Dipalmitoylphosphatidylcholine Multilamellar Vesicles Research Articles

LASER INDUCED CONFORMATIONAL CHANGES OF LIPIDS WITHIN MULTILAMELLAR VESICLES

"The biomedical potential of destabilizing liposomes through photoinduction relies on the use of near-infrared light, which offers inherent therapeutic advantages. Researchers have explored the effects of infrared laser light on dipalmitoyl phosphatidylcholine (DPPC) multilamellar vesicles, specifically investigating the interaction between the laser and zwitterionic dipalmitoyl phosphatidylcholine multilamellar vesicles using Fourier transform infrared spectroscopy and differential scanning calorimetry measurements. The results revealed that laser irradiation increased the number of gauche conformers and led to conformational changes within the acyl chains of the phospholipids. The transition temperature of lyophilized vesicles was also shifted to a lower temperature after laser irradiation, indicating that the laser had a dipalmitoyl phosphatidylcholine multilamellar significant effect on the acyl chains in the dipalmitoyl phosphatidylcholine bilayers and decreased the transition cooperativity of lipid acyl chains. These findings could aid in the development of more effective liposomal drug delivery systems by enhancing our understanding of the interaction between laser and lipid bilayers."

Ruscogenin interacts with DPPC and DPPG model membranes and increases the membrane fluidity: FTIR and DSC studies

Ruscogenin, a kind of steroid saponin, has been shown to have significant anti-oxidant, anti-inflammatory, and anti-thrombotic characteristics. Furthermore, it has the potential to be employed as a medicinal medication to treat a variety of acute and chronic disorders. The interaction of a drug molecule with cell membranes can help to elucidate its system-wide protective and therapeutic effects, and it's also important for its pharmacological activity. The molecular mechanism by which ruscogenin affects membrane architecture is still a mystery. Ruscogenin's interaction with zwitterionic dipalmitoyl phosphatidylcholine (DPPC) and anionic dipalmitoyl phosphatidylglycerol (DPPG) multilamellar vesicles (MLVs) was studied utilizing two non-invasive approaches, including: Fourier Transform Infrared (FTIR) spectroscopy and Differential Scanning Calorimetry. Ruscogenin caused considerable alterations in the phase transition profile, order, dynamics and hydration state of head groups and glycerol backbone of DPPC and DPPG MLVs at all concentrations. The DSC results indicated that the presence of ruscogenin decreased the main phase transition temperature (Tm) and enthalpy (ΔH) values of both membranes and increased half height width of the main transition (ΔT1/2). The FTIR results demonstrated that all concentrations (1, 3, 6, 9, 15, 24 and 30 mol percent) of ruscogenin disordered the DPPC MLVs both in the gel and liquid crystalline phases while it increased the order of DPPG MLVs in the liquid crystalline phase. Moreover, ruscogenin caused an increase in the dynamics of DPPC and DPPG MLVs in both phases. Additionally, it enhanced the hydration of the head groups of lipids and the surrounding water molecules implying ruscogenin to interact strongly with both zwitterionic and charged model membranes.

Cholecalciferol has strong effect on the order and dynamics of DPPC membranes: A combined Fourier transform infrared spectroscopy and differential scanning calorimetry study

The effect of cholecalciferol on dipalmitoyl phosphatidylcholine (DPPC) multilamellar vesicles (MLVs) as a function of temperature and cholecalciferol concentration (1–40 mol%) was studied by using Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The investigation of the CH, C = O, and PO2− antisymmetric double stretching modes in FTIR spectra and DSC studies reveal that addition of cholecalciferol into pure DPPC liposomes shifts the phase transition to lower temperature, disorders and increases the dynamics of the acyl chains in both phases and induces hydrogen bond formation in the interfacial region.

Interaction of the cholesterol reducing agent simvastatin with zwitterionic DPPC and charged DPPG phospholipid membranes

Simvastatin is a lipid-lowering drug in the pharmaceutical group statins. Interaction of a drug with lipids may define its role in the system and be critical for its pharmacological activity. We examined the interactions of simvastatin with zwitterionic dipalmitoyl phosphatidylcholine (DPPC) and anionic dipalmitoyl phosphatidylglycerol (DPPG) multilamellar vesicles (MLVs) as a function of temperature at different simvastatin concentrations using Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The FTIR results indicate that the effect of simvastatin on membrane structure and dynamics depends on the type of membrane lipids. In anionic DPPG MLVs, high simvastatin concentrations (12, 18, 24 mol%) change the position of the CH2 antisymmetric stretching mode to lower wavenumber values, implying an ordering effect. However, in zwitterionic DPPC MLVs, high concentrations of simvastatin disorder systems both in the gel and liquid crystalline phases. Moreover, in DPPG and DPPC MLVs, simvastatin has opposite dual effects on membrane dynamics. The bandwidth of the CH2 antisymmetric stretching modes increases in DPPG MLVs, implying an increase in the dynamics, whereas it decreases in DPPC MLVs. Simvastatin caused broadening of the phase transition peaks and formation of shoulders on the phase transition peaks in DSC curves, indicating multi-domain formations in the phospholipid membranes. Because physical features of membranes such as lipid order and fluidity may be changed with the bioactivity of drugs, opposing effects of simvastatin on the order and dynamics of neutral and charged phospholipids may be critical to deduce the action mechanism of the drug and estimate drug-membrane interactions.

Dimethyl sulfoxide-induced dehydration of the intermembrane space of dipalmitoylphosphatidylcholine multilamellar vesicles: Neutron and synchrotron diffraction study

Small-angle neutron scattering spectra of a polydispersed population of dipalmitoylphosphatidylcholine (DPPC) unilamellar vesicles in heavy water in the presence of dimethyl sulfoxide (DMSO) are analyzed by means of the separated form-factor method. An increase in the mole fraction of DMSO in water from 0 to 15% was shown to lead to an increase in the thickness of the bilayer to the characteristics repeat distances of DPPC multilamellar membranes. This fact is indicative of dehydration of the intermembrane space and a steric contact between adjacent DPPC bilayers at 15% mole fraction of DMSO.

Temperature-dependent bifurcation of cooperative interactions in pure and enriched in β-carotene DPPC liposomes

We examined the influence of temperature on lipid intermolecular interactions and the organization of bilayers within multilamellar dipalmitoylphosphatidylcholine (DPPC) liposomes. We also investigated the effect of 0.5 mol% β-carotene, a non-polar carotenoid, on the adhesive properties of these liposomes. Atomic force microscopy (AFM) and differential scanning calorimetry (DSC) were used to correlate the changes in the physical properties of the liposomal systems with their thermotropic behaviour. Using DSC we detected two transitions in pure DPPC vesicles and in those containing 0.5 mol% β-carotene. In both systems the pretransition occurred at 34.5(1)°C and the main phase transition at 41.4 °C during heating. Upon cooling, the temperatures of the pretransition and the main transition decreased by about 6 °C and 1 °C, respectively. Changes in enthalpy and entropy were also similar in the two investigated systems. Data obtained in parallel AFM force experiments show that the adhesive forces between the liposomal systems and AFM probe strongly depend on the loading rate. Moreover, their characteristic monotonic changes and discontinuities are sensitive to temperature. In the range of temperatures from 27 °C to 31 °C, i.e. below the temperature of phase transition from gel to ripple phase, the adhesive forces measured in a water environment are about an order of magnitude higher in the presence of β-carotene than in pure DPPC liposomes. The observed variable dependence of adhesion on the loading rate suggests that there are changes in the long- and short-range interactions between lipids, and that these may be related to the occurrence of some clustering effects. In addition, the simultaneous existence of different subphases was found in the gel phase of DPPC liposomes. The presence of β-carotene at a level of 0.5 mol% stimulates the structural reorganization of DPPC multilamellar vesicles and enhances the bifurcation phenomenon detected in these systems.

A comparative study of the effects of cholesterol and desmosterol on zwitterionic DPPC model membranes

Desmosterol is a direct biosynthetic precursor of cholesterol in Bloch biochemical pathway of cholesterol biosynthesis and differs with cholesterol only by a double bond in carbon 24. In this study, we aimed to research for the first time comparative effects of cholesterol and its precursor desmosterol on dipalmitoyl phosphatidylcholine (DPPC) multilamellar vesicles (MLVs) by utilizing Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). Our DSC studies reveal that with the addition of increasing desmosterol and cholesterol concentrations into pure DPPC MLVs, the pretransition disappears, the main phase transition shifts to lower temperatures and then disappears. While the main phase transition is abolished at 25mol% concentration of desmosterol, this disappearance of the main phase transition occurs at cholesterol concentration above 30mol%. Our FTIR studies show that both desmosterol and cholesterol decrease the order in the gel phase, whereas they increase it in the liquid crystalline phase. Importantly, we found that the effect of desmosterol on membrane order is weaker than that of cholesterol in both phases. Moreover, desmosterol and cholesterol increase the dynamics of DPPC membranes in the gel phase, while they decrease it in the liquid crystalline phase. Both sterols also induce a decrease in the wavenumber values of the CO stretching and PO2− antisymmetric double stretching bands of DPPC both in the gel and liquid crystalline phases, which points out hydrogen bonding in between the hydroxyl group of both sterols and the carbonyl and phosphate groups of DPPC membranes.

Chain interdigitation in DPPC bilayers induced by HgCl2: Evidences from continuous wave and pulsed EPR

Continuous-wave electron paramagnetic resonance (CW-EPR) spectroscopy and electron spin echo methods of pulsed EPR of phosphatidylcholine spin-labeled at different positions, n, in the sn-2 chain (n-PCSL, n=5, 7, 10, 12, 14, and 16) are used to study the interaction of inorganic mercury chloride HgCl2 with multilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC). For temperatures through the gel phase of DPPC multilayers, the CW-EPR spectra show that an increase of HgCl2 content in the dispersion medium slightly increases the rotational mobility of 5-PCSL and markedly restricts the motion of 16-PCSL. Mercury chloride at 100mM (HgCl2/lipid molar ratio=2:1) removes the gradient of increasing mobility along the chain found in DPPC bilayers in the gel phase. In contrast, HgCl2 does not influence the DPPC chain flexibility profile in the fluid phase. It also suppresses the pre-transition and moderately downshifts the main transition temperature of DPPC membranes. These findings indicate that HgCl2 affects the lipid chain packing of DPPC bilayers and are consistent with the induction of an interdigitated gel phase. Further, D2O-electron spin echo envelope modulation spectroscopy indicates that in the interdigitated phase a higher water permeation is favored at any chain position and the sigmoidal transmembrane water accessibility profile of DPPC bilayers is abolished. Accordingly, the positional dependence of 14N-hyperfine splitting, 2Azz, shows that the typical hydrophobic barrier of DPPC is significantly altered in the interdigitated phase and all the segments of the lipid chains result to be in a more polar environment.

Self-Assembly of a Peptide Amphiphile Containing l-Carnosine and Its Mixtures with a Multilamellar Vesicle Forming Lipid

The self-assembly of the peptide amphiphile (PA) hexadecyl-(β-alanine-histidine) is examined in aqueous solution, along with its mixtures with multilamellar vesicles formed by DPPC (dipalmitoyl phosphatidylcholine). This PA, denoted C(16)-βAH, contains a dipeptide headgroup corresponding to the bioactive molecule L-carnosine. It is found to self-assemble into nanotapes based on stacked layers of molecules. Bilayers are found to coexist with monolayers in which the PA molecules pack with alternating up-down arrangement so that the headgroups decorate both surfaces. The bilayers become dehydrated as PA concentration increases and the number of layers in the stack decreases to produce ultrathin nanotapes comprised of 2-3 bilayers. Addition of the PA to DPPC multilamellar vesicles leads to a transition to well-defined unilamellar vesicles. The unique ability to modulate the stacking of this PA as a function of concentration, combined with its ability to induce a multilamellar to unilamellar thinning of DPPC vesicles, may be useful in biomaterials applications where the presentation of the peptide function at the surface of self-assembled nanostructures is crucial.

Thermal, dynamic and structural properties of drug AT1 antagonist olmesartan in lipid bilayers

It is proposed that AT1 antagonists (ARBs) exert their biological action by inserting into the lipid membrane and then diffuse to the active site of AT1 receptor. Thus, lipid bilayers are expected to be actively involved and play a critical role in drug action. For this reason, the thermal, dynamic and structural effects of olmesartan alone and together with cholesterol were studied using differential scanning calorimetry (DSC), 13C magic-angle spinning (MAS) nuclear magnetic resonance (NMR), cross-polarization (CP) MAS NMR, and Raman spectroscopy as well as small- and wide angle X-ray scattering (SAXS and WAXS) on dipalmitoyl-phosphatidylcholine (DPPC) multilamellar vesicles. 13C CP/MAS spectra provided direct evidence for the incorporation of olmesartan and cholesterol in lipid bilayers. Raman and X-ray data revealed how both molecules modify the bilayer's properties. Olmesartan locates itself at the head-group region and upper segment of the lipid bilayers as 13C CP/MAS spectra show that its presence causes significant chemical shift changes mainly in the A ring of the steroidal part of cholesterol. The influence of olmesartan on DPPC/cholesterol bilayers is less pronounced. Although, olmesartan and cholesterol are residing at the same region of the lipid bilayers, due to their different sizes, display distinct impacts on the bilayer's properties. Cholesterol broadens significantly the main transition, abolishes the pre-transition, and decreases the membrane fluidity above the main transition. Olmesartan is the only so far studied ARB that increases the gauche:trans ratio in the liquid crystalline phase. These significant differences of olmesartan may in part explain its distinct pharmacological profile.

Convulsant agent pentylenetetrazol does not alter the structural and dynamical properties of dipalmitoylphosphatidylcholine model membranes

Pentylenetetrazol (PTZ) is an epileptogenic agent, which is widely used in the determination of epilepsy-induced alterations and in the assessment of anticonvulsant agents in epileptic studies. Even though PTZ is suggested to induce repetitive firing of nerve fibers and shorten the refractory, its mechanism of action is only partially understood. In the literature there are discrepancies for its action mechanism. While some studies stated that primary sites of PTZ are membrane proteins, some reports indicated that PTZ acts on membrane lipids. In order to gain new insight for this we tested the possibility of interaction of PTZ with a simplified model system called dipalmitoylphosphatidylcholine (DPPC) multilamellar vesicles (MLVs) at agent concentrations (0–24 mol%) using differential scanning calorimetry (DSC), Fourier transform infrared (FTIR), electron spin resonance (ESR) and steady-state fluorescence spectroscopy. The results showed that PTZ at concentrations used (1–24 mol%), does not cause any significant change in lipid phase behavior, lipid dynamics (fluidity), lipid acyl chain flexibility (order), hydration state of the head group and/or the region near the head group of DPPC MLVs. These results clearly revealed that PTZ does not change the structural and dynamical parameters of neutral DPPC lipid vesicles and does not locate within the bilayer.

Amyloglucosidase enzymatic reactivity inside lipid vesicles

Efficient functioning of enzymes inside liposomes would open new avenues for applications in biocatalysis and bioanalytical tools. In this study, the entrapment of amyloglucosidase (AMG) (EC 3.2.1.3) from Aspergillus niger into dipalmitoylphosphatidylcholine (DPPC) multilamellar vesicles (MLVs) and large unilamellar vesicles (LUVs) was investigated. Negative-stain, freeze-fracture, and cryo-transmission electron microscopy images verified vesicle formation in the presence of AMG. Vesicles with entrapped AMG were isolated from the solution by centrifugation, and vesicle lamellarity was identified using fluorescence laser confocal microscopy. The kinetics of starch hydrolysis by AMG was modeled for two different systems, free enzyme in aqueous solution and entrapped enzyme within vesicles in aqueous suspension. For the free enzyme system, intrinsic kinetics were described by a Michaelis-Menten kinetic model with product inhibition. The kinetic constants, Vmax and Km, were determined by initial velocity measurements, and Ki was obtained by fitting the model to experimental data of glucose concentration-time curves. Predicted concentration-time curves using these kinetic constants were in good agreement with experimental measurements. In the case of the vesicles, the time-dependence of product (glucose) formation was experimentally determined and simulated by considering the kinetic behavior of the enzyme and the permeation of substrate into the vesicle. Experimental results demonstrated that entrapped enzymes were much more stable than free enyzme. The entrapped enzyme could be recycled with retention of 60% activity after 3 cycles. These methodologies can be useful in evaluating other liposomal catalysis operations.

Structure of (KIAGKIA) 3 Aggregates in Phospholipid Bilayers by Solid-State NMR

The interchain 13C- 19F dipolar coupling measured in a rotational-echo double-resonance (REDOR) experiment performed on mixtures of differently labeled KIAGKIA-KIAGKIA-KIAGKIA (K3) peptides (one specifically 13C labeled, and the other specifically 19F labeled) in multilamellar vesicles of dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol (1:1) shows that K3 forms close-packed clusters, primarily dimers, in bilayers at a lipid/peptide molar ratio (L/P) of 20. Dipolar coupling to additional peptides is weaker than that within the dimers, consistent with aggregates of monomers and dimers. Analysis of the sideband dephasing rates indicates a preferred orientation between the peptide chains of the dimers. The combination of the distance and orientation information from REDOR is consistent with a parallel (N–N) dimer structure in which two K3 helices intersect at a cross-angle of ∼20°. Static 19F NMR experiments performed on K3 in oriented lipid bilayers show that between L/P = 200 and L/P = 20, K3 chains change their absolute orientation with respect to the membrane normal. This result suggests that the K3 dimers detected by REDOR at L/P = 20 are not on the surface of the bilayer but are in a membrane pore.

Thermotropic phase behaviour of α-dipalmitoylphosphatidylcholine multibilayers is influenced to various extents by carotenoids containing different structural features- evidence from differential scanning calorimetry

Carotenoids are the effective modulators of physical properties of model and natural membranes. To demonstrate the relationship between the structure of carotenoids and their effect on the molecular dynamics of membranes, we have investigated the influence of five structurally different carotenoids: β-carotene, lycopene, lutein, violaxanthin, zeaxanthin and additionally carotane- a fully saturated derivative of β-carotene, on thermotropic phase behaviour of dipalmitoylphosphatidylcholine (DPPC) multilamellar vesicles by means of differential scanning calorimetry (DSC). The results obtained indicate that the carotenoids used modulated the thermotropic properties of multibilayers to various extents, broadening the pretransition and the main phase transition peaks and shifting them to lower temperatures. Pronounced decrease of pretransition enthalpy (ΔHp) proves that carotenoids very strongly alter the membrane properties in its gel phase. Comparison of the influence of several carotenoids shows that a rigid, polyisoprenoid chain plays a basic role in altering the thermotropic properties of such membranes and the presence of rings without oxygen-containing groups has a minor significance for the observed interactions. Carotenoids containing epoxy and/or hydroxy groups attached to their rings modify the thermotropic phase behaviour of DPPC multilamellar vesicles stronger than carotenes- a result of their orientation in the DPPC bilayer.

Thermodynamics and Kinetics of the Early Steps of Solid-State Nucleation in the Fluid Lipid Bilayer

Temperature-scanning (10−70 °C) calorimetric, densitometric, and acoustic measurements have been performed in aqueous dispersions of dipalmitoylphosphatidylcholine multilamellar vesicles. In the biologically relevant liquid-crystalline state close to the chain freezing point (within 42−50 °C, in our case), the measured quantities display anomalous deviations from the “ideal” shape of their temperature dependence curves. The deviations indicate the early steps of gel-state nucleation within the stable liquid-crystalline state (Frenkel's heterophase fluctuations). An earlier proposed kinetic model of nucleation (Kharakoz et al. J. Phys. Chem. 1993, 97, 9844−9851) has been refined to achieve more clear physical substantiation. When the deviations are analyzed through the refined model, both the thermodynamic and kinetic characteristics of nucleation are determined. The fluid−solid interface line tension energy is about 0.7 kT per boundary lipid molecule. The tension is strong enough for the phase transition to be first-order and, at the same time, weak enough to allow extensive heterophase fluctuations. The speed of new-phase-front propagation near the transition temperature is virtually proportional to the difference between the current temperature and transition temperature, the proportionality coefficient being 0.2 cm s-1 K-1 (estimated from ultrasonic relaxation data). The experimental data and the nucleation model are consistent with literature data on the anomalous “critical-like” behavior of lipid membranes near the main transition (bending rigidity drop, anomalous swelling, slowing relaxation processes, etc.). Thus, we show that most of the anomalies can be rationalized in terms of the weak first-order transition mechanism without a hypothesis on the proximity of bilayer to a critical state.

Fluorescent Dimers of Merocyanine 540 (MC540) in the Gel Phase of Phosphatidylcholine Liposomes

Abstract— Fluorescence emission from merocyanine 540 (MC540) dimers was observed in dipalmitoylphosphatidylcholine (DPPC) vesicles. This unusual behavior was observed only for vesicles in the gel‐phase state. No dimer fluorescence was observed either in monopalmitoylphosphati‐dylcholine (C16PC) micelles or in liquid‐crystalline DPPC vesicles, indicating that dimer fluorescence efficiency increases in highly packed interfaces. The excitonic theory of Kasha was used to interpret the spectral features. The overall fluorescence quantum yield (φr) decreases with decreasing lipid: probe ratio, not only because of the presence of a weakly fluorescent dimer that absorbs a high fraction of the total absorbed light but also due to quenching of monomer emission. This suggests the existence of probe domains. The dimer fluorescence quantum yields (φm) were estimated in DPPC large unilamellar vesicles (LUV) and DPPC multilamellar vesicles. The dependence of φr with probe concentration is compatible with values of φm lower than 0.05. The dimerization equilibrium of MC540 in C16PC micelles and DPPC‐LUV was also studied. Apparent dimerization equilibrium constants, Kdapp and dimer absorption spectrum were calculated in C16PC micelles for the first time. The dimerization equilibrium constant in DPPC‐LUV was calculated and discussed in terms of the fraction of volume occupied by the lipid phase.

Small angle neutron scattering and calorimetric studies of large unilamellar vesicles of the phospholipid dipalmitoylphosphatidylcholine

High-resolution differential scanning calorimetry (DSC) and small angle neutron scattering (SANS) experiments have been conducted on large unilamellar vesicles (LUV's) of the phospholipid dipalmitoylphosphatidylcholine (DPPC) in excess water. The DSC results indicate a phase transition at temperatures corresponding to the gel ${(L}_{{\ensuremath{\beta}}^{\ensuremath{'}}})$ to ripple ${(P}_{{\ensuremath{\beta}}^{\ensuremath{'}}})$ phase transition seen in multilamellar vesicles of DPPC while the SANS experiments provide direct evidence for the formation of the ${P}_{{\ensuremath{\beta}}^{\ensuremath{'}}}$ phase in these systems. In addition, it is shown that SANS is an effective technique for extracting structural parameters such as vesicle radius and thickness in LUV model membrane systems.

Small-angle scattering studies of the fully hydrated phospholipid DPPC

Small-angle neutron and x-ray scattering studies have been carried out on fully hydrated dipalmitoylphosphatidylcholine (DPPC) multilamellar vesicles. This system is known to exhibit two distinct ripple ${(P}_{{\ensuremath{\beta}}^{\ensuremath{'}}})$ phases, which depend on sample history, at temperatures intermediate to its high-temperature liquid crystalline ${(L}_{\ensuremath{\alpha}}),$ phase, and its low-temperature gel ${(L}_{{\ensuremath{\beta}}^{\ensuremath{'}}}),$ phase. On cooling from the ${L}_{\ensuremath{\alpha}}$ phase, the ${P}_{{\ensuremath{\beta}}^{\ensuremath{'}}}$ phase displays a complex multipeak diffraction pattern that differs significantly from the diffraction pattern seen in the ${P}_{{\ensuremath{\beta}}^{\ensuremath{'}}}$ phase obtained on warming from the ${L}_{{\ensuremath{\beta}}^{\ensuremath{'}}}$ phase. Examining the ${P}_{{\ensuremath{\beta}}^{\ensuremath{'}}}$ phase on cooling using small-angle neutron scattering and x-ray diffraction techniques leads to the conclusion that this phase is characterized by a long wavelength ripple $({\ensuremath{\lambda}}_{r}\ensuremath{\sim}330\AA{})$ and a highly monoclinic unit cell (\ensuremath{\gamma}\ensuremath{\sim}125\ifmmode^\circ\else\textdegree\fi{}). As the ${P}_{{\ensuremath{\beta}}^{\ensuremath{'}}}$ phase is traversed in temperature, the ripple wavelength changes significantly while the monoclinicity remains unchanged. Ripples from the ${P}_{{\ensuremath{\beta}}^{\ensuremath{'}}}$ phase are seen to persist into the ${L}_{{\ensuremath{\beta}}^{\ensuremath{'}}}$ phase on cooling, leading to increased small-angle scattering characteristic of a disordered stacking of the lamellae.

Electron-Spin Resonance Study of Aggregation of Gramicidin in Dipalmitoylphosphatidylcholine Bilayers and Hydrophobic Mismatch

The effect of aggregation of gramicidin A′ (GA) on the phase structure of dipalmitoylphosphatidylcholine (DPPC) multilamellar vesicles was studied by cw-ESR using a chain-labeled lipid (16PC) at temperatures between 30° and 45°C that span the main phase transition of DPPC. Boundary lipids were observed only in dispersions with GA/DPPC molar ratios >1:15, where GA aggregates. Detailed fits by nonlinear least squares (NLLS) methods are consistent with the boundary lipid being characterized by a large negative order parameter (∼−0.4), indicative of a dynamic bending of the end of the acyl chain, and a substantially reduced motion, about an order of magnitude slower than that of the bulk lipid. The NLLS analysis compares favorably with a recent two-dimensional Fourier transform ESR study on DPPC/GA vesicles, which accurately discerned the bulk lipid. The detailed ESR observables are discussed in terms of the ordering effect of GA at low concentration of GA, the dissociation of the GA channel and the dynamic bending of the end chain segment of boundary lipid at high concentration of GA, and of H II phase formation induced by GA. It is suggested that these phenomena can be interpreted in terms of the combined effects of partial dehydration of the lipid headgroup by the GA and of the hydrophobic mismatch between GA and DPPC molecules. Substantial hysteresis is observed for heating versus cooling cycles, but only for a GA/DPPC molar ratio >1:15. This is consistent with the aggregation of GA molecules at high concentrations.

Measurement of the Lateral Diffusion of Dipalmitoylphosphatidylcholine Adsorbed on Silica Beads in the Absence and Presence of Melittin: A 31P Two-Dimensional Exchange Solid-State NMR Study

31P two-dimensional exchange solid-state NMR spectroscopy was used to measure the lateral diffusion, D L, in the fluid phase of dipalmitoylphosphatidylcholine (DPPC) in the presence and absence of melittin. The use of a spherical solid support with a radius of 320 ± 20 nm, on which lipids and peptides are adsorbed together, and a novel way of analyzing the two-dimensional exchange patterns afforded a narrow distribution of D L centered at a value of (8.8 ± 0.5) × 10 −8 cm 2/s for the pure lipid system and a large distribution of D L spanning 1 × 10 −8 to 10 × 10 −8 cm 2/s for the lipids in the presence of melittin. In addition, the determination of D L for nonsupported DPPC multilamellar vesicles (MLVs) suggests that the support does not slow down the lipid diffusion and that the radii of the bilayers vary from 300 to 800 nm. Finally, the DPPC-melittin complex is stabilized at the surface of the silica beads in the gel phase, opening the way to further study of the interaction between melittin and DPPC.