Self-assembled short peptides: Recent advances and strategies for potential pharmaceutical applications

Self-assembled short peptides have intrigued scientists due to the convenience of synthesis, good biocompatibility, low toxicity, inherent biodegradability and fast response to change in the physiological environment. Therefore, it is necessary to present a comprehensive summary of the recent advances in the last decade regarding the construction, route of administration and application of self-assembled short peptides based on the knowledge on their unique and specific ability of self-assembly. Herein, we firstly explored the molecular mechanisms of self-assembly of short peptides, such as non-modified amino acids, as well as Fmoc-modified, N-functionalized, and C-functionalized peptides. Next, cell penetration, fusion, and peptide targeting in peptide-based drug delivery were characterized. Then, the common administration routes and the potential pharmaceutical applications (drug delivery, antibacterial activity, stabilizers, imaging agents, and applications in bioengineering) of peptide drugs were respectively summarized. Last but not least, some general conclusions and future perspectives in the relevant fields were briefly listed. Although with certain challenges, great opportunities are offered by self-assembled short peptides to the fascinating area of drug development.

Self-healing gels triggered by ultrasound with color-tunable emission based on ion recognition

Herein, O-substituted terpyridine motif was used as both rigid fluorescent π core and ion binding site, in order to construct an novel amphiphilic organogelator TEC containing cholesterol unit. We demonstrated a ultrasound induced reversible sol-gel transition approach driven by adjusted non covalent interactions and the resulted gels showed self-healing properties and tunable emission color when incorporating inorganic ions into the gel matrices. By heating-cooling process, the gel transformed to sol again. Simultaneously, the vesicle-tube morphology transition controlled by sonication and heating-cooling was also observed, together with aggregation induced emission enhancement (AIE) property of the gel. The results suggested that ultrasound promoted the J aggregations of terpyridine motifs and enhanced the hydrogen bonding interactions of TEC molecules, leading to the gelation process.

Multiple-Responsive Hierarchical Self-Assemblies of a Smart Supramolecular Complex: Regulation of Noncovalent Interactions

We herein report a smart amphiphilic supramolecular complex ([MimA-EDA-MimA]@[DBS]₂) with stimuli-responsive self-assembly, constructed by 3-(3-formyl-4-hydroxybenzyl)-1-methylimidazolium chloride (MimACl), sodium dodecyl benzene sulfonate (SDBS), and ethylenediamine (EDA). The self-assembly of [MimA-EDA-MimA]@[DBS]₂ shows triple-sensitivities in response to pH, concentration, and salt. At a low pH, only micelles are formed, which can transform into vesicles spontaneously when the pH increases to 11.8. Vesicles can gradually fuse into vesicle clusters and elongated assemblies with increasing concentration of [MimA-EDA-MimA]@[DBS]₂. Chainlike aggregates, ringlike aggregates, or giant vesicles can be formed by adding inorganic salts (i.e., NaCl and NaNO₃), which could be derived from the membrane fusion of vesicles. The noncovalent interactions, including π-π stacking, hydrogen bonding, and electrostatic interactions, were found to be responsible for the topology evolution of assemblies. Thus, it provides an opportunity to construct smart materials through the regulation of the role of noncovalent interactions in self-assembly.

Tunable multicolor emissions in a monocomponent gel system by varying the solvent, temperature and fluoride anion

The facile tuning of the fluorescent properties of organogels is highly desirable for optical switches, light-emitting diodes, chemosensors and bioprobes. The design of organic molecules with multiple emission colors but only one molecular platform remains challenging. Herein, a new cholesterol-based organogelator N1 containing D-A pairs (salicylaldehyde and naphthalimide units) was designed. We successfully obtained multiple solvent-tuned emission colors in both the solution and gel states using a unimolecular platform. Moreover, the effects of the solvent on the gel morphology, rheology and anion-responsive properties were studied. Finally, we showed that the gel in benzene displayed reversible thermochromic properties with changes in emission color from yellow-green to red. Several experiments suggested that a short-distance and ordered array of the D-A pairs facilitated the efficient intermolecular electron transfer of the fluorophores.

Dynamic Evolution of Coaxial Nanotoruloid in the Self-Assembled Naphthyl-Containing l-Glutamide

Supramolecular gelation provides an efficient way of fabricating functional soft materials with various nanostructures. Amphiphiles containing naphthyl group and dialkyl l-glutamide with a methylene spacer, 1NALG and 2NALG, have been designed and their self-assembly in various organic solvents were investigated. Both of these compounds formed organogels in organic solvents. In the case of the alcohol solvents, the initially formed organogel underwent gel-precipitate transformation, which process was monitored by the UV-vis, CD spectra, and SEM observation. It was revealed that both the compounds formed the nanofiber structures in gel phases. Interestingly, in alcohol solvents, during the phase transition from the gel to precipitates, the nanofibers gradually transformed into a series of long coaxial solid nanotoruloid, a unique nanostructure that has never been observed in other self-assembly systems. In addition, during the gel formation, the nanofibers with supramolecular chirality or M-chirality were obtained. However, the coaxial nanotoruloid showed an inversed P-chirality. Comprehensive analysis based on various data and the gelator structure, substituent position, type of organic solvents, it was suggested that the synergistic interactions between the amide H-bond and π-π stacking of the naphthyl groups played important roles in the formation of the gels as well as the nanofiber, while the H-bonding ability of alcohol to the amide group can subtly regulate the gelator-gelator interactions and lead to the dynamic and hierarchical evolution of the unique nanostructures.

Ultrasound-accelerated organogel: application for visual discrimination of Hg2+ from Ag+

The organogelator TN was able to selectively sense Hg²⁺ and Ag⁺via fluorogenic and chromogenic signal outputs; the gel of TN could be further developed to discriminate Hg²⁺ from Ag⁺via visual changes.

Selective and visual Ca(2+) ion recognition in solution and in a self-assembly organogel of the terpyridine-based derivative triggered by ultrasound

A new kind of terpyridine-based Ca(2+) sensor TS was designed and studied based on the internal charge transfer (ICT). In the diluted solution state, TS sensed Ca(2+) and Mg(2+) ions among test ions via an "off-on" approach as seen from fluorescence spectra of test ions. Moreover, TS was able to form stable fluorescent gels in organic solvents accelerated by ultrasound, indicating the ultrasound responsive properties of TS molecules. The S-gel of TS could be successfully used to selectively recognize Ca(2+) through fluorescent emission color and morphological changes, which was different from that of the solution state. It was predicated that the competition between the self-assembly of TS molecules and the host–guest interaction of TS with Ca(2+) or Mg(2+) was responsible for the sensing properties. To the best of our knowledge, this is the first example that organogels could selectively sense Ca2+ ions.

Tunable and Switchable Control of Luminescence through Multiple Physical Stimulations in Aggregation-Based Monocomponent Systems

This report describes how the luminescence of naphthalimide could be tuned by various physical stimuli, including heat, sonication, and grinding. Herein, instant and switchable control of color and fluorescent emissions has been achieved by the sonication-triggered gelation of an organic liquid with naphthalimide-based organogelators (N3-N7). Green emissive suspensions of the gelators in organic liquids are transformed into orange emissive gels upon brief irradiation with ultrasound with an emission wavelength red-shift of approximately 60 nm and fluorescence intensity quenching by a factor of 20, which can subsequently be reversed by heating. When sonication-triggered S-gels are evaporated to S-xerogels, the solid state xerogels (N3, N4, N6, N7) exhibit mechanochromism, the color of which changes from red to yellow and the emission color of which changes from orange to green with enhanced intensity by grinding. This mechanochromic property can be reversed through a regelation process. The mechanochromic character of the S-xerogel of N3 is thus applied to quantitatively sense the mechanical pressure range from 2 to 40 MPa through fluorescence changes, reflecting a new type of application for gelation assembly. The physical stimuli triggered fluorescence changes of these compounds strongly depend on the molecular structure and solvent. The results demonstrate that the different aggregation modes and long-range order arrangement of the molecules regulated by the stimulus may affect the internal charge transfer (ICT) process of the naphthalimide groups, resulting in the tunability of the photophysical properties of the gelators. This report provides a new strategy for tunable and switchable control of luminescence through nonchemical stimuli in aggregation-based monocomponent systems.

Sugar based nanotube assembly for the construction of sonication triggered hydrogel: an application of the entrapment of tetracycline hydrochloride

The tube assembly comprised of naphthalimide and glucose segments could undergo direct suspension-to-hydrogel transformation accelerated by ultrasound.

Ultrasound accelerated sugar based gel for in situ construction of a Eu3+-based metallogel via energy transfer in a supramolecular scaffold

Phase control on the energy transfer process via an “off–on” approach between a 4-amino-naphthalimide derivative and Eu³⁺ ions was achieved in sugar-based organogel tissue.