Multicomponent hydrogels from enantiomeric amino acid derivatives: helical nanofibers, handedness and self-sorting

Abstract

In this study, chiral helical nanofibers have been obtained from suitable, co-assembling, two oppositely charged amino acid based two component hydrogels. An equimolar mixture of an N-terminally protected amino acid Fmoc-(L/D)Glu (Fmoc: N-fluorenyl-9-methoxycarbonyl, Glu: glutamic acid) and (L/D)Lys (Lys: lysine) can co-assemble to form hydrogels. These hydrogels have been characterised using circular dichroism (CD), atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray powder diffraction, fluorescence spectroscopic and rheological studies. CD and AFM studies have been extensively used to examine the chiral/achiral nature of fibers obtained from different hydrogel systems. The equimolar mixture of two L-isomers, {Fmoc-(L)Glu + (L)Lys} in the assembled state, leads to the exclusive formation of left-handed helical nanofibers, whereas an equimolar mixture of two D-isomers, {Fmoc-(D)Glu + (D)Lys}, gives rise to right-handed helical nanofibers. The CD study of the gel obtained from the {Fmoc-(L)Glu + (L)Lys} system is exactly the mirror image of the CD signal obtained from the gel of the {Fmoc-(D)Glu + (D)Lys} system. These results suggest that the molecular chirality is being translated into the supramolecular helicity and the handedness of these fibers depends on the corresponding molecular chirality in the mixture of the two component system. Reversing the handedness of helical fibers is possible by using enantiomeric building blocks. Co-assembly of racemic and equimolar mixtures of all four components, i.e., [{Fmoc-(L)Glu + (L)Lys} + {Fmoc-(D)Glu + (D)Lys}] can also form hydrogels. Interestingly, in this racemic mixture self-sorting has been observed with the presence of almost equal amount of left- and right-handed helical nanofibers. The equimolar mixture of Fmoc-(L)Glu and L-ornithine/L-arginine also produces hydrogel with left-handed helical fibers. Moreover, the straight fiber has been observed from the two component hydrogel {Fmoc-(L)Glu + (L)Lys} system in the presence of Ca²⁺/Mg²⁺ ions. This indicates the straight nanofibers are obtained under suitable conditions and acid–base interaction is responsible for making the helical fibers at the nanoscale.