Ether- versus ester-linked phospholipid bilayers containing either linear or branched apolar chains

Understanding how lipid composition affects membrane behavior remains a central challenge in membrane biophysics. Prior research has shown that bilayer phase behavior and mechanical properties depend on chain length and saturation. However, the specific role of chain branching and ether versus ester linkages in modulating membrane phase transitions and permeability remains unclear. This gap motivated studies to explore how different phospholipid structures influence membrane stability and function. No prior work had resolved how branched versus linear hydrocarbon chains affect phase coexistence or nanomechanical properties. Existing models focus on chain length and saturation but not on branching or linkage types. This uncertainty drove the need to examine lipid mixtures with varying chain geometries and bond types. The absence of data on coexisting gel and fluid phases in mixed lipid systems prompted further investigation. Researchers have yet to fully characterize how branching impacts bilayer rigidity and permeability. This uncertainty underscores the importance of examining both pure and mixed lipid systems.

The study aimed to investigate how lipid chain geometry and bond type influence membrane phase behavior and mechanical properties. Specifically, the focus was on comparing linear and branched hydrocarbon chains in phospholipids with ether or ester linkages. The researchers sought to determine how these structural differences affect bilayer phase transitions and nanomechanical stability. They also aimed to assess how chain geometry influences permeability to nonelectrolytes. The motivation stemmed from the lack of data on coexisting gel and fluid phases in mixed lipid systems. By using a range of phospholipids with distinct chain structures, the study aimed to clarify how branching and linkage types modulate membrane behavior. The goal was to provide experimental evidence on how these structural variations impact bilayer properties. This approach allowed for a systematic comparison of phase coexistence and mechanical stability across different lipid types.

The researchers used differential scanning calorimetry and confocal fluorescence microscopy to analyze phase behavior in giant unilamellar vesicles. These techniques allowed them to observe gel/fluid phase coexistence at room temperature in equimolar mixtures of linear and branched lipids. Atomic force microscopy was employed to measure nanomechanical properties, specifically the average breakdown force (Fb) of supported planar bilayers. They examined four phospholipids: DPPC, DHPC, DPhoPC, and DPhPC, either pure or in mixtures. The study also included imaging of DHPC bilayers to identify coexisting phases of different heights. Permeability to nonelectrolytes was assessed by measuring solute and water diffusion rates in vesicles. The researchers monitored temporal changes in phase morphology using time-lapse fluorescence microscopy. These methods provided a comprehensive view of how lipid structure influences membrane properties.

Equimolar mixtures of linear and branched lipids showed gel/fluid phase coexistence at room temperature. DHPC mixtures transitioned from reticulated domains to extended solid phases within 0.5 hours. The average breakdown force (Fb) varied among lipids, with DHPC ≥ DPPC > DPhoPC >> DPhPC. Two distinct Fb values were observed for each lipid except DPPC. DHPC bilayers exhibited two coexisting phases of different heights, possibly an interdigitated gel phase transforming into a tilted gel phase. Linear-chain phospholipids allowed higher solute and water diffusion rates than branched-chain lipids. However, linear-chain vesicles showed less swelling compared to branched-chain vesicles. Ether or ester bonds had minimal impact on permeability, suggesting chain geometry is the dominant factor.

The study demonstrated that chain geometry and bond type influence bilayer phase behavior and mechanical properties. Gel/fluid phase coexistence was observed in mixtures of linear and branched lipids at room temperature. DHPC mixtures evolved from reticulated to extended solid phases over time. Breakdown forces (Fb) varied significantly, with DHPC showing the highest resistance to mechanical failure. Coexisting phases in DHPC bilayers suggest complex phase transitions over time. Linear-chain lipids facilitated higher solute and water diffusion but limited vesicle swelling. Ether or ester bonds had minor effects on permeability, indicating chain geometry is more critical. These findings align with the authors' observations on phase transitions and mechanical stability. The results provide insights into how lipid structure modulates membrane properties.