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

Abstract

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 (1H MAS, 13C CP/MAS, and 1H–13C 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) 13C 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.

Towards Structure Determination of Self‐Assembled Peptides Using Dynamic Nuclear Polarization Enhanced Solid‐State NMR Spectroscopy

“…Thus, one such throughbond experiment [20] was recorded at natural abundance on the cyclo-FF NTs (Figure 3). All one-bond correlations were observed within 5 h. It is worth noting that no 13 C-13 C correlations were seen in a solution-state NMR correlation experiment recorded in 60 h even though the amount of sample used was 1.5 times larger than that for DNP-enhanced SSNMR spectroscopy.…”

“…This drawback can be circumvented when MAS-DNP is applied to crystalline materials that keep their ordered structures and yield narrow linewidths even at LT. [8][9][10] Therefore, they are a priori favorable for DNP experiments. In fact, a natural-abundance 2D 13 C-13 C correlation experiment using matrix-free (MF) sample preparation with an experimental time of only tens of minutes has recently been reported on crystalline cellulose. [9] This illustrated the realistic feasibility of studying atomicscale structures of unlabeled organic systems with NMR spectroscopy.…”

“…Herein, we demonstrate the possibility of structural study on challenging nanoassemblies. In particular, we chose to tackle systems based on the diphenylalanine (FF) dipeptide [2,[11][12][13] which are currently emerging as a new class of organic semiconductors. [14,15] FF is also a core motif of Alzheimers amyloid-b and plays a key role in the selfassembly of the amyloid.…”

“…Here, we introduce an efficient method to prepare selfassembled peptide NTs suitable for MF-DNP measurements. [9] We perform 2D 13 C- 13 C correlation experiments on cyclo-FF NTs that provide fundamental structural information such as hydrogen-bonding and p-stacking interactions. Furthermore, the naturally low isotopic abundance allows one to detect both intra-and intermolecular long-range interactions during dipolar mediated polarization transfer experiments.…”

“…These interactions are fully consistent with the model of cyclo-FF NWs obtained by Lee et al using X-ray powder diffraction. [18] To further demonstrate the feasibility of supramolecular structure determination using this approach, through-space 13 C-13 C dipolar correlation experiments were performed using SPC5 recoupling [21] at various mixing times ( Figure S6).…”