Reduced fluidity of dipalmitoyl phosphatidic acid membranes by salicylic acid

This paper presents DSC and NMR study of how the kerotolytic drug, salicylic acid (SA), affects the thermotropic and morphological behavior of a model membrane, dipalmitoyl phosphatidic acid (DPPA). The membrane-drug system has been studied in the multilamellar vesicular (MLV) and in the unilamellar vesicular (ULV) forms, for SA/DPPA molar ratios from 0 to 0.5. The mode of interaction of SA molecules with DPPA is similar in MLV and ULV. Chain-melting transition becomes sharper and shifts to higher temperatures in the presence of the drug, implying an enhanced co-operativity of the acyl chains. NMR and DSC data indicate that the drug molecules are located in the aqueous interfacial region neighboring the lipid headgroups. The membrane becomes more rigid in the presence of the drug molecules, due to a stronger interaction between the lipid headgroups leading to reduced permeability. ULVs are destroyed by even a short equilibration at room temperature, whereas prolonged equilibration of the MLV only leads to a slightly reduced interaction between the lipid headgroups due to sequestering of the drug molecules in the interfacial aqueous region.     

Propyl paraben-induced changes in dipalmitoyl phosphatidylethanolamine vesicles

This article reports the influence of the preservative, propyl paraben (PPB), on the phase transition and dynamics of dipalmitoyl phosphatidylethanolamine (DPPE) vesicles both in multilamellar vesicular (MLV) and unilamellar vesicular (ULV) forms using DSC and (¹H and ³¹P) NMR. DSC results indicate that the mechanism by which PPB interacts with DPPE vesicles is similar in both forms. Addition of PPB to DPPE dispersion results in lowering of the gel to liquid crystalline phase transition temperature (T _m) and consequently increases DPPE headgroup fluidity. At high PPB concentration, additional transitions are observed whose intensity increases with increasing PPB concentration. DSC and NMR data indicate that the PPB molecules get intercalated between the DPPE headgroups as the polar group of the PPB molecules interacts with the polar group of PE, and the alkyl chain of PPB penetrates into the acyl chain region. The interesting finding with MLV is that the gel phase of DPPE in the presence of PPB, on equilibration at 25 °C, transforms to a stable crystalline subgel phases and whose intensity increases with increasing PPB concentration. The effect of inclusion of cholesterol in the PPB-free and PPB-doped DPPE dispersion was also studied.     

Interaction of propyl paraben with dipalmitoyl phosphatidylcholine bilayer: a differential scanning calorimetry and nuclear magnetic resonance study

The influence of the preservative, propyl paraben (PPB) on the biophysical properties of dipalmitoyl phosphatidyl choline (DPPC) vesicles, both in multilamellar vesicle (MLV) and unilamellar vesicle (ULV) forms, has been studied using DSC and (¹H and ³¹P) NMR. The mechanism by which PPB interacts with DPPC bilayers was found to be independent of the morphological organization of the lipid bilayer. Incorporation of PPB in DPPC vesicles causes a significant depression in the transition temperature and enthalpy of both the pre-transition (PT) and the gel to liquid crystalline transition. The presence of the PPB also reduces the co-operativity of these transitions. However, at high PPB concentration the PT disappears. DSC and NMR findings indicate that: (i) PPB is bound strongly to the lipid bilayer leading to increased headgroup fluidity due to reduced headgroup–headgroup interaction and (ii) the PPB molecules are intercalated between the DPPC polar headgroups with its alkyl chain penetrate into the co-operative region. MLV incorporated with high PPB concentration shows additional transitions whose intensity increases with increasing PPB concentration. This phase segregation observed could probably be due to co-existence of PPB-rich and PPB-poor phospholipid domains within the bilayers. The effect of inclusion of cholesterol in the PPB-free and PPB-doped DPPC dispersion was also studied. Equilibration studies suggest that PPB molecules are very strongly bound and remain intercalated between the polar headgroup for prolonged time.     

Influence of the leprosy drug, dapsone on the model membrane dipalmitoyl phosphatidylethanolamine

The influence of the sulfone drug, diamino diphenyl sulfone (DDS or dapsone) on the phase transitions and dynamics of the model membrane, dipalmitoyl phosphatidylethanolamine (DPPE)-water/buffer has been studied using DSC and (¹H and ³¹P) NMR. These investigations were carried out with DPPE dispersion in both multilamellar vesicular (MLV) and unilamellar vesicular (ULV) forms for DDS/DPPE molar ratio, R, in the range 0-0.5. DSC results indicate that the mechanism by which DDS interacted with the DPPE membrane is independent of the morphological organization of the lipid bilayer and the solvent (water or buffer) used to form the dispersion. DDS affected both the thermotropic phase transitions and the molecular mobility of the DPPE membrane. Addition of increasing amounts of DDS to the DPPE dispersion, resulted in the lowering of the gel to liquid-crystalline phase transition temperature (T_m) hence increased membrane fluidity. At all concentrations, the DDS is located close to the interfacial region of the DPPE bilayer but not in the acyl chain region. The interesting finding with MLV is that the gel phase of DPPE-water/buffer both in presence and absence of DDS, on prolonged equilibration at 25 °C, transforms to a stable crystalline subgel phase(s). The DPPE-water system forms both crystalline subgel L_LC (with transition temperature T_LC < T_m) and L_HC (with transition temperature T_HC ≥ T_m) phases, while the DPPE-buffer system forms only subgel L_LC phase. The presence of the drug seems to (i) increase the strength of the subgel L_LC phase and (ii) decrease the strength of subgel L_HC (for R < 0.5) phase. However, the value of the transition temperatures T_LC and T_HC does not change significantly with increasing drug concentration.     

Effect of tetracaine on model and erythrocyte membranes by DSC and EPR

Summary DSC and EPR experiments were performed on human erythrocyte membranes and DPPC vesicles in order to study the effect of the anaesthetic drug tetracaine on structure and dynamics of the lipid region. Experiments using spin label technique showed that tetracaine induced fluidity changes of the lipid region in the environment of the fatty acid probe molecules incorporated into the membranes in the vicinity of the lipid-water interface. Similarly to EPR observations, DSC measurements reported decrease of the main melting and the pretransition temperature in comparison to control DPPC vesicles, which is the sign of destabilisation of the structure in the head group region of the lipids. Similar effect was observed in the case of erythrocytes where the protein conformation was also controlled in the presence of drug. A separated membrane melting with well distinguished membrane protein phase transition was found that was affected significantly by tetracaine. These results suggest that tetracaine is able to modify not only the internal dynamics of erythrocyte membranes and produce destabilisation of the lipid structure, but the protein system as well. These might lead to further damage of the biological functions.     

Effect of propyl paraben on the dipalmitoyl phosphatidic acid vesicles

The effect of the preservative propyl paraben (PPB) on the phase transition and dynamics of dipalmitoyl phosphatidic acid (DPPA)-buffer (pH 7.4/9.3) vesicles has been studied using DSC and ((1)H and (31)P) NMR. These investigations were carried out with DPPA dispersion in both multilamellar vesicular (MLV) and unilamellar vesicular (ULV) forms. DSC results indicate that the mechanism by which PPB interact with the DPPA vesicles is similar in MLV and ULV and is independent of pH of the buffer used to form the dispersion. However, for a given concentration of PPB, the perturbation in DPPA bilayer is more when the dispersion is prepared in buffer pH 7.4. PPB affected both the thermotropic phase transition and the molecular mobility of the DPPA membrane. In the presence of PPB, the gel to liquid crystalline phase transition temperature (T(m)) of the DPPA vesicles decreases hence increases membrane fluidity due to reduced headgroup-headgroup interaction. For all concentrations, the PPB molecules seem to get intercalated between the polar groups of the phospholipids with its alkyl chain penetrating into the co-operative region. At high PPB concentration, additional transitions are observed whose intensity increases with increasing PPB concentration. The large enthalpy values obtained at high PPB concentration suggest that presence of PPB makes the DPPA bilayer more ordered (rigid). The interesting finding obtained with MLV is that the stable gel phase of DPPA-buffer (pH 9.3/7.4) system in the presence of high PPB concentration becomes a metastable gel phase, this metastable gel phase on equilibration at 25 degrees C or when cooled to -20 degrees C transforms to a stable crystalline phase(s). The intensity of this new phase(s) increases with increasing PPB concentration. However, the transition temperatures of these new phases are not significantly changed with increasing PPB concentration. The effect of inclusion of cholesterol in the PPB-free and PPB-doped DPPA dispersion was also studied.     

髓鞘碱性蛋白对细胞膜脂质分子DPPC、DPPE单层膜影响机理研究

本文通过Langmuir单层膜的表面压力-平均分子面积(π-A)曲线的测定与分析,分别对髓鞘碱性蛋白(MBP)与细胞膜中不同头部基团脂质分子二棕榈酰基磷脂胆碱(DPPC)和二棕榈酰基磷脂酰乙醇胺(DPPE)在空气/液体界面上的相互作用过程进行了系统研究.实验结果表明:(1)当界面上脂质含量一定时,亚相中随着MBP浓度的增大,DPPC、DPPE单层膜的等温线向平均分子面积较大的方向移动;(2)在单层膜表面压力为10 mN/m时,一个MBP分子分别结合140±3个DPPC分子和100±3个DPPE分子,随着表面压力增大,当MBP分子分别与两种磷脂分子相互作用时,MBP插入到磷脂单层界面的个数逐渐减少;(3)随着蛋白质浓度的增加,脂分子形成的单层膜变得较为疏松,且MBP分子易于插入到分子头部较小的DPPE单层膜中;(4)蛋白质的存在使DPPC单层膜的表面压力逐渐减小,且蛋白质浓度越大表面压力降低越多,DPPC被MBP带入到亚相中越多;(5)对于DPPE单层膜,蛋白质通过与DPPE相互作用插入到界面膜中,引起表面压力增大,且蛋白质浓度越高,压力变化量越大.     

Effects of lipid chain unsaturation and headgroup type on molecular interactions between paclitaxel and phospholipid within model biomembrane

Molecular interactions between paclitaxel, an anticancer drug, and phospholipids of various chain unsaturations and headgroup types were investigated in the present study by Langmuir film balance and differential scanning calorimetry. Both the lipid monolayer at the air-water interface and the lipid bilayer vesicles (liposomes) were employed as model cell membranes. It was found that, regardless of the difference in molecular structure of the lipid chains and headgroup, the drug can form nonideal, miscible systems with the lipids at the air-water interface over a wide range of paclitaxel mole fractions. The interaction between paclitaxel and phospholipid within the monolayer was dependent on the molecular area of the lipids at the interface and can be explained by intermolecular forces or geometric accommodation. Paclitaxel is more likely to form thermodynamically stable systems with 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) and 1,2-dielaidoyl-sn-glycero-3-phosphocholine (DEPC) than with 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC). Investigation of the drug penetration into the lipid monolayer showed that DPPC and DEPC have higher incorporation abilities for the drug than DPPE and DSPC. A similar trend was also evidenced by DSC investigation with liposomes. While little change of DSC profiles was observed for the DPPE/paclitaxel and DSPC/paclitaxel liposomes, paclitaxel caused noticeable changes in the thermographs of DPPC and DEPC liposomes. Paclitaxel was found to cause broadening of the main phase transition without significant change in the peak melting temperature of the DPPC bilayers, which demonstrates that paclitaxel was localized in the outer hydrophobic cooperative zone of the bilayer, i.e., in the region of the C1-C8 carbon atoms of the acyl chain or binding at the polar headgroup site of the lipids. However, it may penetrate into the deeper hydrophobic zone of the DEPC bilayers. These findings provide useful information for liposomal formulation of anticancer drugs as well as for understanding drug-cell membrane interactions.     

Interaction of an anticancer drug, gemcitabine, with phospholipid bilayers

The molecular interaction between gemcitabine, an anticancer pyrimidine analogue, and fully hydrated phospholipid (1,2-dipalmitoyl-sn-3-phosphatidylcholine, DPPC) membrane model has been investigated by Differential Scanning Calorimetry (DSC) and Small and Wide Angle X-ray Diffraction (SWAXS). The behaviour of the charged and uncharged forms of gemcitabine has been examined. Our results show that: (1) at physiological pH, gemcitabine does not modify significantly the thermal and structural properties of DPPC, (2) at pH of 2.4 the drug interacts electrostatically with the polar headgroups, inducing the formation of unilamellar vesicles and (3) at acidic pH the DPPC lamellar phase is recovered when salt is added.     

A mechanistic study of the permeation kinetics through biomembrane models: gemcitabine-phospholipid bilayer interaction

The kinetics of the interaction between Gemcitabine (a new anticancer drug) and phospholipid membrane models was investigated. This kind of study is of particular importance both in hypothesizing the interaction of Gemcitabine with mammalian cell membranes and in evaluating the potentiality of liposomes as a Gemcitabine delivery system. Unilamellar (LUV) and multilamellar (MLV) membrane models were made up of dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidic acid sodium salt (DMPA), or a DMPC-DMPA mixture (1:1 molar ratio). Gemcitabine-phospholipid vesicle interaction was studied by differential scanning calorimetry (DSC) measurements performed at different time intervals. The findings showed slower permeation kinetics of Gemcitabine through MLV than LUV which, at the same lipid/water ratio, are characterized by a larger lipid surface in contact with the drug aqueous solution. Another interesting difference between LUV and MLV is the onset of a transient two-peak structure during the DSC scans of MLVs. The effect is due to the unequal distribution of the drug between the outer and inner bilayers of the multilamellar vesicles during the permeation kinetics. At equilibrium the two-peak structure merges into a unique peak. This finding may provide useful information about the lipid bilayer permeability in model membranes.     

Physicochemical properties of a muramyldipeptide derivative (B30-MDP) related to membrane formation

We investigated the physicochemical properties of B30-MDP [6-O-(2-tetradecylhexadecanoyl)-N-acetyl-muramyl-L-alanyl-D-isoglutamine], a muramyldipeptide derivative having immunoadjuvant activity [1], using polarizing optical microscopy, differential scanning calorimetry (DSC), and electron spin resonance (ESR) spectroscopy. Microscopic observations showed that B30-MDP molecules form myelin figures in phosphate buffered saline (PBS). It was revealed that B30-MDP forms membranous structure because of an increase in the hydrophobicity. In the DSC measurements, the B30-MDP membrane in PBS gave no endothermic peak between 5° to 50°C. Enthalpy change upon the phase transition from the gel to liquid crystalline state or dipalmitoylphosphatidylcholine (DPPC) membrane and its phase transition temperature decreased by the addition of B30-MDP. ESR measurements using 5 doxyl stearic acid showed that the fluidity of the B30-MDP membrane was almost comparable to that of DPPC membrane at the temperature below the phase transition temperature of DPPC, while it was lower than that of DPPC at the temperature higher than this point. The fluidity of DPPC membrane increased upon the addition of B30-MDP. These results indicate that B30-MDP forms membranous structure and that the bulky hydrophilic region of B30-MDP influences its membrane structures, thermal behavior, and membrane fluidity.     

Methotrexate interaction with a lipid membrane model of DPPC

Dipalmitoylphosphatidylcholine (DPPC) liposomes were employed as membrane models for the investigation of the interaction occurring between methotrexate (MTX) and bilayer lipid matrix. Liposomes were obtained by hydrating a lipid film with 50 mM Tris buffer (pH 7.4). The differential scanning calorimetry (DSC) evaluation of the thermotropic parameters associated with the phase transitions of DPPC liposomes gave useful information about the kind of drug-membrane interaction. The results showed an electrostatic interaction taking place with the negatively charged molecules of MTX and the phosphorylcholine head groups, constituting the outer part of DPPC bilayers. No interaction with the hydrophobic phospholipid bilayer domains was detected, revealing a poor capability of MTX to cross through lipid membranes to reach the interior compartment of a lipid bounded structure. These findings correlate well within vitro biological experiments on MTX cell susceptibility.     

Na,K-ATPase reconstituted in liposomes: effects of lipid composition on hydrolytic activity and enzyme orientation

In this paper, the reconstitution of Na,K-ATPase in liposomes (formed by single or mixed phospholipids and cholesterol) was investigated and the enzyme orientation was determined on kinetic basis using only specific inhibitors of ATP hydrolysis.

A condition of foremost importance for enzyme reconstitution is the achievement of complete solubilization of the lipid in the initial stage of the cosolubilization process for the subsequent formation of the liposomes and/or proteoliposomes. PC-liposomes showed that increasing the fatty acid chain length increases the percentage of Na,K-ATPase incorporated. The average diameter of the proteoliposomes also increases in proportion, reaching a maximum with phospholipids with 16 carbon chains, resulting in 75.1% protein reconstitution and 319.4 nm diameter size, respectively. Binary lipid systems with PC and PE were efficient for incorporation of Na,K-ATPase, depending on the lipid:protein ratio used, varying from 15 to 80% recovery of total ATPase activity. The best results for Na,K-ATPase reconstitution using PC and PE mixture were obtained using a lipid:lipid ratio 1:1 (w/w) and lipid:protein 1:3 (w/w). Integrity studies using calcein release mediated by detergent or alamethicin, in association with inhibition of ATPase activity (ouabain and vanadate) showed that the enzyme is oriented inside-out in DPPC:DPPE proteoliposomes. In these vesicular systems, the enzyme is reconstituted with about 78.9% ATPase activity recovery and 89% protein incorporation, with an average diameter of 140 nm. Systems constituted by DPPC:DPPE, DPPC:DLOPE or DLOPC:DLOPE showed approximately 80, 71 and 70% of recovery of total ATPase activity, but no homogeneity in the distribution of Na,K-ATPase orientation. Reconstitution of Na,K-ATPase in DPPC:DPPE:cholesterol or DPPC:DLOPE:cholesterol systems (55% of cholesterol) showed recovery of about 86 and 82%, respectively, of its total ATPase activity.

The results point to an important effect of the lipid acyl chain length and lipid–protein ratio in relation to the composition of the lipid matrix to finely tune the structural asymmetry and the amount of enzyme that can be incorporated a lipid bilayer vesicle while preserving membrane permeability.     

Role of dipolar interaction in the mesoscopic domains of phospholipid monolayers: dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylethanolamine

The role of dipolar interactions in determining the lipid domain shapes at the air-water interface with a change in the chemical structure of the head groups of lipids is theoretically studied. The phospholipids considered are dipalmitoylphosphatidylcholine (D,L-DPPC) and dipalmitoylphosphatidylethanolamine (DPPE). Despite closely similar chemical structures, the domains of the two lipids are strikingly different. The DPPC domains exhibit elongated arms, while the DPPE domains are nearly round-shaped. To compare the dipolar repulsions in the domains of the two phospholipids, different energy-minimized conformers of DPPC and DPPE are studied using the semiempirical quantum chemical method (PM3). It is found that the dipole moment of DPPC is significantly larger than that of DPPE. The in-plane and out-of-plane components of the dipole moments are calculated using grazing incidence X-ray diffraction data at different surface pressure values, as used in the experiment. The result indicates that the magnitude of the dipolar interaction is significantly larger in DPPC than that in DPPE over the surface pressure range considered. The enhanced dipolar repulsion corroborates well with the difference in the domain shapes in the two phospholipid monolayers. The larger dipolar repulsion in DPPC leads to development of elongated domain arms, while relatively less dipolar repulsion allows a closed shape of the condensed-phase DPPE domains.     

The interaction of an antiparasitic peptide active against African Sleeping Sickness with cell membrane models

Zwitterionic peptides with trypanocidal activity are promising lead compounds for the treatment of African Sleeping Sickness, and have motivated research into the design of compounds capable of disrupting the protozoan membrane. In this study, we use the Langmuir monolayer technique to investigate the surface properties of an antiparasitic peptide, namely S-(2,4-dinitrophenyl)glutathione di-2-propyl ester, and its interaction with a model membrane comprising a phospholipid monolayer. The drug formed stable Langmuir monolayers, whose main feature was a phase transition accompanied by a negative surface elasticity. This was attributed to aggregation upon compression due to intermolecular bond associations of the molecules, inferred from surface pressure and surface potential isotherms, Brewster angle microscopy (BAM) images, infrared spectroscopy and dynamic elasticity measurements. When co-spread with dipalmitoyl phosphatidyl choline (DPPC), the drug affected both the surface pressure and the monolayer morphology, even at high surface pressures and with low amounts of the drug. The results were interpreted by assuming a repulsive, cooperative interaction between the drug and DPPC molecules. Such repulsive interaction and the large changes in fluidity arising from drug aggregation may be related to the disruption of the membrane, which is key for the parasite killing property.     

β-lactoglobulin 与磷脂复合膜形成过程中相互 作用的动态研究

利用ADSA系统分别研究了蛋白质β-lactoglobulin在不同pH下与三种磷脂DPPC(中性头部基因)、DPPE(部分正电头部基团)、DPPA(部分带负电头部基因)的吸附动力学,结合AFM技术,讨论在弯曲的液/液界面上蛋白质与磷脂之间各种相互作用对复合膜生成的影响。认为这些相互作用的影响是动态变化着的;在吸附反应的不同阶段各类作用分别成为主导因系。     

Phase behaviour of polymer-grafted DPPC membranes for drug delivery systems design

Summary DPPC dispersions containing DPPE with attached PEG of molecular masses 350, 2000 and 5000 were investigated by DSC in order to determine their phase behaviour and potential use as drug delivery systems. In comparison with previously obtained ESR data, DSC provided a definition of the lipid composition and temperature at which the vesicles are in a liquid crystalline phase. For DPPC DPPE-PEG 350 the composition range is at molar fractions 0&lt;_&khgr;PEG350&lt;0.5.For DPPC DPPE-PEG 2000 the range of applicability is 0&lt;_{&khgr;PEG2000}&lt;0.07 and for DPPC/DPPE-PEG 5000 system it is 0&lt;_{&khgr;PEG5000}&lt;0.05.     

Effects of high electrolyte concentration on DPPC-multilayers: an ESR and DSC investigation

The effects of salinity on the lateral headgroup interactions of dipalmitoylphosphatidylcholine (DPPC) molecules in fully hydrated multilayers have been investigated by spin label electron spin resonance (ESR) spectroscopy and differential scanning calorimetry (DSC).By increasing the NaCl concentration from 0 to 3 M in the multilayers' dispersion medium, the ESR measurements performed with the 5-stearic acid spin label and di-tert-butyl-nitroxide show an increase in the orientational degree of order of the lipid molecules, mainly in the gel phase, and a decrease of the membrane permeability. An upward shift from 31.5° to 36.5°C and from 40.5° to 41.9°C of the pre- and main DPPC phase transition temperatures, respectively, is observed with 5-SASL, while slightly higher values are detected with DTBN. Small effects are evident on the properties of the liquid crystalline phase of the DPPC multilayers.The DSC measurements also reveal an upward shift of the pre- and main transition temperatures. The shifts, however, are more marked if compared to the ones observed with the ESR technique.The findings suggest an increase in the packing density of the DPPC molecules in the multilayers in presence of high salt concentration. Dehydration of the DPPC interfacial region with a variation of the lateral electrostatic interactions between phospholipid polar heads trigger the phenomena observed.     

Effects of poly(ethylene glycol) (PEG) chain length of PEG-lipid on the permeability of liposomal bilayer membranes

The effects of poly(ethylene glycol) (PEG) chain length of PEG-lipid on the membrane characteristics of liposomes were investigated by differential scanning calorimetry (DSC), freeze-fracture electron microscopy (FFEM), fluorescence polarization measurement and permeability measurement using carboxyfluorescein (CF). PEG-liposomes were prepared from mixtures of dipalmitoyl phosphatidylcholine (DPPC) and distearoyl phosphatidylethanolamines with covalently attached PEG molecular weights of 1000, 2000, 3000 and 5000 (DSPE-PEG). DSC and FFEM results showed that the addition of DSPE-PEG to DPPC in the preparation of liposomes caused the lateral phase separation both in the gel and liquid-crystalline states. The fluidity in the hydrocarbon region of liposomal bilayer membranes was not significantly changed by the addition of DSPE-PEG, while that in the interfacial region was markedly increased. From these results, it was anticipated that the CF leakage from PEG-liposomes is accelerated compared with DPPC liposomes. However, CF leakage from liposomes containing DSPE-PEG with a 0.060 mol fraction was depressed compared with regular liposomes, and the leakage decreased with increasing PEG chain length. Furthermore, the CF leakage from liposomes containing DSPE-PEG with a 0.145 mol fraction was slightly increased compared with that of liposomes containing DSPE-PEG with a 0.060 mol fraction. It is suggested that the solute permeability from the PEG-liposomes was affected by not only properties of the liposomal bilayer membranes such as phase transition temperature, phase separation and membrane fluidity, but also the PEG chain of the liposomal surface.     

Mixed monolayers of Bauhinia monandra and concanavalin A lectins with phospholipids, part II

Isotherms of surface pressure and surface potential versus mean molecular area for dibehenoylphosphatidylcholine (DBPC), dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylethanolamine (DPPE), and dioleoylphosphatidylcholine (DOPC) monolayers were shown to be greatly modified when these lipids were cospread with either Bauhinia monandra (BmoLL) or Concanavalin A (Con A) lectins. For the binary films of DBPC, DPPC, and DPPE cospread with each of these two lectins, there was both a displacement of the Pi-A and DeltaV-A isotherms toward higher molecular areas relative to pure lipids and an increase in the maximum surface potential values relative to the DeltaV-A relationships observed for the corresponding single-lectin systems. Both effects can be understood in terms of the occurrence of an explicit interaction between the lipids and the lectins. The plots of the corresponding compressibilities versus molecular areas reveal that, for all lipids but DOPC, the extent of this interaction was always larger for BmoLL than for Con A. The DPPC and DPPE mixed films with BmoLL differed in compressibility. Owing to the small DPPE polar headgroup, the DPPE-BmoLL film was much more incompressible than the DPPC-BmoLL mixed monolayer. However, for the DOPC-BmoLL and DOPC-Con A mixed films there was no evidence that an interaction between the lectins and the lipid took place, a fact attributed to the unsaturated character in the DOPC aliphatic chains, which leads to an expanded Pi-A isotherm.