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

Abstract

1 H CRAMPS (combined rotation and multiple pulse spectroscopy) NMR was applied to structural analysis of polymorphic forms of α-amino acid crystals in order to test the power of 1 H CRAMPS NMR compared with the 13 C and 15 N NMR methods. We have studied two different stages of α-amino acid crystals: α-glycine and γ-glycine, and A-histidine and B-histidine. As a result, it was found that the α-methylene proton (H α) signal of α-glycine splits into two peaks (4.4 and 3.4 ppm), but that of γ-glycine gives a singlet peak (3.3 ppm), which was reasonably explained by the 1 H electrostatic potential charge calculation for this glycine system. Furthermore, it was found that the 1 H 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 1 H chemical shifts of α-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 1 H CRAMPS NMR spectra are very useful for the structural analysis of α-amino acid crystals.