Nevertheless, all compounds gelled in phosphate-buffered saline (PBS) with minimum gelling concentrations (mgc) in the range of 20-30 mM, as shown in Table , although the gel of compound 4 was metastable and converted into crystals over 1 hour. Rheological analyses revealed that the sequences with Nle at the N-terminus (i.e., 1 and 2) formed stiffer gels than those with Phe at the N-terminus (i.e., 3 and 4), and that heterochiral 2 formed stiffer gels than the more hydrophilic homochiral counterpart 1 (Table and ESI, Section S7). Stress sweeps revealed a linear viscoelastic range (LVR) up to 6 Pa for 1 and 2, and up to 2 Pa for 3 (see ESI, Section S7). Finally, all gels were thermoreversible, with melting onset at 55 °C (40 mM), complete dissolution in the range 70-85 °C, and gelation occurring again when the samples were cooled down at room temperature (see ESI, Section S8). Overall, heterochirality and presence of Phe at the C-terminus led to better gelling properties, analogously to dipeptides Leu-Phe and Phe-Leu. Besides, presence of the linear Nle side chain, as opposed to the branched Leu regioisomer, favored selfassembly and reduced the mgc. We inferred that the higher conformational flexibility of Nle side chain favored supramolecular packing, as noted for similar tripeptides. Electron and optical microscopy were used to investigate the ability of compounds 1-4 to selfassemble into nanostructures and microstructures, respectively. All isomers displayed remarkably similar behavior at the nanoscale, assembling into low-contrast thin fibrils that proved sensitive to electron beam irradiation damage, thus they were challenging to image by transmission electron microscopy (TEM). Fibril diameter measurements revealed multimodal distributions that were compatible with fibrils of 4-5 nm width that bundled together (Fig. ). In particular, those from sample 4 (Fig. ) revealed a marked tendency to run parallel to each other in the same direction, as opposed to twist together into bundles. Interestingly, at the microscale, marked differences between the 4 isomers started to emerge. When observed under an optical microscope (Fig. ), all samples displayed bundles of fibrils, although of increasing rigidity from 1 to 4, with those from the latter being rapidly replaced by crystals, which allowed XRD analysis (vide infra). This observation was in agreement with the marked tendency of the fibrils of 4 to mutually interact as they oriented parallel to each other, as seen by TEM (Fig. ).