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.