Thermodynamics and Structures of Amide Phospholipid Monolayers

Langmuir monolayer self-assembled at the air-water interface represents an excellent model for studying phase transition and lipid polymorphism in two dimensions. Compared with numerous studies of phospholipid phase transitions induced by isothermal compression, there are very scarce reports on two-dimensional phase transitions induced by isobaric heating. This is mainly due to technical difficulties of continuously regulating temperature variations while maintaining a constant surface pressure in a classical Langmuir-type film balance. Here, with technological advances in constrained drop surfactometry and closed-loop axisymmetric drop shape analysis, we studied the isobaric heating process of the dipalmitoylphosphatidylcholine (DPPC) monolayer. It is found that temperature and surface pressure are two equally important intensive properties that jointly determine the phase behavior of the phospholipid monolayer. We have determined a critical point of the DPPC monolayer at a temperature of 44 °C and a surface pressure of 57 mN/m. Beyond this critical point, no phase transition can exist in the DPPC monolayer, either by isothermal compression or by isobaric heating. The melting process of the DPPC monolayer studied here provides novel insights into the understanding of a wide range of physicochemical and biophysical phenomena, such as surface thermodynamics, critical phenomena, and biophysical study of pulmonary surfactants.

Section: ■ Introductionmentioning

confidence: 99%

Melting of the Dipalmitoylphosphatidylcholine Monolayer

Bosiljevac

et al. 2018

“…As opposed to this, the critical temperatures of the monolayers differ significantly between AzidoPC ( T cr = 26.5 °C) and DPPC ( T cr = 44.1 °C) . Obviously, T m and T cr only correlate if both the fluid and the condensed phases are structurally comparable in the monolayer and bilayer, which is the case for DPPC . In the case of AzidoPC and PHPC, the lipids in bulk systems do neither form a regular rippled phase ( P β’ ) nor a bilayer gel phase ( L β or L β’ ).…”

Section: Resultsmentioning

confidence: 87%

“…30 Obviously, T m and T cr only correlate if both the fluid and the condensed phases are structurally comparable in the monolayer and bilayer, which is the case for DPPC. 35 In the case of AzidoPC and PHPC, the lipids in bulk systems do neither form a regular rippled phase (P β' ) nor a bilayer gel phase (L β or L β' ). Instead, they form an interdigitated gel phase (L β I), 5,6 which cannot exist at the air− water interface.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

An Azidolipid Monolayer – Transitions, Miscibility, and UV Reactivity Studied by Infrared Reflection Absorption Spectroscopy

Hoffmann¹,

Drescher

Ihling

et al. 2020

In this study, we characterized monolayers of an azide-modified lipid at the air−water interface, pure and in its mixtures with the model lipid DPPC, with the aim of proving its potential to be applied for photo-cross-linking with other molecules. We chose a phospholipid bearing a terminal azide group in one of its hydrophobic tails to study its monolayer characteristics with the Langmuir film balance technique. Furthermore, we performed infrared reflection absorption spectroscopy (IRRAS) to get detailed insights into the organization of those monolayers as well as high-resolution mass spectrometry (HRMS) to see the effects of UV-irradiation on the lipids' chemical structure and organization. Our results suggest that in expanded monolayers of pure azidemodified membrane lipids, the azido-terminated chain folds back toward the air−water interface. Above the LE/LC (liquidexpanded/liquid-condensed) phase transition, the chains stretched, and thus, the azide group detaches from the interface. From temperature-dependent monolayer compressions, we evaluated all relevant thermodynamic parameters of the monolayers, such as the phase transition pressure, the critical temperature, and the triple point, and compare them to those of model lipids. For future applications, we studied the miscibility of the azide-modified lipid with DPPC in monolayers and found at least a certain miscibility over all investigated mixing ratios ranging from 10 to 75% of the azidolipid. Finally, we irradiated the azidolipid monolayer with UV light at 305 nm and measured photodissociation of the azide, leading to chemical cross-linking with other lipids, which shows the potential to be used as a cross-linking agent within self-assembled lipid or lipid/protein layers.

“…Most previous studies of phospholipid phase transitions relied on the classical Langmuir film balance. It was found that there exists a significant hysteresis loop between the compression and expansion isotherms of phospholipid monolayers obtained with the Langmuir balance. − The area enclosed in a π– A hysteresis loop represents the net loss of energy per compression–expansion cycle, which is a crucial biophysical property in evaluating the surface activity of pulmonary surfactant films . A good pulmonary surfactant film should enable reversible compression–expansion cycles that minimize the hysteresis area, thus consuming little to no energy in normal tidal breathing .…”

Section: Results and Discussionmentioning

confidence: 99%

“…Because of losing monolayer materials from the air–water interface at the end of film compression, the subsequent expansion of the monolayer can hardly be reversible. Consequently, an evident hysteresis loop between the compression and expansion isotherms is usually observed with the Langmuir balance. − This artifact of the Langmuir balance prevents the reversibility study of surface phase transitions.…”

Section: Introductionmentioning

confidence: 99%

Reversible Phase Transitions in the Phospholipid Monolayer

Zuo

2018

The polymorphism of phospholipid monolayers has been extensively studied because of its importance in surface thermodynamics, soft matter physics, and biomembranes. To date, the phase behavior of phospholipid monolayers has been nearly exclusively studied with the classical Langmuir-type film balance. However, because of experimental artifacts caused by film leakage, the Langmuir balance fails to study the reversibility of two-dimensional surface phase transitions. We have developed a novel experimental methodology called the constrained drop surfactometry capable of providing a leakage-proof environment, thus allowing reversibility studies of two-dimensional surface phase transitions. Using dipalmitoylphosphatidylcholine (DPPC) as a model system, we have studied the reversibility of isothermal and isobaric phase transitions in the monolayer. It is found that not only the compression and expansion isotherms but also the heating and cooling isobars, completely superimpose with each other without hysteresis. Microscopic lateral structures of the DPPC monolayer also show reversibility not only during the isothermal compression and expansion processes but also during the isobaric heating and cooling processes. It is concluded that the two-dimensional surface phase transitions in phospholipid monolayers are reversible, which is consistent with the reversibility of phase transitions in bulk pure substances. Our results provide a better understanding of surface thermodynamics, phase change materials, and biophysical studies of membranes and pulmonary surfactants.