Evaluation of Molecular Ordering in Bicelle Bilayer Membranes Based on Induced Circular Dichroism Spectra.
2020
Bicelles
are submicrometer-sized disc-shaped molecular self-assemblies
that can be obtained in aqueous solution by dispersing mixtures of
certain amphiphiles. Although phospholipid bicelle and phospholipid
vesicle assemblies adopt similar lipid bilayer structures, the differences
in bilayer characteristics, especially physicochemical properties
such as bilayer fluidity, are not clearly understood. Herein, we report
the lipid ordering properties of bicelle bilayer membranes based on
induced circular dichroism (ICD) and fluorescence polarization analyses
using 1,6-diphenyl-1,3,5-hexatriene (DPH) as a probe. Bicelles were
prepared by using 1,2-dimyristoyl-sn-glycero-3-phosphocholine
(DMPC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine
(DHPC), while pure DMPC vesicles and pure DHPC micelles were used
as references. At temperatures below the phase transition temperature
of DMPC, the bicelles showed lower membrane fluidities, whereas DHPC
micelles showed higher membrane fluidity, suggesting no significant
differences in bilayer fluidity between the bicelle and vesicle assemblies.
The ICD signals of DPH were induced only when the membrane was in
ordered (solid-ordered or ripple-gel) phases. In the bicelle systems,
the ICD of DPH was more significant than that of the DMPC vesicle.
The induced chirality of DPH was dependent on the chirality of the
bilayer lipid. Compared to that of the DMPC/DHPC bicelle, the ICD
of the 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/DHPC
bicelle was higher, while that of the bovine sphingomyelin/DHPC bicelle
was lower. Because the lipids are tightly packed in the ordered phase,
the ICD intensity reflects the molecular ordering state of the lipids
in the bicelle bilayer.
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