Monitoring of Bicelles Spreading into Floating Lipid Bilayers

TL;DR

Researchers studied how bicelles spread to form lipid bilayers on a surface and how the bilayer properties change with electric potential.

Contribution

A novel method to form defect-free floating lipid bilayers from bicelles and study their potential-dependent behavior is presented.

Findings

Bicelles can form compact, defect-free DMPC bilayers on a modified Au(111) surface.

Membrane resistance and acyl chain tilt change linearly with negative potentials but abruptly at positive potentials.

Positive potentials induce defects in the bilayer structure, observed via electrochemical methods.

Abstract

Bicelles are hybrid,
disk-shaped aggregates. Bicelles with a diameter
of 11.8 ± 0.2 nm containing a long-chain lipid (1,2-dimyristoyl-sn-glycero-3-phosphocholine, DMPC) in their core and a short-chain,
rim forming, lipid (1,2-diheptanoyl-sn-glycero-3-phosphocholine,
DHPC) were prepared. A 100-fold dilution of the stock bicelle solution
destabilizes the aggregate structure. Under this condition, the bicelles
spread onto a β-thioglucose:6-mercaptohexanoic acid monolayer
modifying Au(111) surface to form a free-standing floating DMPC bilayer
while the DHPC molecules form micelles and diffuse into the electrolyte
solution. Electrochemical impedance spectroscopy, electrochemically
controlled quartz crystal microbalance, and polarization modulation
infrared reflection absorption spectroscopy were applied to probe
the macroscopic properties and potential-driven molecular scale changes
in the floating DMPC bilayer. They are dependent on the sign of the
membrane potential. The high values of the membrane resistance (ca.
2 MΩ cm2) indicate the formation of a compact, defect-free
bilayer. At negative membrane potentials, the membrane resistance
and tilt of the acyl chains (thickness of the membrane) change linearly
as a function of potential, indicating that ion conduction occurs
through the defect-free bilayer. A transition to positive membrane
potentials leads to an abrupt decrease in the membrane resistance
and tilting of acyl chains. These sudden reorientations lead to the
formation of defects in the membrane structure as observed in electrochemical
impedance spectroscopy experiments. The floating DMPC bilayer spread
from bicelles provides ideal conditions to incorporate transmembrane
proteins.

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