PH-Controlled Supramolecular Chirality And Improved Mechanical Properties Of Phenylalanine Based Hydrogels

Supramolecular hydrogels are significant domain of soft materials consisting of three dimensional network formed by non-covalent interactive forces such as hydrophobic interactions,hydrogen bonding,electrostatic interactions and ?-? stacking forces.Supramolecular hydrogels exhibit self healing and stimuli responsive characteristics as their properties can be effectively modulated by affecting and altering the nature and strength of non covalent interactions.Chirality of the supramolecular hydrogels is an interesting feature in amino acid based hydrogels.Chiral properties of supramolecular hydrogels can be controlled under the influence of external stimuli through modulation of dynamic weak interactions.Due to the controllable chiral properties of low molecular weight supramolecular hydrogels,they have been increasingly used in the area of tissue engineering,asymmetric catalysis,chiral templates and enantiomeric recognition.For all these reasons,the control on chirality through external stimuli has achieved significant research focus.Scientists have used temperature,light,metal ions,solvents and p H to modulate chirality of hydrogels.However,fewer efforts have been devoted to modulate chiral properties of hydrogels by changing p H of the system.The significance of p H controlled chirality cannot be overlooked due to their potential applications in gaining sitespecific control of chiral properties of drugs in biological systems.To achieve desired physicochemical and biochemical activities of optically active molecules at targeted sites in biological systems,tuning of chiral sense by varying internal conditions(e.g.,p H)is of utmost importance.However,it has been a challenge to gain the balance of noncovalent interactions required for chiral transformations merely by deviations in biochemical environment.Herein,p H-controlled chirality of supramolecular nanostructures obtained from co-assembly of phenylalanine derived gelator and achiral cationic polyacrylamide has successfully been achieved.Specifically,any changes in the p H remarkably influence the chirality of these nanostructures.For instance,co-assembled hydrogels possessing L-enantiomer provide right handed nanostructures at p H < 7 while at p H ? 7,handedness of nanostructures switches to the left handed nanostructures.Moreover,viscoelastic properties and twist pitch were also regulated under the influence of p H change.The mechanical properties of co-assembled system of phenylalanine based gelator with achiral cationic polyacrylamide exhibited continuous enhancement of the mechanical properties of hydrogels.The improvement of properties is due the great changes in the electrostatic interactions as a result of formation of carboxylate ions from carboxylic acid groups able to react with quaternary ammonium ions present in the system.This study may provide a methodology to control chirality by simply varying biochemical conditions,which point towards its potential to be used in biological systems as a key to gain preferential chiral sense in different sites of living systems.