Amphipathic peptide-phospholipid nanofibers: Kinetics of fiber formation and molecular transfer between assemblies

Understanding the kinetics of nano-assembly formation is important to elucidate the biological processes involved and develop novel nanomaterials with biological functions. In the present study, we report the kinetic mechanisms of nanofiber formation from a mixture of phospholipids and the amphipathic peptide 18A[A11C], carrying cysteine substitution of the apolipoprotein A-I-derived peptide 18A at residue 11. 18A[A11C] with acetylated N-terminus and amidated C-terminus can associate with phosphatidylcholine to form fibrous aggregates at neutral pH and lipid-to-peptide molar ratio of ∼1, although the reaction pathways of self-assembly remain unclear. Here, the peptide was added to giant 1-palmitoyl-2-oleoyl phosphatidylcholine vesicles to monitor nanofiber formation under fluorescence microscopy. The peptide initially solubilized the lipid vesicles into particles smaller than the resolution of optical microscope, and fibrous aggregates appeared subsequently. Transmission electron microscopy and dynamic light scattering analyses revealed that the vesicle-solubilized particles were spherical or circular, measuring ∼10-20 nm in diameter. The rate of nanofiber formation of 18A with 1,2-dipalmitoyl phosphatidylcholine from the particles was proportional to the square of lipid-peptide concentration in the system, suggesting that the association of particles, accompanied by conformational changes, was the rate-limiting step. Moreover, molecules in the nanofibers could be transferred between aggregates faster than those in the lipid vesicles. These findings provide useful information for the development and control of nano-assembling structures using peptides and phospholipids.