Relation between rheological properties and structural changes in monolayers of model lung surfactant under compression (original) (raw)
Related papers
Biochimica et Biophysica Acta (BBA) - Biomembranes, 1983
Some properties of monolayers of 1-palmitoyl-2-oleoyi-sn-glycero-3-phospho-rac-glycerol (POPG) alone or of POPG in mixtures with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) have been measured near 35°C during dynamic compression and expansion at 3.6 cm2-s-I. (2) The mean values of minimum surface tension (corresponding to maximum surface pressure) which could be obtained with pure POPG monolayers at high compression ranged from 15 to 18 mN.m-i in the presence of Na +, Ca ~+ or low pH (2.0) in the subphase. (3) The presence of Ca 2 + or low pH in the subphase increased the collapse plateau ratios obtained on cyclic compression. This might represent enhanced respreading into the monolayer of pure POPG from a collapsed form during reexpansion of the surface. (4) Monolayers containing 10% or 30% POPG and 90% or 70% DPPC could be compressed to surface tensions approaching zero. (5) In such mixed monolayers, 10% or 30% POPG did not appear to enhance respreading, as measured by collapse plateau ratios, in the presence of Na + or Ca 2+ in the subphase.
Molecular dynamics simulation of phase transitions in model lung surfactant monolayers
Biochimica et Biophysica Acta (BBA) - Biomembranes, 2011
To explore the role of individual lung surfactant components in liquid-condensed (LC)/liquid-expanded (LE) phase transitions the MARTINI coarse-grained (CG) model is used to simulate monolayers containing DPPC and additional lipid or peptide components. Our analysis suggests that the LC phase forms from the LE phase via a nucleation and growth mechanism, while the LC-LE transition occurs by melting that originates from defects in the monolayer. On the time scale of our simulations, DPPC monolayers display a substantial hysteresis between the ordering and disordering transitions, which is decreased by the addition of a second component. In binary mixtures of DPPC with lung surfactant peptide fragment SP-B 1-25 , the ordered side of the hysteresis loop is abolished altogether, suggesting that SP-B 1-25 effectively nucleates disorder in the monolayer on heating. SP-B 1-25 is observed to perturb the packing of the surrounding lipids leading to local fluidization of the monolayer and to aggregate within the LE phase. In 1:1 DPPC:POPC monolayers, a high concentration of unsaturated phospholipid leads to a substantial decrease in the LC-LE and LE-LC transition temperatures. Adding cholesterol to pure DPPC increases the LC-LE and LE-LC transition temperatures and increases the order on the disordered side of the hysteresis loop leading to a phase of intermediate order, which could be the liquid-disordered (Ld) phase. Cholesterol is also observed to show a preference for LC-LE domain boundaries. The results of our molecular dynamics simulations coincide with many experimental observations and can help provide insight into the physiological roles of individual surfactant components. j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / b b a m e m
The melting of pulmonary surfactant monolayers
Journal of Applied Physiology, 2007
Monomolecular films of phospholipids in the liquid-expanded (LE) phase after supercompression to high surface pressures (π), well above the equilibrium surface pressure (π e ) at which fluid films collapse from the interface to form a three-dimensional bulk phase, and in the tilted-condensed (TC) phase both replicate the resistance to collapse that is characteristic of alveolar films in the lungs. To provide the basis for determining which film is present in the alveolus, we measured the melting characteristics of monolayers containing TC dipalmitoyl phosphatidylcholine (DPPC), as well as supercompressed 1-palmitoyl-2-oleoyl phosphatidylcholine and calf lung surfactant extract (CLSE). Films generated by appropriate manipulations on a captive bubble were heated from ≤27°C to ≥60°C at different constant π above π e . DPPC showed the abrupt expansion expected for the TC-LE phase transition, followed by the contraction produced by collapse. Supercompressed CLSE showed no evidence of the TC-LE expansion, arguing that supercompression did not simply convert the mixed lipid film to TC DPPC. For both DPPC and CLSE, the melting point, taken as the temperature at which collapse began, increased at higher π, in contrast to 1-palmitoyl-2-oleoyl phosphatidylcholine, for which higher π produced collapse at lower temperatures. For π between 50 and 65 mN/m, DPPC melted at 48-55°C, well above the main transition for bilayers at 41°C. At each π, CLSE melted at temperatures >10°C lower. The distinct melting points for TC DPPC and supercompressed CLSE provide the basis by which the nature of the alveolar film might be determined from the temperature-dependence of pulmonary mechanics.
Langmuir, 2010
Many biologically relevant monolayers show coexistence of discrete domains of a long-range ordered condensed phase dispersed in a continuous, disordered, liquid-expanded phase. In this work, we determined the viscous and elastic components of the compressibility modulus and the shear viscosity of monolayers exhibiting phase coexistence with the aim at elucidating the contribution of each phase to the observed monolayer mechanical properties. To this purpose, mixed monolayers with different proportions of distearoylphosphatidylcholine (DSPC) and dimyristoylphosphatidylcholine (DMPC) were prepared and their rheological properties were analyzed. The relationship between the phase diagram of the mixture at 10 mN m -1 and the rheological properties was studied. We found that the monolayer shear viscosity is highly dependent on the presence of domains and on the domain density. In turn, the monolayer compressibility is only influenced by the presence of domains for high domain densities. For monolayers that look homogeneous on the micrometer scale (DSPC amount lower that 23 mol %), all the analyzed rheological properties remain similar to those observed for pure DMPC monolayers, indicating that in this proportion range the DSPC molecules contribute as DMPC to the surface rheology in spite of having hydrocarbon chains four carbons longer.
Phase Transitions in Films of Lung Surfactant at the Air-Water Interface
Biophysical Journal, 1998
Pulmonary surfactant maintains a putative surface-active film at the air-alveolar fluid interface and prevents lung collapse at low volumes. Porcine lung surfactant extracts (LSE) were studied in spread and adsorbed films at 23 Ϯ 1°C using epifluorescence microscopy combined with surface balance techniques. By incorporating small amounts of fluorescent probe 1-palmitoyl-2-nitrobenzoxadiazole dodecanoyl phosphatidylcholine (NBD-PC) in LSE films the expanded (fluid) to condensed (gel-like) phase transition was studied under different compression rates and ionic conditions. Films spread from solvent and adsorbed from vesicles both showed condensed (probe-excluding) domains dispersed in a background of expanded (probe-including) phase, and the appearance of the films was similar at similar surface pressure. In quasistatically compressed LSE films the appearance of condensed domains occurred at a surface pressure () of 13 mN/m. Such domains increased in size and amounts as was increased to 35 mN/m, and their amounts appeared to decrease to 4% upon further compression to 45 mN/m. Above of 45 mN/m the LSE films had the appearance of filamentous materials of finely divided dark and light regions, and such features persisted up to a near 68 mN/m. Some of the condensed domains had typical kidney bean shapes, and their distribution was similar to those seen previously in films of dipalmitoylphosphatidylcholine (DPPC), the major component of surfactant. Rapid cyclic compression and expansion of LSE films resulted in features that indicated a possible small (5%) loss of fluid components from such films or an increase in condensation efficiency over 10 cycles. Calcium (5 mM) in the subphase of LSE films altered the domain distribution, decreasing the size and increasing the number and total amount of condensed phase domains. Calcium also caused an increase in the value of at which the maximum amount of independent condensed phase domains were observed to 45 mN/m. It also induced formation of large amounts of novel, nearly circular domains containing probe above of 50 mN/m, these domains being different in appearance than any seen at lower pressures with calcium or higher pressures in the absence of calcium. Surfactant protein-A (SP-A) adsorbed from the subphase onto solvent-spread LSE films, and aggregated condensed domains in presence of calcium. This study indicates that spread or adsorbed lung surfactant films can undergo expanded to condensed, and possibly other, phase transitions at the air-water interface as lateral packing density increases. These phase transitions are affected by divalent cations and SP-A in the subphase, and possibly by loss of material from the surface upon cyclic compression and expansion.
The Collapse of Monolayers Containing Pulmonary Surfactant Phospholipids Is Kinetically Determined
Biophysical Journal, 2005
Prior studies have shown that during and after slow compressions of monomolecular films containing the complete set of purified phospholipids (PPL) from calf surfactant at an air/water interface, surface pressures (p) reach and sustain values that are remarkably high relative to expectations from simple systems with model lipids. Microscopy shows that the liquid-expanded, tilted-condensed, and collapsed phases are present together in the PPL films between 45 and 65 mN/m. The Gibbs phase rule restricts equilibrium coexistence of three phases to a single p for films with two components but not for more constituents. We therefore determined if the surprising stability of PPL reflects release from the thermodynamic restrictions of simple model systems by the presence of multiple components. Experiments with binary films containing dioleoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine first tested the predictions of the phase rule. The onset of three-phase coexistence, determined by fluorescence microscopy, and its termination, established by relaxation of collapsing films on a captive bubble, occurred at similar p. Experiments for PPL using the same methods suggested that the three phases might coexist over a range of p, but limited to ;2 mN/m, and extending below rather than above the coexistence p for the binary films. Our results show that the PPL films at high p must deviate from equilibrium and that they must then be metastable. MATERIALS AND METHODS Materials DPPC and DOPC (Avanti Polar Lipids, Alabaster, AL) and N-(lissamine rhodamine-B sulfonyl)-dipalmitoyl phosphatidylethanolamine (Rh-DPPE; Molecular Probes, Eugene, OR) were purchased and used without further purification or analysis. Extracted calf surfactant (calf lung surfactant extract (CLSE)), purified as described previously (11), was obtained from Dr.
European Biophysics Journal, 1997
Three compounds of the pulmonary surfactant -dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG), and the surfactant associated protein C (SP-C) -were spread at the air-water interface of a Langmuir trough as a model system to mimic the properties of natural surfactant. Fluorescence microscopical images of the film formed at the interface were obtained during compression using a fluorescence dye bound covalently either to phosphatidylcholine or to SP-C. The images were quantified using statistical methods in respect to relative areas and relative fluorescence intensities of the domains found. In the early stage of compression, film pressure rose slightly and was accompanied by a phase separation which could be recognized in the images by the formation of bright and dark domains. On further compression, after a steep increase of film pressure, a plateau region of constant film pressure started abruptly. During compression in the plateau region, fluorescence intensity of the bright domain formed in the early stage of compression increased. The increasing fluorescence intensity, the non-Gaussian intensity distribution of the bright domain, and the small mean molecular area of the film in the plateau region gave rise to the assumption that multilayer structures were formed in the late stage of compression. The formation of the multilayer structures was fully reversible in repeated compression-expansion cycles including the plateau region of the phase diagram. The ability of lipid/SP-C mixtures to form reversible multilayer structures during compression may be relevant to stability in lungs during expiration and inhalation. labeled surfactant associated protein C · SP-C surfactant associated protein C · A b * relative area of the bright domain · A b absolute area of the bright domain · A d * relative area of the dark domain · A d , absolute area of the dark domain · A M molecular area · I* dye relative intensity per fluorescence dye · --I b mean intensity of the bright domain · --I d mean intensity of the dark domain · ∆I -difference of the mean intensities of the bright and the dark domain (∆I --= I b -I d ) · N b intensity distribution of the bright domain · N d intensity distribution of the dark domain · N t total number of pixels of the analyzed frame Eur Biophys J (1997) 26: 359-369
Phospholipid packing and hydration in pulmonary surfactant membranes and films as sensed by LAURDAN
Biochimica et Biophysica Acta (BBA) - Biomembranes, 2011
The efficiency of pulmonary surfactant to stabilize the respiratory surface depends critically on the ability of surfactant to form highly packed films at the air-liquid interface. In the present study we have compared the packing and hydration properties of lipids in native pulmonary surfactant and in several surfactant models by analyzing the pressure and temperature dependence of the fluorescence emission of the LAURDAN (1-[6-(dimethylamino)-2-naphthyl]dodecan-1-one) probe incorporated into surfactant interfacial films or freestanding membranes. In interfacial films, compression-driven changes in the fluorescence of LAURDAN, evaluated from the generalized polarization function (GPF), correlated with changes in packing monitored by surface pressure. Compression isotherms and GPF profiles of films formed by native surfactant or its organic extract were compared at 25 or 37°C to those of films made of dipalmitoylphosphatidylcholine (DPPC), palmitoyloleoylphosphatidylcholine (POPC), DPPC/phosphatidylglycerol (PG) (7:3, w/w), or the mixture DPPC/POPC/palmitoyloleoylphosphatidylglycerol (POPG)/cholesterol (Chol) (50:25:15.10), which simulates the lipid composition of surfactant. In general terms, compression of surfactant films at 25°C leads to LAURDAN GPF values close to those obtained from pure DPPC monolayers, suggesting that compressed surfactant films reach a dehydrated state of the lipid surface, which is similar to that achieved in DPPC monolayers. However, at 37°C, the highest GPF values were achieved in films made of full surfactant organic extract or the mixture DPPC/POPC/POPG/Chol, suggesting a potentially important role of cholesterol to ensure maximal packing/dehydration under physiological constraints. Native surfactant films reached high pressures at 37°C while maintaining relatively low GPF, suggesting that the complex three-dimensional structures formed by whole surfactant might withstand the highest pressures without necessarily achieving full dehydration of the lipid environments sensed by LAURDAN. Finally, comparison of the thermotropic profiles of LAURDAN GPF in surfactant model bilayers and monolayers of analogous composition shows that the fluorophore probes an environment that is in average intrinsically more hydrated at the interface than inserted into free-standing bilayers, particularly at 37°C. This effect suggests that the dependence of membrane and surfactant events on the balance of polar/non-polar interactions could differ in bilayer and monolayer models, and might be affected differently by the access of water molecules to confined or freestanding lipid structures.
Biophysical Journal, 2007
The aqueous lining of the lung surface exposed to the air is covered by lung surfactant, a film consisting of lipid and protein components. The main function of lung surfactant is to reduce the surface tension of the air-water interface to the low values necessary for breathing. This function requires the exchange of material between the lipid monolayer at the interface and lipid reservoirs under dynamic compression and expansion of the interface during the breathing cycle. We simulated the reversible exchange of material between the monolayer and lipid reservoirs under compression and expansion of the interface. We used a mixture of dipalmitoyl-phosphatidylcholine, palmitoyl-oleoyl-phosphatidylglycerol, cholesterol, and surfactantassociated protein C as a functional analog of mammalian lung surfactant. In our simulations, the monolayer collapses into the water subphase on compression and forms bilayer folds. On monolayer reexpansion, the material is transferred from the folds back to the interface. The simulations indicate that the connectivity of the bilayer aggregates to the monolayer is necessary for the reversibility of the monolayer-bilayer transformation. The simulations also show that bilayer aggregates are unstable in the air subphase and stable in the water subphase.
Dynamic properties and relaxation processes in surface layer of pulmonary surfactant solutions
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2019
In this work, dynamic surface properties of spread monolayers of DPPC and adsorbed layers of PS solutions were investigated in a broad range of surface pressure. Application of a modified Langmuir trough and a recently developed approach for the analysis of a non-linear response of the adsorption layer to large surface deformations gives a possibility to determine the dilational surface elasticity in the region of high surface pressures corresponding to the physiological state of the lung alveoli surface. Although DPPC is the main component of the complex pulmonary surfactant (PS) mixture, the dynamic surface properties of the adsorption layers of these two compounds are different. Other components in the PS adsorption layer lead to a decrease of the dynamic surface elasticity due to a looser packing of the layer, as confirmed by ellipsometry and infrared reflection absorption spectroscopy. Moreover, in the region of high surface pressures the main relaxation time of the surface stresses is much lower than that for spread DPPC monolayers. The fast relaxation in the adsorption layer can be connected with the redistribution of molecules between the surface and the subsurface layer in the course of compression. The acceleration of the relaxation processes can lead to a decrease of the number and size of collapsed parts of the layer and thereby to a decrease of the number of molecules with lost functional properties. This effect can be one of the reasons for the lower efficiency of synthetic pharmaceutical drugs, which do not have proteins in their composition, as compared with the natural PS extracted from animal lungs.