Nanodisc-to-Nanofiber Transition of Noncovalent Peptide-Phospholipid Assemblies

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.

Improvement of Thermal Stability of Amphipathic Peptide-Phospholipid Nanodiscs via Lateral Association of α-Helices by Disulfide Cross-Linking

Amphipathic α-helical peptides have been reported to form discoidal particles or nanodiscs with phospholipids, in which a lipid bilayer patch is encircled by peptides. Peptide-based nanodiscs have broad applicability because of their ease of preparation, size flexibility, and structural plasticity. We previously revealed that the nanodiscs formed by apolipoprotein-A-I-derived peptide 18A showed temperature-dependent structural destabilization above the gel-to-liquid-crystalline phase transition temperature of the lipid bilayer. It has been suggested that this destabilization is due to the migration of peptides bound to the edge of the discs to the bilayer surface. In this study, we designed a peptide that could stabilize nanodisc structures against the phase transition of lipid bilayers by disulfide cross-linking of peptides. An 18A-dimer cross-linked by a proline residue, 37pA (Ac-18A-P-18A-CONH₂), also showed thermal destabilization of nanodiscs like 18A. However, cross-linking the sides of the two α-helices of the cysteine-substituted analogue 37pA-C2 with disulfide bonds led to the formation of nanodiscs that were more stable to temperature changes. This stabilizing effect was mainly due to the formation of a cyclic 37pA-C2 monomer by intramolecular disulfide cross-linking. These results suggest that the lateral association of two α-helices, which is the basis of the double-belt structure, is an important factor for the implementation of stable nanodiscs. The results of this study will help in development of more stable nanoparticles with membrane proteins in the future.

Amphipathic Peptide-Phospholipid Nanofibers: Phospholipid Specificity and Dependence on Concentration and Temperature

The design of nanoassemblies is an important part of the development of new materials for applications in nanomedicine and biosensors. In our previous study, cysteine substitution of the apolipoprotein A-I-derived peptide 18A at residue 11, 18A[A11C], bound to 1-palmitoyl-2-oleoylphosphatidylcholine to form fibrous aggregates at a lipid-to-peptide molar ratio of ≤2 and a fiber diameter of 10-20 nm. However, the mechanisms underlying the lipid-peptide interactions that enable nanofiber formation remain unclear. Here, we evaluated the phospholipid specificity, concentration dependence, and temperature dependence of the formation of 18A[A11C]-lipid nanofibers. Nanofibers were found to form in the presence of specific phospholipids and have a constant lipid/peptide stoichiometry of 1.2 ± 0.2. Moreover, an increase in the length of the acyl chain in phosphatidylcholines was found to increase the structural stability of the nanofibers. These results indicate that specific molecular interactions between peptides and both the headgroups and acyl chains of phospholipids are involved in nanofiber formation. Furthermore, the formation and disassembly of the nanofibers were reversibly controlled by changes in temperature and concentration. The results of the present study provide an insight into the creation of nanoassembling structures.

Multicomponent peptide assemblies

Self-assembled peptide nanostructures have been increasingly exploited as functional materials for applications in biomedicine and energy. The emergent properties of these nanomaterials determine the applications for which they can be exploited. It has recently been appreciated that nanomaterials composed of multicomponent coassembled peptides often display unique emergent properties that have the potential to dramatically expand the functional utility of peptide-based materials. This review presents recent efforts in the development of multicomponent peptide assemblies. The discussion includes multicomponent assemblies derived from short low molecular weight peptides, peptide amphiphiles, coiled coil peptides, collagen, and β-sheet peptides. The design, structure, emergent properties, and applications for these multicomponent assemblies are presented in order to illustrate the potential of these formulations as sophisticated next-generation bio-inspired materials.