The Two Faces of the Liquid Ordered Phase

Abstract

Coexisting liquid ordered (L o ) and liquid disordered (L d ) lipid phases in synthetic and plasma membrane-derived vesicles are commonly used to model the heterogeneity of biological membranes, including their putative ordered rafts. However, raft-associated proteins exclusively partition to the L d and not the L o phase in these model systems. We believe that the difference stems from the different microscopic structures of the lipid rafts at physiological temperature and the L o phase studied at room temperature. To probe this structural diversity across temperatures, we performed atomistic molecular dynamics simulations, differential scanning calorimetry, and fluorescence spectroscopy on L o phase membranes. Our results suggest that raft-associated proteins are excluded from the L o phase at room temperature due to the presence of a stiff, hexagonally packed lipid structure. This structure melts upon heating, which could lead to the preferential solvation of proteins by order-preferring lipids. This structural transition is manifested as a subtle crossover in membrane properties; yet, both temperature regimes still fulfill the definition of the L o phase. We postulate that in the compositionally complex plasma membrane and in vesicles derived therefrom, both molecular structures can be present depending on the local lipid composition. These structural differences must be taken into account when using synthetic or plasma membrane-derived vesicles as a model for cellular membrane heterogeneity below the physiological temperature.

Cholesterol Protects the Liquid-Ordered Phase of Raft Model Membranes from the Destructive Effect of Ionic Liquids

“…In comparison with the thermotropic behavior of the dispersion of egg SM (Figure S1), the main phase transition disappeared. This is because of the formation of the L o phase, in which the acyl chain arrangement is less ordered than that of the lamellar gel phase but more ordered than the lamellar liquid-crystal phase. − At 70 °C, the band centers at about 2.07 nm –1 ( d = 0.48 nm). The range of the repeat distances, 0.45–0.48 nm, is consistent with the respective value of the L o phase reported earlier …”

Front Cover: Exploiting Hydrogen Bonding to Direct Supramolecular Polymerization at the Air/Water Interface (ChemNanoMat 1/2023)

“…The aggregates of 2 were composed of anisotropic ordered molecules forming the SPs. [32] The formation of these SPs is promoted by the amide group of 2, given the absence of any SP in 1. Note that, with successive cycles of compression and expansion, the SPs at the interface formed grain-like domains of comparatively smaller sizes for the second cycle, while only a small fraction of bright dots was observed during the third cycle (Figure S10).…”

Exploiting Hydrogen Bonding to Direct Supramolecular Polymerization at the Air/Water Interface

“…0.3 nm 2 per molecule (Figure 3). The aggregates of 2 were composed of anisotropic ordered molecules forming the SPs [32] …”