Article Information

• PDF (1735 kb)
• Full Text with Large Figures
• Cited by in Scopus (4)

PubMed

• Articles by Svetlana Baoukina
• Articles by D. Peter Tieleman

Related Articles

• Liquid-Crystalline Collapse of Pulmonary Surfactant Monolaye...

  Liquid-Crystalline Collapse of Pulmonary Surfactant Monolayers Biophysical Journal, Volume 84, Issue 6, 1 June 2003, Pages 3792-3806 William R. Schief, Meher Antia, Bohdana M. Discher, Stephen B. Hall and Viola Vogel Abstract During exhalation, the surfactant film of lipids and proteins that coats the alveoli in the lung is compressed to high surface pressures, and can remain metastable for prolonged periods at pressures approaching 70mN/m. Monolayers of calf lung surfactant extract (CLSE), however, collapse in vitro, during an initial compression at ∼45mN/m. To gain information on the source of this discrepancy, we investigated how monolayers of CLSE collapse from the interface. Observations with fluorescence, Brewster angle, and light scattering microscopies show that monolayers containing CLSE, CLSE-cholesterol (20%), or binary mixtures of dipalmitoyl phosphatidylcholine(DPPC)-dihydrocholesterol all form bilayer disks that reside above the monolayer. Upon compression and expansion, lipids flow continuously from the monolayer into the disks, and vice versa. In several respects, the mode of collapse resembles the behavior of other amphiphiles that form smectic liquid-crystal phases. These findings suggest that components of surfactent films must collapse collectively rather than being squeezed out individually. Abstract | Full Text | PDF (1297 kb)

• Mechanical Deformation of Spherical Viruses with Icosahedral...

  Mechanical Deformation of Spherical Viruses with Icosahedral Symmetry Biophysical Journal, Volume 91, Issue 3, 1 August 2006, Pages 834-841 Gerard Adriaan Vliegenthart and Gerhard Gompper Abstract Virus capsids and crystalline surfactant vesicles are two examples of self-assembled shells in the nano- to micrometer size range. Virus capsids are particularly interesting since they have to sustain large internal pressures while encapsulating and protecting the viral DNA. We therefore study the mechanical properties of crystalline shells of icosahedral symmetry on a substrate under a uniaxial applied force by computer simulations. We predict the elastic response for small deformations, and the buckling transitions at large deformations. Both are found to depend strongly on the number of elementary building blocks (the capsomers in the case of viral shells), the Föppl-von Kármán number (which characterizes the relative importance of shear and bending elasticity), and the confining geometry. In particular, we show that whereas large shells are well described by continuum elasticity-theory, small shells of the size of typical viral capsids behave differently already for small deformations. Our results are essential to extract quantitative information about the elastic properties of viruses and vesicles from deformation experiments. Abstract | Full Text | PDF (347 kb)

• Effects of Lung Surfactant Proteins, SP-B and SP-C, and Palm...

  Effects of Lung Surfactant Proteins, SP-B and SP-C, and Palmitic Acid on Monolayer Stability Biophysical Journal, Volume 80, Issue 5, 1 May 2001, Pages 2262-2272 Junqi Ding, Dawn Y. Takamoto, Anja von Nahmen, Michael M. Lipp, Ka Yee C. Lee, Alan J. Waring and Joseph A. Zasadzinski Abstract Langmuir isotherms and fluorescence and atomic force microscopy images of synthetic model lung surfactants were used to determine the influence of palmitic acid and synthetic peptides based on the surfactant-specific proteins SP-B and SP-C on the morphology and function of surfactant monolayers. Lung surfactant-specific protein SP-C and peptides based on SP-C eliminate the loss to the subphase of unsaturated lipids necessary for good adsorption and respreading by inducing a transition between monolayers and multilayers within the fluid phase domains of the monolayer. The morphology and thickness of the multilayer phase depends on the lipid composition of the monolayer and the concentration of SP-C or SP-C peptide. Lung surfactant protein SP-B and peptides based on SP-B induce a reversible folding transition at monolayer collapse that allows all components of surfactant to be retained at the interface during respreading. Supplementing Survanta, a clinically used replacement lung surfactant, with a peptide based on the first 25 amino acids of SP-B also induces a similar folding transition at monolayer collapse. Palmitic acid makes the monolayer rigid at low surface tension and fluid at high surface tension and modifies SP-C function. Identifying the function of lung surfactant proteins and lipids is essential to the rational design of replacement surfactants for treatment of respiratory distress syndrome. Abstract | Full Text | PDF (547 kb)

• …more

Copyright © 2007 The Biophysical Society. All rights reserved.
Biophysical Journal, Volume 93, Issue 11, 3775-3782, 1 December 2007

doi:10.1529/biophysj.107.113399

Biophysical Theory and Modeling

The Molecular Mechanism of Monolayer-Bilayer Transformations of Lung Surfactant from Molecular Dynamics Simulations

Svetlana Baoukina^*, Luca Monticelli^*, Matthias Amrein^† and D. Peter Tieleman^*, ,  

^* Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada, T2N 1N4
^† Department of Cell Biology and Anatomy, Faculty of Medicine, University of Calgary, Calgary, Alberta, Canada, T2N 1N4

Address reprint requests to D. Peter Tieleman, Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1N4.

Gas exchange takes place at the internal surface of the alveoli, small globular structures that terminate airways in the lungs. The alveolar epithelial cells are covered by a thin hydration layer exposed to the air. In the absence of surfactant, this air-water interface is unstable because of the high surface tension (∼72mN/m) and the small radius of the alveolus (∼100μm). Thus, the surface tension needs to be reduced to prevent the collapse of the alveoli as a result of the Laplace pressure.

Lung surfactant is a complex mixture of lipids and peptides forming a monolayer at the air-water interface of the alveoli 1. Its main physiological function is the reduction of the surface tension at the air-water interface of the lung. During the breathing cycle, lung surfactant maintains a low surface tension (0–10mN/m) under dynamic compression and expansion of the interface 2. This function is believed to be associated with the exchange of material with lipid reservoirs that are attached to the monolayer 3,4,5,6.

The “squeeze-out” model of lung surfactant suggests that the monolayer undergoes a compositional change on compression: the unsaturated lipids are squeezed out from the interface, and the monolayer becomes highly enriched in dipalmitoyl-phosphatidylcholine (DPPC) 7,8,9. At physiological temperatures DPPC can pack tightly at the interface to form a liquid-condensed phase with a near-zero tension in a monolayer. However, recent experimental findings contradict the “squeeze-out” model 10,11,12 by showing that, at low surface tension, surfactant monolayer has several components and has both liquid-expanded and liquid-condensed phases. Lipid reservoirs connected to the monolayer have also been associated with the stability of the highly compressed monolayer 4,6,13. Atomic force microscopy (AFM) experiments have shown that these reservoirs are discontinuous lipid lamellar stacks containing one or several bilayers 6,13. They are distributed over the surface of the monolayer and formed on monolayer compression.

These monolayer-associated bilayer reservoirs are absent in surfactant with impaired function 4, characteristic of adult respiratory distress syndrome 14,15. Hence, understanding the molecular mechanism of the monolayer-bilayer transformation and the structure of the monolayer-bilayer aggregates may be important for treatment of patients with acute respiratory failure.

Detailed information on the monolayer-bilayer transformation in lung surfactant is not available directly from experiments. First, it is not clear whether the lipid reservoirs are formed in the air subphase or in the water subphase. Second, the structure of highly curved lipid regions connecting lipid reservoirs to the monolayer is difficult to study using current experimental techniques. Third, the role of different lipid and protein constituents in the formation of lipid reservoirs and their connectivity to the monolayer are not fully understood.

Here, we investigate the structure of a lung surfactant model and the mechanism of monolayer-bilayer transformation using molecular dynamics simulations. Previous simulations of lung surfactant focused on specific interactions of the surfactant-associated peptides with lipid monolayers 16,17,18. These simulations used atomistic models and investigated relatively small systems (box lateral size smaller than 10nm) on time scales up to 30ns. Mesoscopic time and length scales are required to study the monolayer-bilayer transition and the stability of the monolayer-associated bilayer aggregates. To reach these time and length scales, we use a coarse-grained (CG) molecular model that accurately reproduces the structure and dynamics of various lipid assemblies and lipid phase transitions 19,20,21. The model also reproduces the pressure-area isotherm for a DPPC monolayer 36. We simulated a model system consisting of DPPC, palmitoyl-oleoyl-phosphatidylglycerol (POPG), cholesterol (8:2:1 DPPC:POPG:cholesterol), and surfactant-associated protein C (SP-C). Despite the simple composition, this system is a functional analog to bovine lipid extract surfactant (BLES) 22. We simulate the formation of monolayer-associated bilayer reservoirs in the water subphase under monolayer compression. We show that lipid aggregates are unstable in the air subphase and thus are likely to be formed only in the water subphase.

The model system for lung surfactant consisted of CG DPPC and POPG lipids and cholesterol in the proportion 8:2:1, and CG SP-C proteins with palmitoylated chains. The simulation setup included a water slab bounded by two vacuum slabs with two symmetric monolayers at the two water-vacuum interfaces (Fig. 1). To study the mechanism of monolayer-bilayer transformation, the system was compressed or expanded to change the area available for the monolayers at the two interfaces without changing the box size in the normal direction. To investigate the properties and the evolution of the structure of the compressed/expanded monolayers, the monolayers were simulated at the constant volume of the simulation box with constant areas of the two interfaces and a fixed box size in the direction normal to the interfaces. To study the stability of lipid reservoirs associated with lung surfactant in the water and in the air subphases, we added preformed lipid aggregates to the system with two surfactant monolayers. In the first case, a circular bilayer patch was placed in the water subphase adjacent to one of the monolayers. In the second case, an inverted circular bilayer patch was placed in the vacuum subphase adjacent to one of the monolayers. In both cases, the lipid reservoirs had the same composition as the monolayers. These two systems were simulated at constant interfacial areas and constant box size in the normal direction.

Display large version of this figure

Figure 1

The simulation setup includes two lung surfactant model monolayers and a water slab. DPPC is shown in green, POPG in yellow, cholesterol in cyan, SP-C in red, water in blue, and Na⁺ ions in gray.

We used an updated version of the CG force field by Marrink et al., 23, which also includes cholesterol. Compared with a previously published version 19, the new version of the CG force field includes more interaction energy levels without changing the form of the interaction potentials. It also has an increased number of particle subtypes compared with the earlier model to represent more accurately the chemical nature of the molecular fragments. For POPG lipids, the head group consists of two hydrophilic particles: the charged phosphate group is represented by a charged particle (Qa), the glycerol group is represented by a polar particle (P4). DPPC and cholesterol are standard components of the force field 23. For the protein, a CG force field was developed following the same strategy as that used by Marrink for lipids and detergents 19. As in the lipid force field, an approximate 4:1 mapping of heavy atoms to CG beads was used, with the exception of the aromatic side chains. A mapping of 2:1 or 3:1 was used for all cyclic structures to preserve their geometry. A reduced mass of 45 was used for all the cyclic-type particles. The N0 particle type was used for the backbone of each amino acid. Between zero and four beads were used to represent the amino acid side chains. The choice of the particle type for the amino acid side chains was based on the free energy of partitioning between water and hydrocarbons. Details of the parameterization and the full force field will be the subject of a separate publication (L. Monticelli, S. K. Kandasamy, X. Periole, R. G. Larson, D. P. Tieleman, S. J. Marrink, unpublished data).

For nonbonded interactions, the standard cutoffs for the CG force field were used: the Lennard-Jones interactions were shifted to zero between 0.9 and 1.2nm, whereas the Coulomb potential was shifted to zero between 0nm and 1.2nm. The relative dielectric constant was 15, which is slightly lower than the value of 20 in the previous version of the force field.

Each monolayer contained 1280 DPPC lipids, 320 POPG lipids, 160 cholesterol molecules, and 14 SP-C proteins. All simulations used periodic boundaries in three dimensions. The water slab contained 126,282 water particles and 598 Na⁺ ions. In the simulations at the constant volume of the box, no pressure coupling was used. The Berendsen barostat (coupling constant τ_P=4.0ps) was used for the semi-isotropic pressure coupling in the compression/expansion runs. The system compressibility was set to 5×10⁻⁵ bar⁻¹ in the lateral direction and to 0 in the normal direction. The latter ensures that the thickness of the vacuum slabs in the box changes as the area of the monolayer changes, without changing the size of the box normal to the membrane. Lipids and water with ions were coupled separately to the Berendsen heat bath 24 at T=310K with a coupling constant τ_T=1ps. A time step of 20 fs was used. The neighborlist was updated every 10 steps.

Molecular dynamics simulations were performed using the GROMACS simulation package 25. A system with two monolayers in a liquid-expanded phase at the surface tension of 35mN/m in the box of 35×35×50nm was used as a starting structure. Initial equilibration was performed for 5ns at constant volume and temperature. The monolayer compression was simulated by applying positive lateral pressure to the system using the semi-isotropic pressure-coupling scheme (constant average pressure in the x-y plane and in the z-direction separately). Positive lateral pressure corresponds to negative surface tension imposed on each interface. The relation between the lateral pressure applied to the system, P_L, and the surface tension, γ_m, in each of the two (symmetric) monolayers reads as γ_m=(P_N–P_L)×L_z/2, where L_z is the box size in the direction normal to the interfaces, P_N is the normal pressure, and P_L=(P_xx+P_yy)/2. The presence of the vacuum region bounding the water slab resulted in low values of the normal pressure (∼0.3 bar). Negative surface tension at each interface destabilizes the flat geometry of the monolayer, inducing monolayer collapse. The compressed monolayers were expanded by applying negative lateral pressure to the system, using the semi-isotropic pressure coupling. Negative lateral pressure corresponds to positive surface tension imposed on each interface. The compression/expansion runs were performed for 50ns using lateral pressures of 10, 2, −10, and −16 bar. An applied pressure of −16 bar corresponds to a surface tension of ∼40 mN/m at each interface, similar to experimental values 3,26. An applied pressure of 10 bar corresponds to a negative surface tension of ∼−25 mN/m imposed on each interface. This tension is not actually reached in the system (at a given lateral size of ∼30nm) because the monolayers collapse. Smaller monolayers (with lateral dimension of ∼5nm) exist in a metastable flat geometry under negative surface tension.

Production runs at constant volume and temperature (for 1μs each) were performed on the systems previously compressed or expanded to interfacial areas of 1414, 1225, 1056, 876, and 645nm², corresponding to surface tensions γ_m of 40, 35, 23, 0, and 23mN/m (the last value refers to the monolayers after collapse). The systems contained either 1), two monolayers only; 2), two monolayers with a bilayer patch in the water subphase; or 3), two monolayers with an inverted bilayer patch in the vacuum subphase.

Each bilayer patch was 12nm in diameter and contained 360 DPPC lipids, 90 POPG lipids, and 45 cholesterol molecules and 2 SP-C proteins. The bilayer patch (similar to a bicelle) was formed by equilibrating a square bilayer in a box of water larger than the bilayer (20×20×20nm) for 10ns with isotropic pressure coupling (1 bar). The box contained 64,911 water particles and 84 Na⁺ ions. During the equilibration, the bilayer became circular, and the lipids bent at the perimeter to minimize the hydrocarbon chain-water contact area. The inverted bilayer patch (with hydrocarbon chains oriented outward) was formed by exchanging the position of the two leaflets of the circular bilayer patch. The inverted patch was equilibrated in vacuum (20×20×20nm box) for 1ns at constant volume and temperature.

We first investigated whether the monolayer-associated lipid reservoirs can be formed in the air and/or water subphases. To this end we simulated small preformed bilayer aggregates in the water and in the vacuum subphases adjacent to a monolayer and followed the change in the structure of these complexes.

In the first system, an inverted bilayer patch was placed in the vacuum subphase close to one of the monolayers in the simulation box (Fig. 2, 0ns). We performed three simulations at different surface tensions in the monolayers (γ_m= 35, 23, 0mN/m) at the constant volume for 1μs each. The patch was unstable and readily fused with the adjacent monolayer (Fig. 2, 0.5ns). Fusion was always observed irrespective of the degree of monolayer compression/expansion. The excess lipids were expelled into the water subphase during the fusion (Fig. 2, 5ns) and remained connected to the monolayer at the interface. No evolution of the structure was observed on the simulation time scale.

Display large version of this figure

Figure 2

Two lung surfactant monolayers with an inverted circular bilayer patch in the vacuum subphase at 0, 0.5, and 5ns. The inverted bilayer patch is unstable and readily fuses with the adjacent monolayer. Colors are as in Fig. 1.

In the second system, a lipid bilayer patch was placed in the water subphase close to one of the monolayers (Fig. 3, 0ns). We performed four simulations at different surface tensions in the monolayers (γ_m= 40, 35, 23, 0mN/m) at the constant volume of the box for 1μs each. In most cases, the patch was stable in the water subphase (Fig. 3, 50ns). Only in the case of a highly expanded monolayer (at γ_m= 40 mN/m) with large pores (Fig. 4, 0ns) was the bilayer patch adsorbed on the interface in the pore region (Fig. 4, 5ns). This process was, however, distinct from the fusion of a bilayer patch with a monolayer because the patch adsorbed directly at the bare water-vacuum interface. The lipids from the patch filled the pore region; however, the adsorption was not complete, and the residual bilayer aggregate with proteins remained on the interface (Fig. 4, 100ns). On compression of the monolayer with the adsorbed bilayer patch (lateral pressure 2 bar, 50-ns simulation), the excess material was expelled into the water subphase and formed a circular bilayer aggregate. This bilayer aggregate did not detach from the monolayer in a subsequent simulation (constant volume, 1μs) and remained connected via a highly curved lipid region.

Display large version of this figure

Figure 3

Two lung surfactant monolayers with a circular bilayer patch in the water subphase (water not shown) at (A) 0ns and (B) 50ns. Colors are as in Fig. 1.

Display large version of this figure

Figure 4

Adsorption of a bilayer patch in the pore region of an expanded monolayer (water not shown) at (A) 0ns, (B) 5ns, and (C) 100ns. Colors are as in Fig. 1.

Next, we investigated the transformations between the monolayer and bilayer reservoirs during compression and expansion of the monolayer. The compression of the system with two monolayers (γ_m=35mN/m) was simulated by applying a positive lateral pressure to the system, which corresponds to imposing to a negative surface tension on each interface (see Methods). In two 50-ns simulations of monolayer compression, two different lateral pressures were applied (2 bar and 10 bar). Fig. 5 shows the evolution of the actual surface tension in the monolayer at an imposed surface tension on the interface of −25mN/m (corresponding to a lateral pressure of 10 bar). On compression, the tension in the monolayer decreases gradually from the starting value (35mN/m) to a slightly negative value but does not reach the applied value. When an expanded surfactant monolayer (Fig. 6, 0ns, top and side view) is compressed, its surface becomes rippled (Fig. 6, 2ns) when the surface tension approaches zero. If the applied lateral pressure is small (2 bar), the monolayer does not collapse. For larger values of the applied lateral pressure (10 bar), the monolayer begins to buckle as the tension in the monolayer becomes negative (Fig. 6, 5ns). Buckling is characterized by out-of-plane deviations of the monolayer (corresponding to one bending wave per simulation box), which grow in amplitude with time (Fig. 6, 10ns). Once the buckles form, the monolayer will collapse even if the compression is stopped. The monolayer collapses by folding into the water subphase (Fig. 6, 15ns); collapse into the air subphase was never observed. After the monolayer buckling/collapse occurred, the system was simulated at constant volume for 1μs.

Display large version of this figure

Figure 5

Surface tension in the monolayer compressed by a lateral pressure of 10 bar as a function of simulation time.

Display large version of this figure

Figure 6

Collapse of the lung surfactant monolayer and formation of bilayer folds in the water subphase. On compression, an expanded monolayer with pores (0ns, top and side views) forms a liquid-condensed phase (2ns); the surface of the monolayer buckles (5ns); the buckles grow in amplitude (10ns); and the monolayer folds to form a bilayer (15ns). DPPC is shown in green, POPG in yellow, cholesterol in cyan, SP-C in red, water in blue, and Na⁺ ions in gray. Lipid head groups are represented as spheres, and lipid chains, cholesterol, and SP-C as sticks; the size of several selected lipid molecules is increased to show their conformation.

He bilayer folds in the water subphase are flat and are oriented approximately perpendicular to the monolayer. The folds either have a circular shape or are rectangular and periodic, spanning the simulation box (Fig. 7). They are connected to the monolayer through lipid regions of high negative curvature. The bilayer folds have the same lipid composition as the monolayer, and no segregation of SP-C or lipids is observed on the simulation time scale. The folds are stable during the simulation time; they do not change the shape and remain connected to the monolayer. The surfactant monolayer coexists with the bilayer folds at a surface tension of 23mN/m, which is the equilibrium spreading tension for the studied system.

Display large version of this figure

Figure 7

Bilayer folds (circular on top and rectangular at the bottom) attached to lung surfactant monolayers. Molecular representations are as in Fig. 6.

Using the compressed monolayers with folds as starting structures, we simulated expansion and further compression of the system. When the monolayer was compressed further, more material was expelled from the monolayer into the bilayer folds. Monolayer expansion was achieved by applying a negative lateral pressure to the system (−10 and −16 bar; 50ns in both cases). On lateral expansion of the monolayer, the bilayer folds respread at the interface. Complete respreading of the material from the reservoirs at the interface occurs on the simulation time scale only if the imposed surface tension is larger than the monolayer equilibrium spreading tension.

Lipid reservoirs connected to surfactant monolayer are essential for lung surfactant function 3,4,5,6. AFM experiments confirm that these reservoirs are lipid lamellar stacks containing one or several bilayers 4,13,27. However, in the AFM experiments lung surfactant is deposited on a substrate, and lipid aggregates are flattened; thus, information about the actual geometry and orientation of lipid reservoirs is lost by the deposition on the substrate. Hence, it is difficult to define the detailed structure of the lipid aggregates adopted on the alveolar gas exchange interface. It is also not clear whether these reservoirs are formed in the water or in the air subphase and what the molecular mechanism of their formation is.

The formation of lipid reservoirs connected to the lung surfactant monolayer is associated with reversible monolayer collapse by monolayer buckling and folding 13,28. The mechanism of monolayer collapse depends on the monolayer composition, which in turn determines molecular solubility and monolayer structure, morphology, and elastic properties 13. It also depends on the compression rate 29. Monolayer collapse by folding into the water subphase has been observed experimentally for single-component and binary lipid mixtures 13,28, where the folds were identical in composition to the monolayer, as well as for lipid mixtures containing surfactant proteins SP-B and SP-C 11,30,31, where the proteins segregated in the folds. Earlier simulations of a monolayer collapse were carried out for the arachidic acid at the air-water interface using an atomistic model 32. In these simulations the molecules were ejected either into the air subphase or into the water subphase, forming small monolayer aggregates. However, this study was limited to a small box size (∼6nm), which did not allow monolayer bending and buckling to develop. Monolayer collapse has also been simulated for the long-chain PC lipids using a CG model 33, where a lipid bridge was formed in the air subphase. These simulations were also limited to a small system size (250 lipids in the monolayer) and a short simulation time (350ps).

In this study we have simulated the collapse of a model lung surfactant monolayer on much larger length and time scales than previous studies. Collapse proceeds through buckling and folding into the water subphase and results in bilayer folds in the water subphase, never in the air subphase. The molecular composition of these folds is the same as in the monolayers, even though our simulations are long enough to allow significant lipid diffusion. When a bilayer fold is formed from a buckled monolayer in the water subphase, the contact area of the lipid chains with the air is reduced by approximately twice the area of the fold. This leads to a decrease of the surface energy of the lipid chain-air interface and thus to a decrease of the free energy of the system. The surface energy of the lipid chain-air interface can be estimated as the area times the surface tension at the hydrocarbon chain-air interface 29.

In the alternative scenario, lipid aggregates would form in the air subphase. The simulations clearly indicate that preformed lipid aggregates are unstable in the air subphase. In this case, the hydrocarbon chains of the excess lipids continue to be exposed to the air, and thus the surface energy would not decrease. The simulation results show that lipid reservoirs form in the water subphase, and we suggest that lipid reservoirs associated with a lung surfactant monolayer are likewise formed only in the water subphase.

Our simulations also provide a detailed molecular view of the structures of the monolayer-bilayer complexes and the transformation pathway. The simulations show that the bilayer folds remain connected to the lung surfactant monolayer, which coexists with folds at the equilibrium spreading tension of 23 mN/m. This behavior is consistent with experimental results on lung surfactant, where a plateau region is observed at ∼22–25 mN/m in the tension-area isotherm, indicative of the monolayer coexistence with reservoirs. The simulations show that the folding of the monolayer is initiated by monolayer buckling. The monolayer buckling implies bending of a flat monolayer and thus requires overcoming an energy barrier. This energy barrier decreases with the decrease of the monolayer tension but remains finite until the monolayer tension becomes negative. In the simulations, this energy barrier also increases with the decrease of the monolayer lateral size (because the bending deformation is limited to one wavelength per simulation box because of the imposed three-dimensional periodicity). For the considered system size, the monolayer buckling and collapse were observed on the simulation time scale only for negative monolayer tensions. The monolayer compression rates obtained in the simulations (∼0.05–0.0005Ų/ns·molecule) are much faster than the experimental compression rates. They are also much faster than the monolayer relaxation rates, which drives the system substantially out of equilibrium. This can affect the value of the collapse tension 34 and also the mechanism of monolayer collapse 29.

The simulations also show that on repeated compression and expansion of the monolayer, there is a reversible exchange of material between the monolayer and the associated bilayer reservoirs. If the connectivity of the monolayer with the material in the bulk is lost, then the transfer of the material from the bulk to the interface is possible in two ways: either via fusion of the bulk lipid structures with the monolayer or via adsorption of the lipid structures on the bare air-water interface of pores in the highly expanded monolayer. To obtain the highly expanded monolayer with pores, the surface tension has to increase to nonphysiological levels above 35 mN/m. Thus, the lipid adsorption on the bare air-water interface of the pores in an expanded monolayer is probably not relevant for the functional lung. On the other hand, the process of fusion usually involves a high activation barrier 35, which would slow down the transfer of material from the bulk structures to the monolayer. Ineffective transfer of material to the interface would lead to an increase in the surface tension on monolayer reexpansion and may cause the loss of function. Based on these considerations, we propose that the connectivity of the bilayer aggregates in the water subphase to the monolayers is necessary for the reversibility of the monolayer-bilayer transformation. The latter maintains the low value of the surface tension in the monolayer at the air-water interface, which is essential for the function of lung surfactant.

In conclusion, we have simulated the reversible monolayer-bilayer transformations of a lung surfactant model system. Bilayer folds are formed in the water subphase on monolayer compression and respread from the bulk at the interface on monolayer expansion. The bilayer folds are stable lipid reservoirs that remain connected to the monolayer. The simulations indicate that the connectivity of lipid reservoirs to the monolayer is required for the reversibility of the exchange of material at low surface tensions.