Nanovesicles based on self-assembly of conformationally constrained aromatic residue containing amphiphilic dipeptides

Fabrication of hollow self-assembled peptide microvesicles and transition from sphere-to-rod structure

This paper presents the construction of hollow peptide microspheres and the mechanism of transition of microspheres to rod-like vesicles at low concentration. The tripeptides Boc-Phe-Maba-Phe-OMe 1 and Boc-Phe-Maba-Tyr-OMe 2, each of them containing a rigid m-aminobenzoic acid (Maba) template at the central position, forms microspheres at a concentration of 1.6 mM in methanol. At low concentration, these vesicular structures are fused through neck formation, and this leads to sphere-to-rod transition of vesicles. Sizes of these microspheres increase with increasing concentration. We have successfully characterized this transition by fluorescence spectroscopy, DLS, and electron microscopic study. The scanning electron microscopy clearly shows that these spheres are hollow. One important property of these microvesicular structures is the encapsulation of a potent anticonvulsant and mood stabilizing drug carbamazepine, which holds future promise to use these microvesicles as delivery vehicles.

Effects of side-chain orientation on the backbone conformation of the dehydrophenylalanine residue. Theoretical and X-ray study

Two E isomers of α,β-dehydro-phenylalanine, Ac-(E)-ΔPhe-NHMe (1a) and Ac-(E)-ΔPhe-NMe(2) (2a), have been synthesized and their low temperature structures determined by single-crystal X-ray diffraction. A systematic theoretical analysis was performed on these molecules and their Z isomers (1b and 2b). The ϕ,ψ potential energy surfaces were calculated at the MP2/6-31+G(d,p) and B3LYP/6-31+G(d,p) levels in the gas phase and at the B3LYP/6-31+G(d,p) level in the chloroform and water solutions with the SCRF-PCM method. All minima were fully optimized by the MP2 and DFT methods, and their relative stabilities were analyzed in terms of π-conjugation, internal H-bonds, and dipole-dipole interactions between carbonyl groups. The results indicate that all the studied compounds can adopt the conformation H (ϕ, ψ ≈ ±40°, ∓120°) which is atypical for standard amino acids residues. A different arrangement of the side chain in the E and Z isomers causes them to have different conformational preferences. In the presence of a polar solvent both Z isomers of ΔPhe (1b and 2b) are found to adopt the 3(10)-helical conformation (left- and right-handed are equally likely). On the other hand, this conformation is not accessible or highly energetic for E isomers of ΔPhe (1a and 2a). Those isomers have an intrinsic inclination to have an extended conformation. The conformational space of the Z isomers is much more restricted than that of the E derivative both in the gas phase and in solution. In the gas phase the E isomers of ΔPhe have lower energies than the Z ones, but in the aqueous solution the energy order is reversed.

De novo design of α,β-didehydrophenylalanine containing peptides: from models to applications

The de novo design of peptides and proteins has emerged as an approach for investigating protein structure and function. The success relies heavily on the ability to design relatively short peptides that can adopt stable secondary structures. To this end, substitution with α,β-dehydroamino acids, especially α,β-didehydrophenylalanine (ΔPhe or ΔF) has blossomed in manifold directions, providing a rich diversity of well-defined structural motifs. Introduction of α,β-didehydrophenylalanine induces β-bends in small and 3(10)-helices in longer peptide sequences. Most favorable conformation of ΔF residues are (φ,ψ) ∼(60°, 30°), (-60°, -30°), (-60°, 150°), and (60°, -150°). These features have been exploited in designing helix-turn-helix, helical bundle arrangements, and glycine zipper type super secondary structural motifs. The unusual capability of α,β-didehydrophenylalanine ring to form a variety of multicentered interactions (N-H…O, C-H…O, C-H…π, and N-H…π) suggests its possible exploitation for future de novo design of supramolecular structures. This work has now been extended to the de novo design of peptides with antibiotic, antifibrillization activity, etc. More recently, self-assembling properties of small dehydropeptides have been explored. This review focuses primarily on the structural and functional behavior of α,β-didehydrophenylalanine containing peptides.

Probing the role of aromaticity in the design of dipeptide based nanostructures

Self-assembly of peptide into nanostructures is believed to be stabilized primarily by aromatic interactions. Using a minimalistic approach, we probed the importance of aromatic interactions in the self-assembly of simple model dipeptides. Our results suggest that aromaticity may not be absolutely essential for self-assembly, even though it tends to provide directionality to the assembly. We found that peptides containing cyclic/linear side chain hydrophobic residues were also capable of forming stable self-assemblies that are stabilized by hydrophobic interactions. Our observations will find relevance in the design of small peptide based nanoparticles.

Designed peptides as model self-assembling nanosystems: characterization and potential biomedical applications

Synthesis of nanomaterials via ‘molecular self-assembly’ allows one to define the properties of the nanomaterial by rational design of the individual constituents. Use of peptides for self-assembly offers the ease of design and synthesis, and provides higher biofunctionality and biocompatibility to nanomaterials. Our work focused on the synthesis, characterization and potential biomedical applications of small self-assembled peptide-based nanosystems. We demonstrated that dipeptides containing the conformational restricting residue α,β-dehydrophenylalanine, self-assembled into nanovesicular and nanotubular structures. The nanosystems could encapsulate and release anticancer drugs, showed enhanced stability to proteinase K degradation, a property crucial for them to have a high in vivo half-life, and exhibited no cytotoxicity towards cultured mammalian cells. The dipeptide nanostructures were easily taken up by cells and could evade uptake by reticuloendothelial systems when injected into healthy laboratory animals. Thus, small self-assembling peptides may offer novel scaffolds for the future design of nanostructures with potential applications in the field of drug delivery.

Concentration dependent transformation of oligopeptide based nanovesicles to nanotubes and an application of nanovesicles

The concentration dependent transformation of an oligopeptide nanostructure from nanovesicles to nanotubes at neutral pH is presented. The oligopeptide Acp-Tyr-Glu (Acp: 6-aminohexanoic acid) forms nanovesicles at a concentration of 6.9 mg mL(-1). At a concentration of 2.3 mg mL(-1) these vesicular structures completely disappear and nanotubular structures are observed. We have also successfully optimized an intermediate concentration (3.4 mg mL(-1)) where an ordered array of fused vesicular structures are formed, which actually leads to the transition from nanovesicles to nanotubes. These vesicular structures are very much sensitive toward metal ions and pH. Biocompatible calcium ions and high pH (10.7) can trigger the rupturing of these nanovesicles. One important property of these nanovesicular structures is the encapsulation of a potent anticancer drug doxorubicin, which can also be released in the presence of calcium ions promising a future use of these nanovesicles as vehicles for carrying biologically important molecules.

Self-assembling materials for therapeutic delivery

A growing number of medications must be administered through parenteral delivery, i.e., intravenous, intramuscular, or subcutaneous injection, to ensure effectiveness of the therapeutic. For some therapeutics, the use of delivery vehicles in conjunction with this delivery mechanism can improve drug efficacy and patient compliance. Macromolecular self-assembly has been exploited recently to engineer materials for the encapsulation and controlled delivery of therapeutics. Self-assembled materials offer the advantages of conventional crosslinked materials normally used for release, but also provide the ability to tailor specific bulk material properties, such as release profiles, at the molecular level via monomer design. As a result, the design of materials from the "bottom up" approach has generated a variety of supramolecular devices for biomedical applications. This review provides an overview of self-assembling molecules, their resultant structures, and their use in therapeutic delivery. It highlights the current progress in the design of polymer- and peptide-based self-assembled materials.