End-Sealing of Peptide Nanotubes by Cationic Amphiphilic Polypeptides and Their Salt-Responsive Accordion-like Opening and Closing Behavior

One strategy to prepare phase-separated co-assembly is to use the existing assembly as a platform to architect structures. For this purpose, the edge of a sheet or tube-shaped molecular assembly, which is less hydrophilic than the bulk region can become a starting point to build assembly units to realize more complex structures. In this study, we succeeded in preparing rod-shaped nanocapsules with previously unachieved sealing efficiency (>99%) by fine-tuning the properties of cationic amphiphilic polypeptides to seal the ends of neutral charge nanotubes. In addition, we demonstrated the nanocapsule's reversible responsiveness to salt. In high salt concentrations, a decrease in electrostatic repulsion between cationic polypeptides caused tearing and shrinking of the nanocapsule's sealing dome, which resulted in an opened nanotube. On the other hand, when salt was removed, the electrostatic repulsion among the cationic peptides localizing on the edge of opened nanocapsules was recovered, and the sealing membrane swelled up like an accordion to create a distance between the peptides, resulting in the restoration of the seal.

Supramolecular Hybrids from Cyanometallate Complexes and Diblock Copolypeptide Amphiphiles in Water

The self-assembly of discrete cyanometallates has attracted significant interest due to the potential of these materials to undergo soft metallophilic interactions as well as their optical properties. Diblock copolypeptide amphiphiles have also been investigated concerning their capacity for self-assembly into morphologies such as nanostructures. The present work combined these two concepts by examining supramolecular hybrids comprising cyanometallates with diblock copolypeptide amphiphiles in aqueous solutions. Discrete cyanometallates such as [Au(CN)2]-, [Ag(CN)2]-, and [Pt(CN)4]2- dispersed at the molecular level in water cannot interact with each other at low concentrations. However, the results of this work demonstrate that the addition of diblock copolypeptide amphiphiles such as poly-(L-lysine)-block-(L-cysteine) (Lysm-b-Cysn) to solutions of these complexes induces the supramolecular assembly of the discrete cyanometallates, resulting in photoluminescence originating from multinuclear complexes with metal-metal interactions. Electron microscopy images confirmed the formation of nanostructures of several hundred nanometers in size that grew to form advanced nanoarchitectures, including those resembling the original nanostructures. This concept of combining diblock copolypeptide amphiphiles with discrete cyanometallates allows the design of flexible and functional supramolecular hybrid systems in water.

Solution Self-Assembly of Coil-Crystalline Diblock Copolypeptoids Bearing Alkyl Side Chains

Polypeptoids, a class of synthetic peptidomimetic polymers, have attracted increasing attention due to their potential for biotechnological applications, such as drug/gene delivery, sensing and molecular recognition. Recent investigations on the solution self-assembly of amphiphilic block copolypeptoids highlighted their capability to form a variety of nanostructures with tailorable morphologies and functionalities. Here, we review our recent findings on the solutions self-assembly of coil-crystalline diblock copolypeptoids bearing alkyl side chains. We highlight the solution self-assembly pathways of these polypeptoid block copolymers and show how molecular packing and crystallization of these building blocks affect the self-assembly behavior, resulting in one-dimensional (1D), two-dimensional (2D) and multidimensional hierarchical polymeric nanostructures in solution.

Solvent Effects on the Self-Assembly of an Amphiphilic Polypeptide Incorporating α-Helical Hydrophobic Blocks

The self-assembly of biological molecules is an important pathway to understanding the molecular basis of complex metabolic events. The presence of a cosolvent in an aqueous solution during the self-assembly process can promote the formation of kinetically trapped metastable intermediates. In nature, a category of cosolvents termed osmolytes can work to strengthen the hydrogen-bond network of water such that the native states of certain proteins are favored, thus modulating their function and stability. However, identifying cosolvents that act as osmolytes in biomimetic applications, such as the self-assembly of soft materials, remains challenging. The present work examined the effects of ethanol (EtOH) and acetonitrile (ACN) as cosolvents on the self-assembly of the amphiphilic polypeptide PSar₃₀-(l-Leu-Aib)₆ (S30L12), which incorporates α-helical hydrophobic blocks, in aqueous solution. The results provided a direct observation of morphological behavior of S30L12 as a function of solvent composition. Morphological transitions were investigated using transmission electron microscopy, while the packing of peptide molecules was assessed using circular dichroism analyses and evaluations of membrane fluidity. In the EtOH/H₂O mixtures, the EtOH strengthened the hydrogen-bond network of the water, thus limiting the hydrophobic hydration of S30L12 assemblies and enhancing hydrophobic interactions between assemblies. In contrast, ACN formed self-associated nanoclusters in water and at the hydrophobic cores of peptide assemblies to stabilize the edges exposed to bulk water and enhance the assembly kinetics. Fourier transform infrared (FT-IR) analysis indicated that both EtOH and ACN can modify the self-assembly of biomaterials in the same manner as osmolyte protectants or denaturants.

End-Sealed High Aspect Ratio Hollow Nanotubes Encapsulating an Anticancer Drug: Torpedo-Shaped Peptidic Nanocapsules

Nanomaterial morphology is important for the targeted delivery of drugs to tissues as well as subsequent cellular uptake. Hollow nanotubes composed of peptides, with a diameter of 80 nm and various lengths (100, 200, 300, 600 nm), were successfully capped and sealed with a peptide hemisphere to encapsulate the anticancer drug, cisplatin. The torpedo-shaped nanocapsules with an aspect ratio (length/diameter) of 2.4 showed more rapid cellular uptake and accumulation at the tumor site compared with spherical analogues. Successful delivery of cisplatin to tumors was achieved in a mouse model and tumor growth was efficiently suppressed.

Spontaneous Formation of Gating Lipid Domain in Uniform-Size Peptide Vesicles for Controlled Release

Hybrid assemblies composed of phospholipids and amphiphilic polymers have been investigated previously as a biomimetic model of biological cells. However, these studies focused on the functions of polymers in a sea of membrane lipids. Here, we prepared a highly stable peptide-lipid hybrid vesicle from a combination of an amphiphilic polypeptide and the phospholipid, 1,2-dimyristoyl- sn-glycero-3-phosphocholine, with a mixing molar ratio of 1:1. The phase-separated structure of the hybrid vesicle was demonstrated by fluorescence resonance energy transfer analysis. The lipid domain of the hybrid vesicle had a phase-transition temperature of 38 °C and allowed the permeation of a hydrophilic molecule, fluorescein isothiocyanate-labeled polyethylene glycol ( M_w: 2000), above 38 °C. The designed peptide-lipid hybrid vesicle and a "lipidic gate" are a promising tool for smart drug delivery.

Phase-Separated Molecular Assembly of a Nanotube Composed of Amphiphilic Polypeptides Having a Helical Hydrophobic Block

Amphiphilic block polypeptides of poly(sarcosine)-b-(l- or d-Leu-Aib)₆ (SL12OMe or SD12OMe) and poly(sarcosine)-b-(l-Leu-Aib)₇ (SL14OMe) were reported to self-assemble into a nanotube morphology. Herein, we tried to construct a phase-separated nanotube by sticking two different kinds of nanotubes. SD12OMe nanotubes were found to stick to SL14OMe nanotubes with a heat treatment at 50 °C, but the sticking yield was limited. The amphiphilic polypeptides were functionalized by replacement of methyl ester with aromatic groups of N-ethylcarbazole (SL12Ecz) and naphthalimide (SD12NpiTEG), but they lost the ability to form homogeneous nanotubes. A fraction of the functionalized amphiphilic polypeptides mixing in the nanotube-forming amphiphilic polypeptides, a mixture of SL12OMe and SL12Ecz (9:1) as well as a mixture of SD12OMe and SD12NpiTEG (9:1), allowed nanotube formation. These two kinds of nanotubes partly stuck together with a heat treatment at 15 °C to maintain a segregated state of two kinds of aromatic groups along the nanotube, resulting in the formation of a phase-separated nanotube.

Amphiphilic polypeptides with prolonged enzymatic stability for the preparation of self-assembled nanobiomaterials

Aib residue distribution in Lys/Aib polymers influences the morphology of forming nanoparticles and the rate of their enzymatic degradation.