Structural study of α-amino-acid crystals by 1H CRAMPS NMR spectroscopy

H-1 CRAMPS (combined rotation and multiple pulse spectroscopy) NMR was applied to structural analysis of polymorphic forms of alpha-amino acid crystals in order to test the power of H-1 CRAMPS NMR compared with the C-13 and N-15 NMR methods. We have studied two different stages of alpha-amino acid crystals: alpha-glycine and gamma-glycine, and A-histidine and B-histidine. As a result, it was found that the alpha-methylene proton (H-alpha) signal of alpha-glycine splits into two peaks (4.4 and 3.4 ppm), but that of gamma- glycine gives a singlet peak (3.3 ppm), which was reasonably explained by the H-1 electrostatic potential charge calculation for this glycine system. Furthermore, it was found that the H-1 chemical shift difference between the H-2 and H-5 peaks from the imidazole ring of A-histidine (0.4 ppm) could easily be distinguished from that of B-histidine (0.9 ppm). Thus, the H-1 chemical shifts of alpha-amino acids are very sensitive to a slight difference in magnetic surroundings of protons as well as to differences of the hydrogen bond network. Therefore, the H-1 CRAMPS NMR spectra are very useful for the structural analysis of alpha-amino acid crystals. (C) 1998 Elsevier Science B.V. All rights reserved.