Protecting Group-Directed Diversity in the Morphology of Self-Assembled Ant-Aib Dipeptides: Garland-Like Architecture and Nanovesicle Formation

The morphology and molecular organization of a set of different N-terminal protecting groups containing dipeptides were investigated. The dipeptides consisted of two rigid noncanonical amino acids, Ant and Aib (X-Ant-Aib-OMe; Ant: anthranilic acid and 2-aminobenzoic acid, Aib: 2-aminoisobutyric acid). The change of the N-terminal protecting groups (X = Boc (peptide 1), N^α-fluorenylmethoxycarbonyl (Fmoc) (peptide 2), o-NBS (peptide 3), and p-NBS (peptide 4); NBS = nitrobenzyl sulfonyl group) displayed a characteristic morphological variety. Field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) experiments suggested that while t-butyloxycarbonyl (Boc) and p-NBS containing peptides exhibited distinct rod-like fiber structures, Fmoc and o-NBS containing peptides displayed remarkable vesicular structures. FE-SEM and thermogravimetric analysis (TGA) suggested that peptide nanostructures demonstrated excellent thermal stability in dry conditions. Interestingly, peptides 2 and 4 exhibited a type-III N₂ gas adsorption isotherm. Fluorescence microscopy analysis revealed that nanovesicles formed by peptides 2 and 3 have drug encapsulation properties exemplified by curcumin, rhodamine B, and carboxyfluorescein. These results will help in designing peptide-based nanomaterials for diverse applications.