Polylactide (PLA) is a biodegradable polyester formed by the ring-opening polymerization of lactide, the cyclic dimer of lactic acid. Many of the physical properties of PLA are influenced by the amount and distribution of the R and S stereocenters in the polymer chain. NMR spectroscopy is the most common technique used to determine the stereosequence distribution of the polymer. The correct determination of the stereosequence distribution is contingent upon the assignment of the peaks in the NMR spectrum to specific stereosequences. Recently, alternative assignments to the commonly accepted stereosequence assignments have been proposed. If any of these alternative assignments are found to be correct, it would invalidate the conclusions of much of the previous work in understanding the kinetics of lactide polymerization and determining the stereoselectivity of new catalysts. The most significant problem is reconciling the commonly accepted peak assignments, which were based upon statistical probabilities, with contradictory connectivity data observed in a HETCOR NMR experiment. To describe the directionality in PLA, we have modified the nomenclature used to describe directionality in peptides and proteins. For PLA, the end containing the carboxylic group is referred to as the C terminus, and the end with the hydroxyl group is the O terminus. We had previously proposed that the central pairwise relationship (isotactic or syndiotactic, denoted i or s) in the 1 H NMR spectrum is determined by the stereocenter in the lactic acid unit attached to the O terminus and that in the 13 C NMR spectrum it is determined by the central pairwise relationship of the stereocenter in the lactic acid unit attached to the C terminus. One-and two-dimensional NMR techniques, in combination with selective isotopic labeling, were used to show that this relationship is correct and that the commonly accepted assignments are correct. In addition, all of the nondegenerate resonances in the 1 H and 13 C NMR spectrum of polylactide at the tetrad stereosequence level have been assigned.
Three forms of crystalline aspartame have been observed: two hemihydrate polymorphs and a dihemihydrate. The 13C CP/MAS NMR spectra of two of the forms of aspartame showed that certain carbons have up to three resonances due to different conformations/arrangements of molecules in the asymmetric unit cell. Techniques for assigning resonances based upon the number of attached protons or J couplings were not effective because the multiple resonances arise from the same carbon in the molecule. We used two-dimensional exchange experiments on uniformly 13C-labeled aspartame to assign the spectra of aspartame. Experiments performed with typical MAS rates (7 kHz) and 1H decoupling powers (63 kHz) of uniformly 13C-labeled aspartame were uninformative because 1H−13C and 13C−13C dipolar couplings significantly broadened these resonances. Increasing the spinning rate to 28 kHz and the 1H decoupling power to 263 kHz increased the resolution sufficiently to observe crystallographically inequivalent sites. Two-dimensional radio frequency driven dipolar recoupling (RFDR) and exchange experiments using very high spinning speed and decoupling power gave complimentary assignment information for short (1−2 bond) and long (>3 bonds) range interactions in the two polymorphic forms. For one form of aspartame, peaks were assigned to aspartame molecules in three inequivalent crystalline environments.
Polymorphism is defined as the ability of a compound to adopt two or more conformations and/or arrangements in its crystalline state. Polymorphism and pseudopolymorphism (i.e., a change in solvation state) are important because the different crystal forms of a compound can have different physical properties, such as density, melting point, and solubility. Powder X-ray diffraction is currently the most common method for determining the existence of polymorphism in crystalline organic compounds. Solid-state 13 C NMR spectroscopy has emerged as another powerful analytical technique for determining polymorphism. Both solid-state 13 C NMR spectroscopy and powder X-ray diffraction have been used to analyze mixtures of the solid forms of neotame. Neotame, N-(3,3dimethylbutyl)-L-aspartyl-L-phenylalanine methyl ester, is a new high-potency sweetener that exists in multiple solid forms. The most stable form of neotame under ambient conditions in the presence of moisture is a monohydrate. Altering the crystallization and drying conditions can generate mixtures of solid forms of neotame. A systematic study has been performed to observe the conversion under vacuum of the monohydrate to a mixture of forms and then 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, suggesting that no change in form had occurred. The solidstate 13 C NMR spectra, however, indicated the presence of many forms of neotame during the reconversion. One possible reason that solid-state 13 C 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.Polymorphism is defined as the ability of a substance to exist in two or more crystalline forms that differ in the arrangement and/or conformation of the molecules in the crystal lattice. 1 Over one-third of all organic compounds exist in two or more polymorphic forms. 2 An even greater number of organic substances exhibit pseudopolymorphism, which is defined as a change in solvation state. 1 Polymorphism and pseudopolymorphism are important because the different forms of a compound can have different physical properties, such as density, melting point, and solubility, 3 which are important in the pharmaceutical, food, and agrochemical industries.Many techniques, such as microscopic methods, thermal analytical techniques, and spectroscopic methods, have been used to study polymorphism. Microscopic methods include hot-stage microscopy 4-6 and scanning electron microscopy. [7][8][9] Thermal techniques are frequently employed and include thermogravimetric analysis, 10 differential thermal analysis, 11 differential scanning calorimetry, 2 and solution calorimetry. 12,13 Spectroscopic methods of analysis include infrared, 14-16 near-infrared, 17,18 Raman, 19,20
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