Investigation of Polymorphism in Aspartame and Neotame Using Solid-State NMR Spectroscopy

We have been studying the artificial sweeteners aspartame (l-aspartyl-l-phenylalanine methyl ester) and neotame (N-(3,3-dimethylbutyl)-l-aspartyl-l-phenylalanine methyl ester) as compounds which exhibit polymorphism. 13C CP/MAS NMR shows that aspartame exists in three distinct forms at room temperature, depending on preparation conditions. For two of the forms, there exists three resonances for each carbon, indicating three crystallographically inequivalent sites and therefore three distinct conformations and/or arrangements of aspartame molecules within the unit cell. Two-dimensional exchange spectroscopy using high-speed MAS and very high-power 1H decoupling on uniformly 13C labeled aspartame is a very powerful tool for unambiguously assigning each resonance in the NMR spectrum of aspartame. Even for forms of aspartame that possesses multiple crystallographically inequivalent sites, it is possible to identify connectivities between the nuclei of each conformation and/or arrangement of molecules using two-dimensional NMR techniques. 13C CP/MAS NMR also shows that neotame exists in multiple solid forms. The most stable form of neotame under ambient conditions is a monohydrate. However, other forms can be prepared by heating or using reduced pressures. We have found that high-speed magic-angle spinning can cause a change in polymorphic forms. Three different forms were produced upon spinning at 29 kHz for several days. The monohydrate was identified as the second form produced. Also, altering the crystallization and drying conditions can generate mixtures of the solid forms of neotame. When the monohydrate form of neotame was heated under vacuum, a mixture of anhydrate forms was produced. In the reconversion to the monohydrate upon exposure to moisture under ambient conditions no significant changes were observed in the powder X-ray diffraction patterns during part of the reconversion process. This suggests that no change in form had occurred. The 13C CP/MAS NMR spectra, however, indicated the presence of many forms of neotame during the reconversion. One possible reason that solid-state NMR spectroscopy detected the changes in forms and powder X-ray diffraction did not is that the conformation of the neotame molecules changes between forms but the unit cell parameters do not change significantly.