Effect of membrane composition on surface states of ganglioside GM1/dipalmitoylphosphatidylcholine/dioleoylphosphatidylcholine monolayers

The surface states of ganglioside GM1 (GM1)/dipalmitoylphosphatidylcholine (DPPC)/dioleoylphosphatidylcholine (DOPC) monolayers having various compositions were investigated using atomic force microscopy (AFM), and the effect of the composition on the surface states of the membrane was examined. The AFM images for the ternary system showed a DPPC-rich phase containing GM1 in the DOPC matrix, which indicated that the morphology varied as the composition of the monolayers changed. The AFM images for the GM1/DPPC/DOPC monolayers having (2:9:9) and (4:18:9) molar ratios showed a percolation pattern similar to that observed for the GM1/DPPC (1:9) monolayer. The AFM image for the GM1/DPPC/DOPC (2:18:9) monolayer showed a dotted pattern with a high topography. Monolayers having a higher content of DOPC than DPPC and/or having a higher content of GM1 showed dot-like domains in the DPPC-rich phase containing GM1. In conclusion, the surface states of GM1/DPPC/DOPC monolayers changed depending on the composition. These results may be related to a diversity of GM1 in various organs.     

Atomic force microscopy studies of ganglioside GM1alpha in dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine mixed monolayers and hybrid bilayers

The membrane states of the alpha-series ganglioside GM1alpha in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) mixed monolayers and hybrid bilayers were investigated using atomic force microscopy (AFM). The AFM image for the GM1alpha/DOPC/DPPC ternary monolayers showed the formation of GM1alpha-raft in the DOPC matrix. As increase of the surface pressure, GM1alpha are condensed in DPPC-rich domains; long and slender GM1alpha-rafts are separated from the DPPC-rich domains into the DOPC matrix. The GM1alpha/DOPC/DPPC ternary monolayers were deposited on mica coated with the first layer (1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine: DPPE) using the Langmuir-Schaeffer technique. The AFM image for the hybrid bilayers showed that same molecules were heterogeneously concentrated according to increase of the surface pressure to form GM1alpha-raft, DPPC-rich domain and DOPC matrix, being in agreement with the observation on the monolayer experiment. The found phenomenon implies that a binding of lectin to GM1alpha causes the increase of the surface pressure, the localization of GM1alpha and the succeeding formation of the raft as a first step of a specific signal transduction.     

DOPC/DPPC单层膜中分子间的相互作用及形态特征

利用Langmuir-Blodgett(LB)技术制备了不同表面压力下的1,2-二油酸-甘油-3-磷脂酰胆碱(DOPC)/1,2-二棕榈酸甘油-3-磷脂酰胆碱(DPPC)(摩尔比为1:1)和DOPC/DPPC/Chol(摩尔比为2:2:1)单层膜, 对单层膜内分子间的相互作用进行了热力学分析, 并用荧光显微镜和原子力显微镜对其形态进行了观测.热力学分析表明, DOPC与DPPC分子在单层膜结构中相互作用为排斥力, 诱导单层膜出现相变; DOPC, DPPC与胆固醇(Chol)间的相互作用均为吸引力, 当表面压力(π)大于18 mN/m时, DPPC与胆固醇的作用力大于DOPC.荧光显微镜观测表明, DOPC/DPPC单层膜出现明显相分离现象, 富含DPPC微区成“花形”结构, 且随着表面压力的升高微区逐渐增大, “花瓣”增多; 当胆固醇加入到DOPC/DPPC体系时, 单层膜相态由液相与凝胶相共存转变为液态无序相与液态有序相共存结构, 富含DPPC的微区形状从“花形”转变成“圆形”.原子力显微镜对单层膜的表征验证了荧光显微镜的观测结果, 表明胆固醇加入到DOPC/DPPC体系中对单层膜排列具有明显的影响, 压力和溶液状态等是影响脂膜结构的重要因素.     

Membrane properties of mixed ganglioside GM1/phosphatidylcholine monolayers

The interaction between ganglioside G_M1 (GM1) and --dipalmitoylphosphatidylcholine (DPPC) in mixed monolayers was investigated using surface pressure measurements and atomic force microscopy (AFM), and the effects of GM1, surface pressure and temperature on the properties of the membranes were examined. Mixed GM1/DPPC monolayers were deposited on mica using the Langmuir–Blodgett (LB) technique for AFM. GM1 and DPPC were miscible below the 0.2 mole fraction of GM1 and there was attractive interaction between GM1 and DPPC. The AFM images for the GM1/DPPC monolayers (X_GM1 < 0.2) at 30 mN m⁻¹ and 25 °C indicated a percolation pattern which means a micro phase separation: namely, the mixed film composed of GM1 and DPPC phase-separated from the DPPC liquid-condensed film. The AFM images for the mixed monolayers at 33 mN m⁻¹ indicated a specific morphology when the surface pressure was varied from 30 to 40 mN m⁻¹. The percolation pattern in the AFM image at 25 °C came to be destroyed with increasing temperature and completely disappeared at 45 °C. The change in the morphology of mixed GM1/DPPC monolayers on varying the surface pressure and temperature is thought to be related to signal transduction and a preventive mechanism against viral infections in the human body.     

Effect of hydrocarbon chain and pH on structural and topographical characteristics of phospholipid monolayers

Structural characteristics (structure, elasticity, topography, and film thickness) of dipalmitoyl phosphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers were determined at the air-water interface at 20 degrees C and pH values of 5, 7, and 9 by means of surface pressure (pi)-area (A) isotherms combined with Brewster angle microscopy (BAM) and atomic force microscopy (AFM). From the pi-A isotherms and the monolayer elasticity, we deduced that, during compression, DPPC monolayers present a structural polymorphism at the air-water interface, with the homogeneous liquid-expanded (LE) structure; the liquid-condensed structure (LC) showing film anisotropy and DPPC domains with heterogeneous structures; and, finally, a homogeneous structure when the close-packed film molecules were in the solid (S) structure at higher surface pressures. However, DOPC monolayers had a liquid-expanded (LE) structure under all experimental conditions, a consequence of weak molecular interactions because of the double bond of the hydrocarbon chain. DPPC and DOPC monolayer structures are practically the same at pH values of 5 and 7, but a more expanded structure in the monolayer with a lower elasticity was observed at pH 9. BAM and AFM images corroborate, at the microscopic and nanoscopic levels, respectively, the same structural polymorphism deduced from the pi-A isotherm for DPPC and the homogeneous structure for DOPC monolayers as a function of surface pressure and the aqueous-phase pH. The results also corroborate that the structural characteristics and topography of phospholipids (DPPC and DOPC) are highly dependent on the presence of a double bond in the hydrocarbon chain.     

Ganglioside partitioning and aggregation in phase-separated monolayers characterized by bodipy GM1 monomer/dimer emission

The distribution of Bodipy GM1 in monolayers of binary and ternary lipid mixtures with coexisting fluid and ordered phases has been examined using a combination of atomic force microscopy and near-field scanning optical microscopy. Monolayers deposited at high (30 mN/m) and low (5 or 10 mN/m) surface pressures were examined and compared to those containing the same concentration of unlabeled ganglioside. Measurements of monomer and dimer Bodipy emission were used to distinguish aggregated from dilute ganglioside levels. For binary DPPC/DOPC monolayers, Bodipy GM1 is distributed throughout both the fluid and ordered phases at low surface pressures, and both labeled and unlabeled gangliosides result in a reduction in the size of ordered DPPC domains at 0.4% and the appearance of small aligned ganglioside-rich domains at 4%. In agreement with earlier studies, GM1 is heterogeneously distributed in small islands in the condensed DPPC domains at high surface pressure. By contrast, Bodipy GM1 causes the disappearance of large DPPC domains at 0.4% and the formation of a new GM1-rich phase at 4%. The addition of both gangliosides leads to a comparable loss of large ordered domains at low surface pressure and the appearance of a new GM1-rich phase at 30 mN/m for ternary lipid mixtures containing cholesterol. The results demonstrate the complexity of GM1 partitioning and illustrate the utility of complementary AFM and high spatial resolution two-color fluorescence experiments for understanding Bodipy GM1 aggregation and distribution.     

Penetration of bovine serum albumin into dipalmitoylphosphatidylglycerol monolayers: direct observation by atomic force microscopy

The penetration of bovine serum albumin (BSA) into dipalmitoylphosphatidylglycerol (DPPG) monolayers was observed using atomic force microscopy (AFM) and surface pressure measurements. The effects of surface pressure, amount of BSA and the addition of ganglioside GM1 (GM1) were investigated. The surface pressure of the DPPG monolayer was increased by the penetration of BSA, and the increase in surface pressure was greater in the liquid-expanded film than that in the liquid-condensed film. The AFM images indicated that BSA penetrated into the DPPG monolayer. The amount of BSA that penetrated into the DPPG monolayer increased with time and with the amount of BSA added. On the contrary, the AFM image showed that BSA penetration into the mixed DPPG/GM1 (9 : 1) monolayer scarcely occurred. GM1 inhibited the penetration of BSA into the DPPG monolayer.     

维生素E与磷脂酰胆碱的单分子膜研究

测定了下列气水界面单分子膜的表面压－平均分子面积等温线：（１）ｄ－α，ｄ－β，ｄ－γ－，和ｄ－δ－生育酚等４种维生素Ｅ与ＤＰＰＣ，ＤＯＰＣ及ＤＬＰＣ的混合物，（２）ｄ－α－生育酚等摩尔比的ＤＰＰＣ和ＤＬＰＣ的混合物，（３）胆固醇与ＤＰＰＣ，ＤＬＰＣ的混合物，讨论了维生素Ｅ色满环上甲基对其在ＰＣ单分子膜中物理化作用的影响，实验结果表明：（１）维生素Ｅ以ｄ－α〉ｄ－β－≈ｄ－γ〉ｄ－δ的次序引起的ＤＰ     

Membrane resistance to Triton X-100 explored by real-time atomic force microscopy

Lateral segregation of lipids and proteins in biological membranes leads to the formation of detergent-resistant domains, also called "rafts". Understanding the mechanisms governing the biomembrane's resistance to solubilization by detergents is crucial in biochemical research. Here, we used real-time atomic force microscopy (AFM) imaging to visualize the behavior of a model supported lipid bilayer in the presence of different Triton X-100 (TX-100) concentrations. Mixed dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DOPC/DPPC) supported bilayers were prepared by vesicle fusion. Real-time AFM imaging revealed that, at concentrations below the critical micelle concentration (CMC), TX-100 did not solubilize the bilayer, but the DPPC domains were eroded in a time-dependent manner. This effect was attributed to the DPPC molecular packing disorganization by the detergent starting from the DOPC/DPPC interface. Just above the CMC, the detergent led to a complete solubilization of the DOPC matrix, leaving the DPPC domains unaltered. At higher TX-100 concentrations, the DOPC was also immediately removed just after detergent addition, and the DPPC domains remaining on the mica surface appeared to be more swollen and were gradually solubilized. This progressive solubilization of the DPPC remaining phase did not start at the edge of the domains but from holes appearing and expanding at the center of DPPC patches. The swelling of the DPPC domains was directly correlated with TX-100 concentration above the CMC and with detergent intercalation between DPPC molecules. We are convinced that this approach will provide a key system to elucidate the physical mechanisms of membrane solubilization by nonionic detergents.     

Syntheses of new model compounds related to an antigenic epitope from Bupleurum falcatum L. and their distributions in various ganglioside-phospholipid monolayers

6-N-[2-(Tetradecyl)hexadecanamido]hexyl beta-D-glucopyranosyluronic acid-(1-->6)-beta-D-galactopyranosyl-(1-->6)-beta-D-galactopyranoside (1) and its clustering compound (2) carrying a tetravalent sugar unit, which are new model compounds related to a major antigenic epitope from antiulcer pectic polysaccharide of Bupleurum falcatum L., were synthesized and the distributions of 1 and 2 in mixed ganglioside (GM1, GD1a or GT1b)/phospholipid (DPPC) monolayers were observed using atomic force microscopy (AFM). AFM images showed that 1 was distributed in the GM1, GD1a and GT1b region of the mixed monolayers, in which 1 was miscible with GD1a. Specific distribution of 1 was observed in the mixed GM1/DPPC monolayer. Compound 2 was miscible with GM1, while 2 formed associations with GD1a and GT1b in the mixed monolayers. The distribution mode of 1 and 2 was different among the mixed ganglioside/DPPC monolayers.     

Solubilization of supported lipid membranes by octyl glucoside observed by time-lapse atomic force microscopy

Detergents are very useful for the purification of membrane proteins. A good detergent for protein extraction has to prevent denaturation by unfolding, and to avoid aggregation. Therefore, gaining access to the mechanism of biomembranes’ solubilization by detergents is crucial in biochemical research. Among the wide range of detergents used to purify membrane proteins, n-octyl β-d-glucopyranoside (OG) is one of the most important as it can be easily removed from final protein extracts.

Here, we used real-time atomic force microscopy (AFM) imaging to visualize the behavior of a model supported lipid bilayer in the presence of OG. Two kinds of supported model membranes were prepared by fusion of unilamellar vesicles: with an equimolar mixing of dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DOPC/DPPC) or with DPPC alone. Time-lapse AFM experiments evidenced that below its critical micelle concentration (CMC), OG was not able to solubilize the bilayer but the gel DPPC domains were instantly dissolved into the DOPC matrix. This result was interpreted as a disorganization of the DPPC molecular packing induced by OG. When membranes were incubated with OG at concentrations higher than CMC, the detergent immediately provoked the complete and immediate desorption of the whole bilayer for both compositions: DPPC and DOPC/DPPC. After a while, some patches appeared onto the bare mica surface. This redeposition activity, together with fusion events, progressively led to the recovery of a continuous bilayer. These results provide a new insight on the unique properties of OG employed in membrane reconstitution protocols.     

Nonfibrous beta-structured aggregation of an Abeta model peptide (Ad-2alpha) on GM1/DPPC mixed monolayer surfaces

Adsorption and aggregation of transformed peptides and proteins onto the cell membrane surface is commonly associated with forms of amyloidosis such as Alzheimer's disease and prion disease. To address dynamic features of these pathological phenomena molecularly, the in situ Ad-2alpha model peptide deposition on glycolipid-containing monolayers was studied by using a 9 MHz quartz-crystal microbalance (QCM). The Ad-2alpha peptide has two amphiphilic alpha-helix segments, each modified with a 1-adamantanecarbonyl group at the N-terminal as a hydrophobic defect. The peptide folds in a 2alpha-helix structure in the bulk solution. In the presence of mixed monolayers of glycolipids (GM1, asialo-GM1, GM3, or LacCer) and/or dipalmitoyl phosphatidylcholine (DPPC) laminated on the QCM plate, the peptide deposition and the conformational change to beta-structure on the monolayers were accelerated. The adsorption kinetics and the amount of Ad-2alpha were dependent on the sort and contents of the glycolipid in the DPPC matrix. Although the Ad-2alpha peptide adsorbs onto most of the glycolipid membranes as monolayer coverage, it adsorbed largely onto the GM1/DPPC (30/70 mol%) mixed monolayer with characteristic kinetic behaviors. The accumulation of beta-structured nonfibrous aggregations was confirmed by AFM and fluorescence microscopy with Thioflavin T (ThT).     

Mode of interaction of amphiphilic alpha-helical peptide with phosphatidylcholines at the air-water interface

Surface pressure (pi)-, surface potential (deltaV)-, and dipole moment (mu(perpendicular))-area (A) isotherms and morphological behavior were examined for monolayers of a newly designed 18-mer amphiphilic alpha-helical peptide (Hel 13-5), DPPC, and DPPC/egg-PC (1:1) and their combinations by the Wilhelmy method, ionizing electrode method, fluorescence microscopy (FM), and atomic force microscopy (AFM). The newly designed Hel 13-5 showed rapid adsorption into the air-liquid interface to form interfacial films such as a SP-B function. Regardless of the composition and constituents in their multicomponent system of DPPC/egg-PC, the collapse pressure (pi(c); approximately 42 mN m(-1)) was constant, implying that Hel 13-5 with the fluid composition of egg-PC is squeezed out of Hel 13-5/DPPC/egg-PC monolayers accompanying a two- to three-dimensional phase transformation. FM showed that adding a small amount of Hel 13-5 to DPPC induced a dispersed pattern of ordered domains with a "moth-eaten" appearance, whereas shrinkage of ordered domains in size occurred for the DPPC/egg-PC mixture with Hel 13-5. Furthermore, AFM indicated that (i) the intermediate phase was formed in pure Hel 13-5 systems between monolayer states and excluded nanoparticles, (ii) protrusions necessarily located on DPPC monolayers, and (iii) beyond the collapse pressure of Hel 13-5, Hel 13-5 was squeezed out of the system into the aqueous subphase. Furthermore, hysteresis curves of these systems nicely resemble those of the DPPC/SP-B and DPPC/SP-C mixtures reported before.     

Label-free coherent anti-stokes Raman scattering imaging of coexisting lipid domains in single bilayers

Laser-scanning coherent anti-Stokes Raman scattering (CARS) microscopy was used to image lipid domains in single bilayers without any labeling. On the basis of the molecular packing density difference between liquid-disordered (Ld), liquid-ordered (Lo), and gel (So) phases, clear vibrational contrasts were generated between coexisting domains in a single bilayer of DOPC/DPPC (1:1) and DOPC/DPPC/cholesterol (4:4:2). The method reported here can be potentially applied to study phase segregation in live cell membranes which are highly heterogeneous and dynamic.     

Effects of Fullerenes on Phospholipid Membranes: A Langmuir Monolayer Study

We investigate two‐component Langmuir monolayers of dipalmitoylphosphatidylcholine (DPPC)/C₆₀ by recording surface pressure/area (π/A) and surface potential/area (ΔV/A) isotherms and by direct Brewster angle microscopy (BAM) imaging. Atomic force microscopy (AFM) is employed to study morphologies of the mixed monolayers transferred to a solid substrate by the Langmuir–Blodgett technique. C₆₀ is shown to have little influence on isotherms of the DPPC/C₆₀ monolayers even at a molar fraction as high as X_C60=0.3. The elastic modulus ( ) versus π curves of the DPPC/C₆₀ monolayers almost overlay each other, as well as that of pure DPPC, that is, the elasticities of pure DPPC monolayers and DPPC/C₆₀ monolayers are remarkably similar. AFM studies reveal that fullerene flocs form at low surface pressures (π≤15 mN m^?1), are gradually disaggregated and dispersed in the DPPC monolayer with increasing surface pressure up to 35 mN m^?1, and are then progressively squeezed out to form protruded islands as the surface pressure increases up to 65 mN m^?1. Our work provides experimental support to the computational result that C₆₀ can dissolve in lipid bilayers without significantly compromising their mechanical properties, a finding which has important implications for the toxicity and development of drug vehicles from fullerene materials.     

Remodeling of ordered membrane domains by GPI-anchored intestinal alkaline phosphatase

Glycosylphosphatidyl-inositol (GPI)-anchored proteins preferentially localize in the most ordered regions of the cell plasma membrane. Acyl and alkyl chain composition of GPI anchors influence the association with the ordered domains. This suggests that, conversely, changes in the fluid and in the ordered domains lipid composition affect the interaction of GPI-anchored proteins with membrane microdomains. Validity of this hypothesis was examined by investigating the spontaneous insertion of the GPI-anchored intestinal alkaline phophatase (BIAP) into the solid (gel) phase domains of preformed supported membranes made of dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DOPC/DPPC), DOPC/sphingomyelin (DOPC/SM), and palmitoyloleoylphosphatidylcholine/SM (POPC/SM). Atomic force microscopy (AFM) showed that BIAP inserted in the gel phases of the three mixtures. However, changes in the lipid composition of membranes had a marked effect on the protein containing bilayer topography. Moreover, BIAP insertion was associated with a net transfer of phospholipids from the fluid to the gel (DOPC/DPPC) or from the gel to the fluid (POPC/SM) phases. For DOPC/SM bilayers, transfer of lipids was dependent on the homogeneity of the gel SM phase. The data strongly suggest that BIAP interacts with the most ordered lipid species present in the gel phases of phase-separated membranes. They also suggest that GPI-anchored proteins might contribute to the selection of their own microdomain environment.     

Mode of interaction of two fluorinated-hydrogenated hybrid amphiphiles with dipalmitoylphosphatidylcholine (DPPC) at the air-water interface

Two-component Langmuir monolayers formed on 0.02M Tris buffer solution (pH 7.4) with 0.13M NaCl at 298.2K were investigated for two different fluorinated-hydrogenated hybrid amphiphiles (F6PH5PPhNa and F8PH5PPhNa or F6 and F8, respectively) with DPPC. Surface pressure (pi), surface potential (DeltaV) and dipole moment (mu( perpendicular)) as a function of molecular surface area (A) were measured by employing the Whilhelmy method and an ionizing electrode method. From the A- and DeltaV-X(F6) (or X(F8)) curves, partial molecular surface area (PMA) and apparent partial molecular surface potential (APSP) were determined as a function of surface mole fraction (X(Fn)) at discrete surface pressures. Then, the behavior of occupied surface areas and surface potentials of the respective components could be made clearer. Compressibility (C(s)), elasticity (C(s)(-1)), and excess Gibbs energy (DeltaG((ex))) as a function of X(F6) (or X(F8)) were estimated at definite pressures. These physico-chemical parameters were found to reflect the mechanical strength of monolayer films formed. The regular solution theory being applied to DeltaG((ex)), the activity coefficients (f) as well as the interaction parameter (I(p)) between DPPC and two hybrid amphiphiles in the binary monolayers were evaluated. I(p) values thus obtained indicated that F8 molecules interact more strongly with DPPC molecules than F6. Moreover, in order to better understand the morphological monolayer state, Langmuir-Blodgett (LB) films made from DPPC and fluorinated-hydrogenated hybrid amphiphiles were examined by atomic force microscopy (AFM). The miscibility of the two components in the monolayer state is evidenced by these thermodynamic quantities and AFM observations. Furthermore, AFM images demonstrated that F8 could more effectively disperse the ordered domains of DPPC than F6.     

Properties of two-component Langmuir monolayer of single chain perfluorinated carboxylic acids with dipalmitoylphosphatidylcholine (DPPC)

The surface pressure (π)– and the surface potential (ΔV)–area (A) isotherms were obtained for two-component monolayers of four different perfluorocarboxylic acids (FCns; perfluorododecanoic acid: FC12, perfluorotetradecanoic acid: FC14, perfluorohexadecanoic acid: FC16, perfluorooctadecanoic acid: FC18) with dipalmitoylphosphatidylcholine (DPPC) on substrate solution of 0.15 M NaCl (pH 2.0) at 298.2 K as a function of compositions in the mixtures by employing the Wilhelmy method, the ionizing electrode method, the fluorescence microscopy, and the atomic force microscopy. The data for the two-component monolayers on these systems were analyzed in terms of the additivity rule. Assuming a regular surface mixture, the Joos equation which allows one to describe the collapse pressure of a two-component monolayer with miscible components was used to declare the miscibility of the monolayer state, and an interaction parameter and an interaction energy were calculated. The new finding was that FCns and DPPC are miscible or immiscible depending on chain length increment of fluorocarbon part. That is, FC12/DPPC monolayer was perfectly miscible, and FC14/DPPC, and FC16/DPPC (0 ≤ X_FC16 ≤ 0.3) monolayers were partially miscible. While FC16/DPPC (0.3 < X_FC16 < 1) and FC18/DPPC systems are immiscible in the monolayer state. Furthermore, the mean molecular area, the surface dipole moment, and the phase diagrams enabled us to estimate the molecular orientation of four different perfluorocarboxylic acids/DPPC in the two-component monolayer state. One type of phase diagrams was obtained and classified into the positive azeotropic type. The miscibility of FCns and DPPC in the monolayer was also supported by fluorescence microscopy and atomic force microscopy. FC12/DPPC, FC14/DPPC and FC16/DPPC (0 ≤ X_FC16 ≤ 0.3) two-component monolayers on 0.15 M NaCl (pH 2) showed that FC12, FC14 and FC16 (0 ≤ X_FC16 ≤ 0.3) can dissolve or partially dissolve the ordered solid DPPC domains formed upon compression. This indicates that these fluorinated amphiphiles soften or harden the lipid depending on their chain length.     

Effect of D-α-tocopheryl polyethylene glycol 1000 succinate (TPGS) on surfactant monolayers

In the present study, the effects of an amphiphilic polymer, d-α-tocopheryl polyethylene glycol 1000 succinate (TPGS) on model surfactant monolayers dipalmitoylphosphatidylcholine (DPPC), a binary mixture of DPPC with palmitoyloleoyl phosphatidylglycerol (DPPC-POPG) 9:1 (w/w) and binary mixture of DPPC and oleic acid (DPPC-OA) were evaluated. The ability of TPGS to act as an antioxidant adjuvant for pulmonary surfactants was also evaluated. Compression isotherms of surfactant monolayers at 37 °C in a Langmuir-Blodgett trough showed that DPPC and DPPC:TPGS mixed monolayers (1:0.25-1:1, w/w) exhibited low minimum surface tensions (MST) of 1-2 mN/m. Similarly [DPPC:POPG (9:1, w/w)]:TPGS mixed films of 1:0.25-1:1 weight ratios reached 1-2 mN/m MST. DPPC:POPG:TPGS liposomes adsorbed to surface tensions of 29-31 mN/m within 1s. While monolayers of DPPC:OA (1:1, w/w) reached high MST of ～11 mN/m, DPPC:OA:TPGS (1:1:0.25, w/w) film reached near zero MST suggesting that low concentrations of TPGS reverses the effect of OA on DPPC monolayer. Capillary surfactometer studies showed DPPC:TPGS and [DPPC:POPG (9:1, w/w)]:TPGS liposomes maintained 84-95% airway patency. Fluorescence spectroscopy of Laurdan loaded DPPC:TPGS and DPPC:POPG:TPGS liposomes revealed no segregation of lipid domains in the lipid bilayer. Addition of TPGS to soybean liposome significantly reduced thiobarbituric acid reactive substance (TBARS) by 29-39% confirming its antioxidant nature. The results suggest a potential use of TPGS as an adjuvant to improve the surfactant activity as well as act as an antioxidant by scavenging free radicals.