Multistimuli‐Responsive Supramolecular Organogels Formed by Low‐Molecular‐Weight Peptides Bearing Side‐Chain Azobenzene Moieties

Abstract

This work demonstrates that the incorporation of azobenzene residues into the side chain of low-molecular-weight peptides can modulate their self-assembly process in organic solvents leading to the formation of stimuli responsive physical organogels. The major driving forces for the gelation process are hydrogen bonding and π-π interactions, which can be triggered either by thermal or ultrasound external stimuli, affording materials having virtually the same properties. In addition, a predictive model for gelation of polar protic solvent was developed by using Kamlet-Taft solvent parameters and experimental data. The obtained viscoelastic materials exhibited interconnected multistimuli responsive behaviors including thermal-, photo-, chemo- and mechanical responses. All of them displayed thermoreversability with gel-to-sol transition temperatures established between 33-80 °C and gelation times from minutes to several hours. Structure-property relationship studies of a designed peptide library have demonstrated that the presence and position of the azobenzene residue can be operated as a versatile regulator to reduce the critical gelation concentration and enhance both the thermal stability and mechanical strength of the gels, as demonstrated by comparative dynamic rheology. The presence of N-Boc protecting group in the peptides showed also a remarkable effect on the formation and properties of the gels. Despite numerous examples of peptide-based gelators known in the literature, this is the first time in which low-molecular-weight peptides bearing side chain azobenzene units are used for the synthesis of "intelligent" supramolecular organogels. Compared with other approaches, this strategy is advantageous in terms of structural flexibility since it is compatible with a free, unprotected amino terminus and allows placement of the chromophore at any position of the peptide sequence.

Supramolecular Phase-Selective Gelation by Peptides Bearing Side-Chain Azobenzenes: Effect of Ultrasound and Potential for Dye Removal and Oil Spill Remediation

“…Herein, we report the discovery of selective and quantitative gelation of organic phases from oil/aqueous mixtures using model tetrapeptide-based gelators (Figure 1) that we described earlier [38]. Moreover, these materials could also be used for the uptake of toxic dyes from aqueous solutions.…”

“…The synthesis of the tetrapeptides used in this work (Figure 1) was carried out in solution employing standard EDC/HOBt as coupling agents and following the experimental procedures previously reported [38]. In brief, required C -terminal methyl esters 2 (Scheme 1, top ) were prepared by simple reaction of the corresponding N -Boc-protected amino acid 1 with MeI.…”

“…A preliminary screening of the peptide library synthesized in our previous work [38] showed a significant ability to selectively gel organic solvents from their mixtures with water after heating-cooling treatment. In these experiments the water phase remained in the fluid state.…”

“…Within this context, and as part of our main research program devoted to the development of new soft gel-based materials for different applications, we have recently reported a new series of multistimuli-responsive supramolecular gels using for the first time LMW peptides bearing side-chain azobenzene moieties [38]. Interestingly, the azobenzene residue can be used as a versatile regulator to reduce the critical gelation concentration and enhance both the thermal stability and mechanical strength of the gels.…”

Multistimuli‐Responsive Supramolecular Gels: Design Rationale, Recent Advances, and Perspectives

“…The incorporation of various functional groups, for example, azobenzene, into the dendritic backbone, leads to the formation of MRSGs. Díaz and coworkers [14] have also reported a class of azobenzene-based gelators which exhibit multistimuli responsiveness to various external stimuli, including thermal, photo, chemo, and mechanical changes.…”