Spontaneous physical gelation from functionally modified tripeptides [C16-Ser(OBz)-X-Ser(OBz)-C16] via supramolecularly assisted polarophobic association in diverse organic media

• Supramolecular interaction via polarophobic association is the driving force for gelation.
• Gelation behavior was affected by the nature of the central amino acid of the tripeptide unit.
• Self assembly of the gelators indicates an antiparallel β-sheet type molecular organization.

Spontaneous physical gelation via self assembly was observed in a wide range of aromatic and non aromatic polar organic solvents from functionally modified tripeptides C16-Ser(OBz)-X-Ser(OBz)-C16 where the central amino acid (X) has been varied by Gly (1), Ala (2) and Phe (3). The self assembly of these three peptide derivatives has been studied in different organic solvents using various physical methods. Presence of three dimensional molecular networks, obtained from TEM and FESEM studies, and rheological experiments confirms the gelation phenomena of peptides 1–3. Critical gelation concentration (CGC) was determined by vial inversion method and thereby calculated their gelation numbers. The gel melting temperature (Tgel) was determined by differential scanning calorimetry (DSC) and dropping ball method. The presence of non-covalent interaction like H-bonding and π–π interaction was found in infrared (IR) spectroscopy and X-ray powder diffraction (XRD) of the xerogels. The chiral nature of the gel was evident from circular dichroism (CD) studies. The side chain substitution of the central amino acid plays an important role in gelation behavior. Gelation number (GN) apparently follows a decreasing trend like, 1 > 2 > 3 in aromatic solvents where the steric bulk of the side chain residues affect the self assembly; however in alcoholic media, it is 1 > 3 > 2, where the molecular organization is supported by an additional π–π interactive motif from 3. To understand the gelator–solvent interaction and the mechanism involved behind the gelation process, the effect of solvents on gelation was analyzed using Hansen Solubility Parameters (HSP) of the solvents. The over all understanding of the experimental results suggest that the present gelation phenomena is triggered by a predominant polarophobic effect with a simultaneous assistance of supramolecular interactions among the gelator molecules with an anti-parallel β-sheet type molecular network.

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