Three-Dimensional Heterogeneous Structure Formation on a Supported Lipid Bilayer Disclosed by Single-Particle Tracking

Three-dimensional
(3D) single-particle tracking was employed to
study the lipid membrane morphology change at different pHs on glass
supported lipid bilayers (SLBs) [1,2-dioleoyl-sn-glycero-3-phosphoethanolamine/1,2-dioleoyl-sn-glycero-3-phospho-l-serine (sodium salt)/1,2-dioleoyl-sn-glycero-3-phosphocholine = 5:3:2]. Fluorescently tagged,
carboxylated polystyrene nanoparticles (of 100 nm) were used as the
probes. At neutral pHs, the particles’ diffusion was close
to two-dimensional Brownian motion, indicating a mainly planar structure
of the SLBs. When the environmental pH was tuned to be basic at 10.0,
transiently confined diffusions within small areas were frequently
observed. These confinements had a lateral dimension of 100–200
nm. Most interestingly, they showed 3D bulged structures protruding
from the planar lipid bilayer. The particles were trapped by these
3D structures for a short period of time (∼0.75 s), with an
estimated escape activation energy of ∼4.2 kBT. Nonuniform distribution of pH-sensitive
lipids in the membrane was proposed to explain the formation of these
3D heterogeneous structures. This work suggests that the geometry
of the 3D lipid structures can play a role in tuning the particle–lipid
surface interactions. It sheds new light on the origin of lateral
heterogeneity on the lipid membrane.