Amino Acid-Derived Supramolecular Assembly and Soft Materials

Amino acids (AAs), serving as the primary monomer of peptides and proteins, are widely present in nature. Benefiting from their inherent advantages, such as chemical diversity, low cost, ease of modification, chirality, biosafety, and bio-absorbability, AAs have been extensively exploited to create self-assembled nanostructures and supramolecular soft materials. In this review article, we systematically describe the recent progress regarding amino acid-derived assembly and functional soft materials. A brief background and several classified assemblies of AAs and their derivatives (chemically modified AAs) are summarized. The key non-covalent interactions to drive the assembly of AAs are emphasized based on the reported systems of self-assembled and co-assembled AAs. We discuss the molecular design of AAs and the general rules behind the hierarchical nanostructures. The resulting soft materials with interesting properties and potential applications are demonstrated. The conclusion and remarks on AA-based supramolecular assemblies are also presented from the viewpoint of chemistry, materials, and bio-applications.

Probing Complex Chemical Processes at the Molecular Level with Vibrational Spectroscopy and X-ray Tools

Understanding the origins of structure and bonding at the molecular level in complex chemical systems spanning magnitudes in length and time is of paramount interest in physical chemistry. We have coupled vibrational spectroscopy and X-ray based techniques with a series of microreactors and aerosol beams to tease out intricate and sometimes subtle interactions, such as hydrogen bonding, proton transfer, and noncovalent interactions. This allows for unraveling the self-assembly of arginine-oleic acid complexes in an aqueous solution and growth processes in a metal-organic framework. Terahertz and infrared spectroscopy provide an intimate view of the hydrogen-bond network and associated phase changes with temperature in neopentyl glycol. The hydrogen-bond network in aqueous glycerol aerosols and levels of protonation of nicotine in aqueous aerosols are visualized. Future directions in probing the hydrogen-bond networks in deep eutectic solvents and organic frameworks are described, and we suggest how X-ray scattering coupled to X-ray spectroscopy can offer insight into the reactivity of organic aerosols.

Temperature and oxidation-sensitive dioleoylphophatidylethanolamine liposome stabilized with poly(ethyleneimine)/(phenylthio)acetic acid ion pair

Temperature and oxidation-sensitive liposomes were prepared by stabilizing dioleoylphosphatidylethanolamine (DOPE) bilayers with the ion pair of poly(ethyleneimine)/(phenylthio)acetic acid (PEI/PTA). An upper critical solution temperature (UCST) behavior was observed when PEI/PTA ion pair was suspended in an aqueous solution. It was observed that the UCST increased with increasing PTA content. The ion pair was self-assembled into nanospheres owing to its amphiphilic property which was confirmed by transmission electron microscopy. The FT-IR spectroscopic spectrum showed that the ion pair formed a salt bridge between the amino group and the carboxyl group and the PTA content in the ion pair was readily oxidized by H₂O₂. Further, DOPE liposomal membranes could be stabilized with PEI/PTA ion pair. Due to the amphiphilic property, the ion pair played a role as a stabilizer for the formation of DOPE liposomes. The liposome released its payload in a temperature-responsive manner, possibly because when the temperature is raised, the ion pair loses its amphiphilic property and can be detached from the liposomal membrane. The liposome was also oxidation-responsive in terms of release, possibly because the amphiphilic property of the ion pair disappears when the PTA is oxidized.

Arginine induces protein self-assembly into nanofibers for triggering osteogenic differentiation of stem cells

Although silk proteins are considered promising in building a scaffold for tissue engineering, one of the silk proteins, Bombyx mori silk sericin (BS), has limited processability in producing nanofibrous scaffolds because its surface charge anisotropy promotes gelation instead. To overcome this daunting challenge, we developed a mild and simple procedure for assembling BS into nanofibers and nanofibrous scaffolds. Briefly, arginine was added to the aqueous BS solution to reduce the negative charge of BS, thereby inducing BS to self-assemble into nanofibers in the solution. Circular dichroism (CD) and Fourier transform infrared (FT-IR) spectra showed that arginine promoted the formation of β-sheet conformation in BS and increased its thermal stability. Furthermore, the arginine-induced BS nanofiber solution could be casted into scaffolds made of abundant network-like nanofibrous structures. The BS scaffolds promoted cell adhesion and growth and stimulated osteogenic differentiation of the bone marrow mesenchymal stem cells (BMSCs) in the absence of differentiation inducers in culture media. Our study presents a new strategy for assembling proteins into osteogenic nanofibrous scaffolds for inducing stem cell differentiation in regenerative medicine.

Probing Self-Assembly in Arginine-Oleic Acid Solutions with Terahertz Spectroscopy and X-ray Scattering

A study of the formation of microstructures in the reaction of oleic acid with arginine elucidates dynamical self-assembly processes at the molecular level. Terahertz spectroscopy combined with density functional calculations reveals the initial hydrogen-bonding motifs in the assembly process, leading to the formation of micelles and vesicles. Small-angle X-ray scattering measurements allow for kinetic analysis of the growth processes of these nanostructures, revealing a prenucleation pathway of vesicles and micelles which lead to spongelike structures. This final stage of the assembly into spongelike aggregates is investigated with optical microscopy. The formed structures only occur at pH > 8 and are resistant to extreme acidic and basic conditions. A mechanistic pathway to the formation of the spongelike aggregates is described.

Mechanism and Improved Dissolution of Glycyrrhetinic Acid Solid Dispersion by Alkalizers

The purpose of this study was to increase the dissolution of glycyrrhetinic acid (GA) by preparing ternary solid dispersion (TSD) systems containing alkalizers, and to explore the modulating mechanism of alkalizers in solid dispersion systems. GA TSDs were prepared by hot melt extrusion (HME) with Kollidon^® VA64 as the carrier and L-arginine/meglumine as the alkalizers. The in vitro release of the TSD was investigated with a dissolution test, and the dissociation constant (pKa) was used to describe the ionization degree of the drug in different pH buffers. Scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray powder diffraction (XRPD), Fourier Transform Infrared Spectroscopy (FTIR), Raman spectra, X-ray photoelectron spectroscopy (XPS), and a molecular model were used for solid-state characterizations and to study the dissolution mechanism of the TSDs. It was evident that the dissolution of GA significantly increased as a result of the TSD compared to the pure drug and binary solid dispersion. SEM, DSC, and XPRD data showed that GA transformed into an amorphous form in TSD. As illustrated by FTIR, Raman, XPS, and molecular docking, high binding energy ion-pair complexes formed between GA and the alkalizers during the process of HME. These can destroy the H-bond between GA molecules. Further, intermolecular H-bonds formed between the alkalizers and Kollidon^® VA64, which can increase the wettability of the drug. Our results will significantly improve the solubility and dissolution of GA. In addition, the lower pKa value of TSD indicates that higher ionization is beneficial to the dissolution of the drug. This study should facilitate further developments of TSDs containing alkalizers to improve the dissolution of weakly acidic drugs and gain a richer understanding of the mechanism of dissolution.

Hollow doughnut shaped mesoporous silica nanoparticles for reduction of the thermal expansion coefficient of poly(ether sulfone) films

A hollow doughnut shaped mesoporous silica nanoparticle filler that significantly enhances the dimensional thermal stability without sacrificing the optical properties of poly(ether sulfone) films is reported.