C₂-symmetric Supramolecular Chiral Phenylalanine Based Hydrogelators As Molecular Recognition Units

In recent years,C₂-symmetric supramolecular chiral phenylalanine based hydrogelators(PF)have emerged as a powerful tool for the construction of functional nanostructures to be applied in different fields.Myriad applications of these nanoscale architectures,in fields such as cell culture,dye adhesion and release,bacterial adhesion,tissue engineering,drug delivery,and chiroptical switches are being envisaged.Due to highly symmetric structure and more non-covalent interaction sites,this study provides the use of C₂-symmetric supramolecular chiral gelators as recognition unit.The first chapter of the thesis is a general introduction of hydrogels and their stimuli responsiveness causing molecular recognition.The second chapter provides the details of materials used,synthesis procedure,and structural characterizations.The rest of the chapters(3,4and 5)cover the following abstract:1)The molecular recognition of two harmful structural analogues i.e.melamine(MA)and cyanuric acid(CA)is accomplished by using PF.Two distinct molecular recognition events between hydrogelator fibrils and triazine based achiral molecules are demonstrated: phenylalanine-co-melamine(PF-co-MA)gels showed chirality amplification while chirality inversion was observed for phenylalanine-co-cyanuric acid(PF-co-CA)gels.Such phenomenon is mainly mediated by hydrogen bonding(H-bonding)among the two components,which may subtly change the arrangement of the interacting molecules,in turn,the self-assembled nanostructures.This method of using PF in the recognition of the achiral molecules may be broadly applicable for a wide variety of molecular recognition partners such as sugars.2)Molecular recognition of simple sugars is crucial due to their essential roles in most living organisms.However,it remains extremely challenging to achieve a visual recognition of simple sugars like sucrose in water media under physiological conditions.In this chapter,the visual recognition of sucrose is accomplished by a chiral supramolecular hydrogel formation on introduction of sucrose to a two-component fibrous solution(l-phenylalanine based gelator co-diaminopyridine,LDAP).H-bonding between the amino group of LDAP and the hydroxyl group of sucrose facilitates the gelation byloading sucrose into the LDAP solution.The formed hydrogel showed an amplified inversion of circular dichroism(CD)signals as compared to the corresponding LDAP solution.In addition,the effective chirality transfer was accompanied by a bathochromic shiftin Ultraviolet/Visible(UV-Vis)and fluorescence(FL)spectra of the gel.Such a simple and straightforward chiral co-assembled strategy to visually recognize sucrose will have the potential use of smart gelators in saccharides separation and proteomics to be further applied in medical diagnostics and cell imaging.3)Enantio-differentiation is vital in organic chemistry,material chemistry,pharmaco-chemistry,and life science.However,a wide range visual enantio-differentiation by a single material design remains a challenge.Here,we report PF based self-assembled supramolecular hydrogel strategy with chirality transfer to make an enantio-selective generality achievable.By this enantioselective study,it was observed that the addition of S-additives to left-handed phenylalanine based C₂-symmetric supramolecular hydrogelator (LPF),resulted in gel with amplified CD signal.While visual gel-collapse with decrease in CD signal is demonstrated on introduction of R-counterparts to LPF.Based on this phenomenon,it's concluded that the system becomes highly sensitive,enabling a visual sensing toward the single enantiomer of a diverse series of chiral species(including axial,point,and planar chirality)via gel-to-sol transition.Such a strategy facilitates a simple and straight-forward way to differentiate a wide optical activity independent of devices based on a considerable amplification of the stereo-selective interactions.This work could help in the future study of different molecular and chiral species by PF as recognition motif.It can be applied in-vitro and in-vivo sensing of wide range of molecules from carbohydrates to hazardous melamine etc.Such a study is easy to handle and involves feasible synthetic procedure with less complications.We anticipate that the use of PF as recognition unit can further provide much information in real life studies to be applied for real time disease monitoring and so on.