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.