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.

Diastereoisomers of diethyl 5-substituted (2-thioxo-imidazolidin-4-yl)phosphonates, which can be regarded as protected diethyl 1,2-diaminoalkylphosphonates, have been analyzed by electron ionization mass spectrometry. Significant differences in the fragmentation of cis- and trans-diastereoisomers were found. The stereospecificity of the elimination of diethyl phosphonate and the loss of the diethoxyphosphoryl group were studied using specific labeled compounds and collision-induced dissociation. The relative abundances of ions formed via these fragmentation processes can be used for differentiation of both diastereoisomers.

Electron ionization mass spectrometry and density functional theory (DFT) calculations have been used to study the fragmentation of diastereoisomers of protected 1,2-diaminoalkylphosphonic acids. The loss of a diethoxyphosphoryl group and the elimination of diethyl phosphonate were found to be competitive fragmentation processes, which can be used to differentiate both stereoisomers. Selective deuterated analogs and product- and precursor-ion mass spectra allowed the elucidation of the fragmentation mechanisms. The structures of the transition states and product ions were optimized using the density functional theory (DFT), and free energy calculations confirmed the observed differences in the formation and relative intensities of specific fragment ions.FigureᅟElectronic supplementary materialThe online version of this article (doi:10.1007/s13361-012-0556-y) contains supplementary material, which is available to authorized users.

Thermal reactions in two Tyr-Ala-Phe (YAF) tripeptide crystals with different molecular packing (monoclinic and hexagonal), distinct stereochemistry of central amino acid (D or L alanine) and specific arrangement of molecules in the crystal lattice (head-to-tail) were investigated. Samples were heated up to 180 °C, while the melting point for YAF crystals is above the 220 °C. Below the melting temperature, in both cases the chemical reactions leading to formation of cyclic dipeptides (YA diketopiperazine) and leaving of phenylalanine were observed. Two possible mechanisms of chemical reaction in the crystal lattice assuming intra- and/or intermolecular pathways were considered. (13)C and (15)N enriched YAF samples were employed to study of mechanism of solid state reactivity using mass spectrometry and advanced solid state NMR techniques (2D DARR (Dipolar Assisted Rotational Resonance) and 2D Double CP (Cross-Polarization) correlations).