infrared spectroscopy, deuterium NMR, and DSC. The IR method was used for measuring the transfer kinetics and the amount of lipid transferred to the supported bilayer, while NMR was employed for the assessment of molecular order and for the occurrence of lipid asymmetries due to the transfer. We find that the composition of a supported planar dimyristoylphosphatidylcholine (DMPC-d54) bilayer can be modified by incubation with high concentrations of sonicated vesicles consisting of the donor lipid. Three cases were studied. First, the incubation was done with DMPC as donor lipid. The kinetics of this process is double exponential and comparatively slow, with a half-time in the range of several hours. The activation energy was estimated as 50 +/- 2 kJ/mol. In a second set of measurements, cationic DODAB or anionic DMPG was used as donor lipid. The kinetics of this transfer is 1 order of magnitude faster than for DMPC and can be described by a single exponential. For DMPG transfer, we obtained an activation energy of 35 +/- 2 kJ/mol. Independent of the headgroup charge of the donor lipid, 25-35% of the (acceptor) DMPC in the supported bilayer is not accessible for exchange with the donor lipid. The transfer of either DMPG or DODAB causes drastic changes of the phase transition behavior of the supported bilayer without significantly altering the lipid packing density of the lipids.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipophilic derivatization and its effect on the interaction of cholecystokinin (CCK) nonapeptide with phospholipids

N-terminal lipophilic derivatization of the fully active cholecystokinin analogue (Thr,Nle)-CCK-9 with the di-myristoyl-raz-thioglyceryl moiety leads to spontaneous self-aggregation of the lipopeptide into polydispersed vesicles at the liquid state. The high degree of fluidification of the vesicles favors a fast and net transfer of monomers to phospholipid bilayers even below the phase-transition temperature of the acceptor vesicles. Surprisingly, the process is accompanied by formation of peptide clusters. The peptide-rich domains exhibit a high affinity for Ca2+, a fact which may be correlated to the biological function of CCK as this hormone is known to stimulate calcium outflux from reserves in the cell membranes. Moreover, induced membrane affinity allowed to study more precisely the interaction of the CCK peptide with lipid bilayers as mimicry of cell membranes. Differently from what was observed with a similar lipogastrin derivative in which the peptide tail was found to be permanently exposed to the aqueous phase, in the case of the lipo-CCK compound insertion of its C-terminal active site into more hydrophobic compartments of the bilayer is occurring, as well as a folding into beta-type structures, thus confirming the role of cell membranes in displaying peptide hormones for specific receptor recognition.

Interaction of myelin basic protein with single bilayers on a solid support: an NMR, DSC and polarized infrared ATR study

The interaction of myelin basic protein (MBP) with single bilayers on a solid support (planar and spherical support) is studied by deuterium nuclear magnetic resonance (2H-NMR), differential scanning calorimetry (DSC) and polarized attenuated total reflection infrared spectroscopy (ATR-IR). The single bilayer consisted of either DMPC or of a binary mixture of DMPC with 10-20 mol% of an acidic phospholipid (DMPG, DMPS or DMPA). All methods applied indicate that MBP strongly interacts with the binary lipid systems but not with the pure DMPC bilayers. The interaction is predominantly electrostatic in nature and does not depend on the choice of a particular acidic lipid (for the binary systems). In particular, the results give no indication for a hydrophobic interaction of MBP with the membrane. Our data provide evidence that, in contrast to previous findings, no demixing and/or domain formation in the binary systems is induced due to the MBP coupling. The infrared order parameter was determined for both lipid components of the binary systems and shows a remarkable change for both lipids due to the interaction with MBP while the NMR order parameter remained essentially unchanged. This is discussed in terms of the different timescales characteristic for both methods. The single supported bilayer responds to the MBP coupling as a whole although only 50% of the bilayer surface is accessible to the protein, indicating a strong coupling between the two bilayer leaflets via the hydrophobic chain region. Moreover, the asymmetric coupling of MBP to the single supported bilayer does not result in a significant redistribution of lipids between the two bilayer leaflets. NMR relaxation time measurements in the headgroup and chain region of DMPG and DMPC suggest that the lateral diffusion coefficient of the acidic lipid decreases significantly due to the coupling with MBP while the zwitterionic DMPC is not affected.

Peptide hormone-membrane interactions. Intervesicular transfer of lipophilic gastrin derivatives to artificial membranes and their bioactivities

Incorporation of di-fatty acylglycerol moieties at the N-terminus of human little-gastrin-(2-17) leads to self-aggregation of the resulting lipo-gastrins into stable, most probably fluid vesicles. Net intervesicular transfer of the lipo-gastrins to phosphatidyl-choline model bilayers occurs at high rates whereby the chain length of the gastrin lipid moiety was found to affect the transfer rate more decisively than the nature of the acceptor vesicle. Similarly, the bioactivity of the lipo-gastrins is again affected by the nature of the lipid moiety suggesting differentiated interdigitation with the natural bilayer components and thus, different two-dimensional migration rates to the target receptors. Embedment of the lipo-gastrins in phosphatidylcholine bilayers at high lipid/gastrin ratios as mimicry of the cell membrane bound state does not result in onset of ordered structure, but leads to full exposure of the gastrin in essentially randomly coiled form at the water/lipid interface. This may result from the artificial N-terminal anchorage of the gastrin molecules to the bilayers, but also from the relatively tight packing of the phosphatidylcholine vesicles. Nevertheless, this observation might suggest that in the present case membrane-induced conformation and orientation may not represent a pre-requisite for the hormone receptor binding process. However, the results of this study clearly confirm even for the non-amphiphilic hormone gastrin a membrane-bound pathway for receptor recognition and occupancy.

Coupling of spectrin and polylysine to phospholipid monolayers studied by specular reflection of neutrons

The technique of specular reflection of neutrons is applied for the first time to study the charge-dependent interaction of the protein spectrin and the polypeptide poly-L-lysine with model phospholipid monolayers in the condensed phase state. We first established the structure of a pure monolayer of dimyristolyphosphatidylcholine (DMPC) in both the expanded and condensed fluid phase states without protein in the subphase. The thickness of the hydrocarbon chains increases from 11.4 +/- 1.5 A in the expanded state to 15.8 +/- 1.5 A in the condensed state, whereas the head group region is approximately 10 A thick for both phase states. When spectrin is present in the subphase, the dimensions of DMPC in the condensed state are not significantly affected, but there is approximately 0.09 volume fraction spectrin in the head group region. Lipid-spectrin coupling is enhanced by electrostatic interaction, as the volume fraction of spectrin in the head group region increases to 0.22 in a mixed monolayer of DMPC and negatively charged dimyristolyphosphatidylglycerol in the condensed state. In contrast to spectrin, polylysine does not penetrate the head group region, but forms a layer electrostatically adsorbed to the charged head groups.

Structure of an adsorbed dimyristoylphosphatidylcholine bilayer measured with specular reflection of neutrons

Using specular reflection of neutrons, we investigate for the first time the structure of a single dimyristoylphosphatidylcholine bilayer adsorbed to a planar quartz surface in an aqueous environment. We demonstrate that the bilayer is strongly adsorbed to the quartz surface and is stable to phase state changes as well as exchange of the bulk aqueous phase. Our results show that the main phase transition is between the L alpha phase and the metastable L beta'* phase, with formation of the P beta' ripple phase prevented by lateral stress on the adsorbed bilayer. By performing contrast variation experiments, we are able to elucidate substantial detail in the interfacial structure. We measure a bilayer thickness of 43.0 +/- 1.5 A in the L alpha phase (T = 31 degrees C) and 46.0 +/- 1.5 A in the L beta'* phase (T = 20 degrees C). The polar head group is 8.0 +/- 1.5 A thick in the L alpha phase. The water layer between the quartz and bilayer is 30 +/- 10 A for the lipid in both the L alpha and L'* phase. Our results agree well with those previously reported from experiments using lipid vesicles and monolayers, thus establishing the feasibility of our experimental methods.

Physical properties of single phospholipid bilayers adsorbed to micro glass beads. A new vesicular model system studied by 2H-nuclear magnetic resonance

Spherical supported vesicles (SSVs), a new model system consisting of single dimyristoyl phosphatidylcholine (DMPC) bilayers adsorbed to spherical glass beads with a narrow size distribution, were prepared at two different sizes (0.5 and 1.5 microns) and their physical properties were studied by deuterium nuclear magnetic resonance (2H-NMR). Such SSV samples can be prepared at any desired size between 0.3 and 10 microns. The 2H-NMR measurements provide evidence for a strong dependence of the spectra and the transverse relaxation times on the curvature of the SSVs in a diameter range between 0.5 and 1.5 microns. For larger SSVs (1.5 microns diameter) their powder spectra and their calculated oriented spectra are similar to those obtained for multilamellar dispersions of DMPC-d54. The lineshape of the smaller SSVs exhibits a temperature dependence which is not found in multilamellar samples. The SSVs are stable in the liquid crystalline phase over days but irreversibly change to multilamellar vesicles in the gel state. The average thickness of the water layer between the single bilayer and the glass bead surface was estimated by 1H-NMR to e 17 +/- 5 A.

Specular reflection of neutrons at phospholipid monolayers. Changes of monolayer structure and headgroup hydration at the transition from the expanded to the condensed phase state

The specular neutron reflection technique has been applied for the first time to study the structure and head group hydration of a phospholipid monolayer (dimyristoylphosphatidylcholine containing negatively charged phospholipids) in the condensed and expanded state. By variation of the contrast of the subphase to that of the air it is shown that at the transition from the condensed to the expanded state the carboxyl bonds of the glycerol backbone are hydrated leading to a strong structural change of the headgroup. The total monolayer thickness in the condensed state is 22.5 +/- 1 A (tilt angle 32 +/- 6 degrees) and decreases to 19.5 +/- 1 A in the expanded state. The mean molecular volume increases from 1190 +/- 50 A3 to 1250 +/- 50 A3.

On the modulation of a high-enthalpy pretransition in binary mixtures of DMPC and DMPG by polar headgroup interaction

Employing high-sensitivity differential scanning calorimetry (DSC), we discovered a pretransition in binary mixtures of dimyristoyl phosphatidylcholine and dimyristoyl phosphatidylglycerol, the main feature of which is its extraordinarily high transition enthalpy of 6.3 Kcal/mol, nearly an order of magnitude higher than those values previously found for such transitions. Using DSC, deuterium nuclear magnetic resonance, and electron microscopy, it is shown that the energetic origin of this type of pretransition is caused by interactions between the phospholipids in their headgroup region. The most likely interaction involves the formation of a hydrogen bond between the headgroups of the two phospholipid species in the gel (L beta') phase which is disrupted at the transition to the "ripple" (P beta') phase. The finding that this large pretransition is unique for mixtures of phosphocholine and phosphoglycerol with myristoyl chains indicates a dependence of the headgroup long range order of such mixtures in the gel phase on the acyl chain length.

Solubilization of DMPC and DPPC vesicles by detergents below their critical micellization concentration: high-sensitivity differential scanning calorimetry, Fourier transform infrared spectroscopy and freeze-fracture electron microscopy reveal two interaction sites of detergents in vesicles

The interaction of sodium deoxycholate, sodium cholate and octyl glucoside with sonicated vesicles of L alpha-dimyristoyl-phosphatidylcholine (DMPC) and L alpha-dipalmitoylphosphatidylcholine (DPPC) at concentrations below the critical micellization concentration (cmc) of the detergents was studied by high-sensitivity DSC (hs-DSC), Fourier transform infrared spectroscopy (FT-IR) and freeze-fracture electron microscopy. The two phospholipids exhibited a striking different thermotropic behaviour in the presence of these detergents. For DPPC vesicles, the detergents were found to interact exclusively in the aqueous interface region of the bilayer below the membrane saturation concentration Rsat while in DMPC vesicles two coexisting interaction sites below this concentration persist. These are detergents which interact at the aqueous interface region (site 1) and in the acyl chain region (site 2) of the DMPC vesicles. The partition coefficients K of the detergents between DPPC vesicles and the water phase were calculated from the hs-DSC results at two detergent/phospholipid molar ratios Rtot less than or equal to Rsat as 0.35, 0.049 and 0.040 mol-1 for sodium deoxycholate, sodium cholate and octyl glucoside, respectively. In contrast, for DMPC the K values for Rtot less than or equal to Rsat were found to be dependent on Rtot due to the occupation of site 2 by the detergents above a certain Rtot. The model is discussed on the basis of the detergents free energies of transfer from the water phase to site 1 and site 2 of the vesicles, respectively. The solubilization behaviour of DPPC vesicles, dependent on whether the total detergent concentration is above or below the cmc at Rsat, differed significantly as revealed by hs-DSC. This suggests that in the latter case an additional hydrophobic effect could facilitate the formation of disc shaped mixed micelles. Moreover, this different behaviour was employed to measure the cmc values of the detergents studied in the presence of the vesicles by hs-DSC.