Self-Assembly Properties of Xylene-Derived Constitutional Isomers of Fmoc-Phenylalanine

Fluorenylmethoxycarbonyl-phenylalanine (Fmoc-Phe) derivatives are a privileged molecular class that readily undergoes supramolecular self-assembly into hydrogel networks. Herein, we characterize the self-assembly properties of xylene-derived constitutional isomers of Fmoc-Phe, demonstrating the impact of molecular configuration on the emergent structure and properties of supramolecular assemblies of these derivatives. The self-assembly properties of Fmoc-Phe and a cationic derivative of Fmoc-Phe that has been modified at the C terminus with diaminopropane (DAP), Fmoc-Phe-DAP, were compared to those of several corresponding xylene derivatives in which Fmoc-functionalized amine and carboxylic acid or DAP-functionalized carboxylic acid are organized around a central benzene ring, with the appended functionality oriented in ortho, meta, or para spatial arrangements. Under conditions where Fmoc-Phe and Fmoc-Phe-DAP derivatives undergo self-assembly into fibrillar supramolecular hydrogel networks, it was found that corresponding xylene derivatives assemble into distinctive nanoribbon/nanotape morphologies that fail to form supramolecular networks that elicit emergent hydrogel formation. The assemblies formed are dependent on the spatial arrangement of the xylene core structure. These studies provide insight into the significant effects of molecular arrangement on the supramolecular self-assembly properties of constitutional isomers of phenylalanine derivatives.