Positionally Isomeric Organic Gelators: Structure–Gelation Study, Racemic versus Enantiomeric Gelators, and Solvation Effects

Abstract

Low molecular weight gelator molecules consisting of aliphatic acid, amino acid (phenylglycine), and omega-aminoaliphatic acid units have been designed. By varying the number of methylene units in the aliphatic and omega-aminoaliphatic acid chains, as defined by descriptors m and n, respectively, a series of positionally isomeric gelators having different positions of the peptidic hydrogen-bonding unit within the gelator molecule has been obtained. The gelation properties of the positional isomers have been determined in relation to a defined set of twenty solvents of different structure and polarity and analyzed in terms of gelator versatility (G(ver)) and effectiveness (G(eff)). The results of gelation tests have shown that simple synthetic optimizations of a "lead gelator molecule" by variation of m and n, end-group polarity (carboxylic acid versus sodium carboxylate), and stereochemistry (racemate versus optically pure form) allowed the identification of gelators with tremendously improved versatility (G(ver)) and effectiveness (G(eff)). Dramatic differences in G(eff) values of up to 70 times could be observed between pure racemate/enantiomer pairs of some gelators, which were manifested even in the gelation of very similar solvents such as isomeric xylenes. The combined results of spectroscopic ((1)H NMR, FTIR), electron microscopy (TEM), and X-ray diffraction studies suggest similar organization of the positionally isomeric gelators at the molecular level, comprising parallel beta-sheet hydrogen-bonded primary assemblies that form inversed bilayers at a higher organizational level. Differential scanning calorimetry (DSC) studies of selected enantiomer/racemate gelator pairs and their o- and p-xylene gels revealed the simultaneous presence of different polymorphs in the racemate gels. The increased gelation effectiveness of the racemate compared to that of the single enantiomer is most likely a consequence of its spontaneous resolution into enantiomeric bilayers and their subsequent organization into polymorphic aggregates of different energy. The latter determine the gel fiber thickness and solvent immobilization capacity of the formed gel network.

Selective aliphatic/aromatic organogelation controlled by the side chain of serine amphiphiles

“…In this regard, solvent polarity, solubility parameters and structural relationships have been investigated. [13][14][15][16][17][18][19][20] These studies highlight the impact that subtle structural modications can have in gelation efficacy. [21][22][23] Despite these efforts and the recent advances in the understanding of kinetic and thermodynamic aspects of the selfassembly process 24,25 it is not yet possible to formulate a model for the design of functional molecular gels.…”

The Special “Morse Code” between Solvent Polarity and Morphology Flexibility in a POSS–Dendrimer Supramolecular System

“…In Q. Jin's research, the same gelator molecule can self‐assemble into diverse nanostructures over a wide scale range from nanofibers, nanotwists, to nanotubes and microtubes in different solvents, depending on the solvent polarity and their hydrogen‐bonding nature. Nevertheless, research on solvent‐induced morphology has always focused on common solvents rather than the LCs, which may be due to the lack of knowledge of LCs solvent parameters. Herein, by measuring the solvent polarity of LCs and quantitatively comparing fiber flexibility, we connect solvent polarity with morphology flexibility by a fitting function, which can even be generalized to some common solvents.…”

The Metal Effect on Self-Assembling of Oxalamide Gelators Explored by Mass Spectrometry and DFT Calculations

“…The organization in gel assemblies deduced from spectroscopic structural studies are in certain cases closely related to the organizations found in the crystal structures of selected gelators, confirming similar organizations in gel assemblies and in the solid state. Mixing of the constitutionally different bis(amino acid) and bis(amino alcohol)oxalamide gelators resulted, in some cases, in highly improved gelation efficiency denoted as synergic gelation effect (SGE) [11], being also highly dependent on the stereochemistry of the component gelators. Applications of oxalamide units in crystal-and bioengineering may open new perspectives in the utilization of these systems as powerful and versatile organizational and directional element for the future construction of novel abiotic and biotic supramolecular systems.…”