The melting of pulmonary surfactant monolayers

Abstract

Yan W, Biswas SC, Laderas TG, Hall SB. The melting of pulmonary surfactant monolayers. J Appl Physiol 102: 1739 -1745, 2007. First published December 28, 2006; doi:10.1152/japplphysiol.00948.2006.-Monomolecular films of phospholipids in the liquid-expanded (LE) phase after supercompression to high surface pressures (), well above the equilibrium surface pressure ( e) at which fluid films collapse from the interface to form a three-dimensional bulk phase, and in the tiltedcondensed (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 Յ27°C to Ն60°C at different constant above 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 , in contrast to 1-palmitoyl-2-oleoyl phosphatidylcholine, for which higher produced collapse at lower temperatures. For between 50 and 65 mN/m, DPPC melted at 48 -55°C, well above the main transition for bilayers at 41°C. At each , CLSE melted at temperatures Ͼ10°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. captive bubble; dipalmitoyl phosphatidylcholine; jammed; monolayer; pulmonary mechanics; supercompressed THE BEHAVIOR OF PULMONARY surfactant indicates that films at the air/water interface in the alveoli are solid. Multiple approaches consistently indicate that, when compressed by the decreasing alveolar surface area during exhalation, the films of pulmonary surfactant reach high surface pressures () (13,19,30,35,37,39). The observed values are well above the equilibrium spreading pressure ( e ) of ϳ46 mN/m, at which two-dimensional monomolecular films coexist at equilibrium with their three-dimensional bulk phases (14). Films under equilibrium conditions never reach Ͼ e , because compression produces only flow of constituents into the bulk phase, with no increase in the density of material within the interface. The Ͼ e observed in static lungs, therefore, indicate that the alveolar films deviate from equilibrium and must by definition be metastable. The rate at which a film flows into the bulk phase in response to the thermodynamic driving force of ( Ϫ e ) can be used to calculate an effective viscosity (27). The very slow rates at which films in the lungs undergo this p...

Phase Transitions in Dipalmitoylphosphatidylcholine Monolayers

“…32 Hall and co-workers determined T M of the DPPC monolayer in the range of 48–55 °C. 3,33 Hence, the three characteristic temperatures pertinent to DPPC monolayers and bilayers rank as T m < T c < T M . These differences suggest the importance of molecular packing in affecting phospholipid phase behavior.…”

Automated Droplet Manipulation Using Closed-Loop Axisymmetric Drop Shape Analysis

“…It can be seen that the Π– A curve is consistent with well-characterized DPPC compression isotherms reported in the literature. 14,33,34 At large surface area and low surface pressure, the DPPC monolayer is in a fluid-like liquid-expended (LE) phase, while the surface pressure only increases slowly with area reduction. After passing a phase transition plateau at about 8.5 mN/m, the DPPC monolayer is transformed into a solid-like tilted-condensed (TC) phase at which the surface pressure increases rapidly with compression, indicating a very low film compressibility for the TC phase DPPC monolayer.…”

Lamellar Bodies Form Solid Three-dimensional Films at the Respiratory Air-Liquid Interface

“…Presence of enough DPPC would be important to permit segregation of highly packed membrane patches, which provide mechanical stability to some extent. Some controversy exists with respect to the actual existence of such phase segregation in the lung, particularly at physiological temperature, humidity, and compression states (24). On the other hand, mechanical properties of surfactant films do not depend only on the behavior of single monolayered structures.…”