Precise Evaluation of the Effects of a Small Amount of D-histidine in L-histidine Crystal Form B Using High-Frequency-Accurate Terahertz Spectroscopy

Abstract

The effects of small amounts of enantiomeric impurities on the vibrational modes of polycrystalline L-histidine (His) were examined using high-frequency-accurate terahertz (THz) spectroscopy. Samples of polycrystalline L-His in its monoclinic form (form B) were grown from an ethanol-rich solvent. D-histidine, ranging from 0.05 to 10.0% in content, was added to the L-His/ethanol-rich solution prior to recrystallization. The crystal forms of all recrystallized samples were first confirmed by high-frequency-accurate THz spectra recorded at 70 K and room-temperature powder X-ray diffraction spectra. A detailed analysis of two spectral features (1.54 and 1.80 THz) measured at 10 K showed that the absorption lines had a narrow linewidth, as well as broad spectral components that could be fitted by Gaussian curves corresponding to defects within the crystals. The shifts in peak frequency with varying amounts of D-His additive in the polycrystalline L-His samples were examined in detail. It was observed that the peak frequency shifted to lower frequencies when small amounts of D-histidine were present and then to higher frequencies when relatively large amounts were present. At low concentrations, D-His molecules are thought to act as impurities that disturb the normal mode vibrations of L-His crystals; however, above certain concentrations, D-His contributes to the formation of a DL racemic crystal. Although the effects of small amounts of D-His molecules in L-His crystals were detectable, quantitative determination of trace amounts of impurity was limited by the complex crystallography of L-His, which includes defects and racemic compounds.