Spectroscopic characterization of chiral substances: proline and glutamine

Abstract

Chiral substances such as proline and glutamine have garnered increasing attention due to their distinct biological roles in different enantiomeric forms. In this study, proline and glutamine were investigated using powder X-ray diffraction (PXRD), Raman spectroscopy, and terahertz time-domain spectroscopy (THz-TDS). PXRD confirmed the crystalline structures of the samples, while Raman spectroscopy showed limited ability to distinguish between enantiomers. In contrast, THz-TDS enabled clear differentiation based on peak positions and intensities. Quantitative analysis reveals a strong linear correlation between concentration and THz response. For L-proline, the average prediction accuracy attains 96% based on peak amplitude and 98% based on peak area. Similarly, for L-glutamine, the corresponding accuracies are 96% and 92%, respectively. Notably, the most effective indicators are the peak area of L-proline at 2.08 THz (R² = 0.99980) and the peak amplitude of L-glutamine at 1.71 THz (R² = 0.98521). Furthermore, THz spectral characteristics consistent with those of pure samples were observed in two commercial dietary supplements. These findings demonstrate that THz spectroscopy offers a rapid and effective method for identifying chiral active ingredients in dietary supplements.