Solid-State NMR Characterization of the Structure of Self-Assembled Ile-Phe-OH

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 C-13 and N-15 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 C-13 and N-15 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.