Solid State NMR Study of Thermal Processes in Nanoassemblies Formed by Dipeptides

Three linear dipeptides Phe-Phe (FF), Tyr-Ala (YA) and Asp-Phe­(OMe) (DF-OMe, also known as aspartame) were investigated via solid state (SS) NMR spectroscopy, differential scanning calorimetry (DSC), mass spectrometry, and scanning electron microscopy (SEM). Both 1D and 2D SS NMR techniques (<sup>1</sup>H MAS, <sup>13</sup>C CP/MAS, and <sup>1</sup>H–<sup>13</sup>C inverse HETCOR ultrafast MAS) were used to study the thermal stability and chemical processes of the self-assembled structures: peptide nanotubes (PNTs) and peptide nanowires (PNWs). Each of the investigated dipeptides underwent thermal rearrangement to cyclic dipeptides, also known as diketopiperazines (DKP). Employment of variable temperature (VT) <sup>13</sup>C NMR measurements revealed that the cyclization of Phe-Phe (FF) PNT began at a temperature of 373 K, which is lower than the temperature reported previously. The process to form FF DKP would be anticipated to occur because of the removal of water from the hydrophilic channel of the PNTs. When FF PNT is thermally treated carefully and the subtle nanostructure is not damaged, the empty channel can be refilled with water during the diffusion process. An analysis of the thermal stability of YA dipeptide revealed that, as in case of FF, a synthesis of YA DKP is a facile process and can be performed in NMR rotor. YA DKP forms PNTs, which are more thermally stable than FF PNTs. Finally, aspartame forms fibrils and peptide nanowires, which is particulary important because it is commonly applied in the food industry.