4 research outputs found
Energetics of the Mixing of Phospholipids in Bilayers Determined Using Vesicle Solubilization
Here,
we report an experimental approach for determining the change
in the free energy and the enthalpy that accompanies the mixing of
the anionic phosphatidylglycerol and the zwitterionic phosphatidylcholine.
The enthalpy change originates in the thermal changes of disrupting
lipid bilayer vesicles titrated into a surfactant micelle solution
and is monitored using isothermal titration calorimetry. The difference
in the solubilization enthalpies between pure and mixed lipid vesicles
yields the lipid mixing enthalpy. The Gibbs free energy changes are
estimated by determining the thermodynamic equilibrium constants of
forming a molecular complex between phospholipids and methyl-β-cyclodextrin.
We provide direct experimental evidence that mixing of the anionic
lipid and the zwitterionic lipid is explained well by the entropic
term of the electrostatic free energy of a charged surface in the
Gouy–Chapman model. The present strategy enables us to determine
the precise energetics of lipid–lipid interactions in near-native
environments such as liposomes without any chemical modification to
lipid molecules
Self-Reproduction of Nanoparticles through Synergistic Self-Assembly
We
describe a self-reproduction mechanism of nanometer-sized particles
(i.e., nanodiscs) through chemical ligation of the precursors and
self-assembly of the building blocks. The ligation reaction was accelerated
on lipid bilayer surfaces, and the products spontaneously assembled
into nanodiscs with lipid molecules. With the increase in the number
of nanodiscs, a rapid proliferation of the nanodiscs occurred through
the spatial rearrangements of the molecules between the pre-existing
nanodiscs and the unreacted materials, rather than template- or complex-enhanced
ligation of the precursors. The subsequent process of surface-enhanced
ligation of integrated precursors matured the nanoparticles into identical
copies of the pre-existing assembly. Our study showed that the synergistic
self-assembly mechanism probably underlie the self-replication principles
for heterogeneous multimolecular systems
High Membrane Curvature Enhances Binding, Conformational Changes, and Fibrillation of Amyloid‑β on Lipid Bilayer Surfaces
Aggregation
of the amyloid-β (Aβ) protein and the formation
of toxic aggregates are the possible pathogenic pathways in Alzheimer’s
disease. Accumulating evidence suggests that lipid membranes play
key roles in protein aggregation, although the intermolecular forces
that drive the interactions between Aβ-(1–40) and the
membranes vary in different membrane systems. Here, we observed that
a high positive curvature of lipid vesicles with diameters of ∼30
nm enhanced the association of Aβ with anionic phosphatidylglycerol
membranes in the liquid-crystalline phase and with zwitterionic phosphatidylcholine
membranes in the gel phase. The binding modes of Aβ to these
membranes differ in terms of the location of the protein on the membrane
and of the protein secondary structure. The fibrillation of Aβ
was accelerated in the presence of the vesicles and at high protein-to-lipid
ratios. Under these conditions, the protein accumulated on the surfaces,
as demonstrated by a high (10<sup>7</sup> M<sup>–1</sup>) binding
constant. Our findings suggest that packing defects on membranes with
high curvatures, such as the intraluminal vesicles in multivesicular
bodies and the exosomes, might result in the accumulation of toxic
protein aggregates
Nanodisc-to-Nanofiber Transition of Noncovalent Peptide–Phospholipid Assemblies
We report a novel
molecular architecture of peptide–phospholipid
coassemblies. The amphiphilic peptide Ac-18A-NH<sub>2</sub> (18A),
which was designed to mimic apolipoprotein α-helices, has been
shown to form nanodisc structures with phospholipid bilayers. We show
that an 18A peptide cysteine substitution at residue 11, 18AÂ[A11C],
forms fibrous assemblies with 1-palmitoyl-2-oleoyl-phosphatidylcholine
at a lipid-to-peptide (L/P) molar ratio of 1, a fiber diameter of
10–20 nm, and a length of more than 1 μm. Furthermore,
18AÂ[A11C] can form nanodiscs with these lipid bilayers at L/P ratios
of 4–6. The peptide adopts α-helical structures in both
the nanodisc and nanofiber assemblies, although the α-helical
bundle structures were evident only in the nanofibers, and the phospholipids
of the nanofibers were not lamellar. Fluorescence spectroscopic analysis
revealed that the peptide and lipid molecules in the nanofibers exhibited
different solvent accessibility and hydrophobicity from those of the
nanodiscs. Furthermore, the cysteine substitution at residue 11 did
not result in disulfide bond formation, although it was responsible
for the nanofiber formation, suggesting that this free sulfhydryl
group has an important functional role. Alternatively, the disulfide
dimer of 18AÂ[A11C] preferentially formed nanodiscs, even at an L/P
ratio of 1. Interconversions of these discoidal and fibrous assemblies
were induced by the stepwise addition of free 18AÂ[A11C] or liposomes
into the solution. Furthermore, these structural transitions could
also be induced by the introduction of oxidative and reductive stresses
to the assemblies. Our results demonstrate that heteromolecular lipid–peptide
complexes represent a novel approach to the construction of controllable
and functional nanoscale assemblies