Multistimuli-responsive supramolecular organogels formed by low-molecular-weight peptides bearing side-chain azobenzene moieties.

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.

Access to enantiomerically pure cis- and trans-ß-phenylproline by high-performance liquid chromatography resolution.

The preparation of all four stereoisomers of the proline analog that bears a phenyl group attached to the ß carbon either cis or trans to the carboxylic acid (cis- and trans-ß-phenylproline, respectively) has been addressed. The methodology developed allows access to multigram quantities of the target amino acids in enantiomerically pure form and suitably protected for use in peptide synthesis. Racemic precursors of cis-ß-phenylproline and trans-ß-phenylproline were prepared from easily available starting materials and subjected to high-performance liquid chromatography enantioseparation. Semipreparative columns (250 × 20 mm) containing chiral stationary phases based on amylose (Chiralpak IA) (Daicel-Chiral Technologies Europe, Illkirch, France) or cellulose (Chiralpak IC) were used respectively for the resolution of the cis- and trans-ß-phenylproline precursors.

ß-Phenylproline: the high ß-turn forming propensity of proline combined with an aromatic side chain.

The conformational propensities of the proline analogue bearing a phenyl substituent attached to the ß carbon, in either a cis or a trans configuration relative to the carbonyl group, have been investigated. The behaviour of cis- and trans(ßPh)Pro has been compared with that of proline in homochiral and heterochiral dipeptide sequences. NMR and IR studies as well as X-ray diffraction analysis provide evidence that the ß-phenyl substituent does not disrupt the tendency of proline to occupy the i+1 position of a ß-turn. The puckering of the pyrrolidine ring is significantly affected by the presence of the aromatic substituent, which tends to occupy positions that minimize steric repulsions. As a consequence, this substituent adopts specific well-defined orientations, which are more restricted for the cis derivative. Interactions between this aromatic group and that in the adjacent phenylalanine residue may be responsible for some of the conformational differences observed among the different peptides studied.