The design and control of self-assembling biomaterials have significantly attracted attention over the last decades because of their broad ranges of applications. Here, we introduce the self-assembled fibers of the pyrene connected dipeptides, l-pyrenylalanine-l-phenylalanine (l-Pyr-l-Phe) and d-pyrenylalanine-l-phenylalanine (d-Pyr-l-Phe), and their structural analysis using experimental and computational techniques. While l-Pyr-l-Phe self-assembled into solid fibers, d-Pyr-l-Phe self-assembled into hydrogels with different morphologies. Fluorescence spectroscopy revealed monomer and red-shifted excimer emissions of the self-assembled l-Pyr-l-Phe and d-Pyr-l-Phe dipeptide nanostructures, respectively. This result was related to different 13C and 15N solid-state nuclear magnetic resonance (NMR) data on the backbone and side-chains of the self-assembled dipeptides. Molecular dynamics simulations demonstrated detailed information about the chirality effects of the dipeptides on their self-assembled structures.
Solid-state nuclear magnetic resonance (NMR) spectroscopy provides significant structural information regarding the conformation and dynamics of a variety of solid samples. In this study, we recorded the 13 C and 15 N solid-state NMR spectra of a self-assembled isoleucine-phenylalanine (Ile-Phe-OH) dipeptide. Immediately after the addition of hexane to a solution of concentrated peptide in ethyl acetate, the peptide visually aggregated into a nanofiber. Then, we obtained well-resolved 13 C and 15 N NMR signals of the natural, isotopic-abundant Ile-Phe-OH peptide in the nanofiber. Furthermore, we calculated the chemical shift values of the reported crystal structure of the Ile-Phe dipeptide via the density functional theory (DFT) calculation and compared these results with the experimental values. Notably, the two sets of values were in good agreement with each other, which indicated that the self-assembled structure closely reflected the crystal structure. Therefore, herein, we demonstrated that solid-state NMR characterization combined with DFT calculations is a powerful method for the investigation of molecular structures in self-assembled short peptides.
A new dipeptide with an acridone chromophore, namely 2-acridonylalanine-phenylalanine (AAP) was synthesized, and a fluorescent hydrogel was produced under physiological pH conditions by the AAP self-assembly process. The hydrogel exhibited a blue emission and reasonable thermoresponsiveness. Therefore, dipeptides containing an acridone side chain of the first residue were found to be effective hydrogelators.
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