Dehydration Process of the Monohydrates of Asparagine Enantiomers Observed by Terahertz Spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) has been used to investigate the absorption spectra of L-, D-, and DL-asparagine in the frequency range of 0.3-2.4 THz, as well as their monohydrates. The spectra exhibit distinct differences in peak frequencies among the three enantiomers and three monohydrates. The observed results show that the chirality and the presence of crystallization water both lead to spectral differences, which are attributed to the intermolecular interactions. The differences are even more significant at low temperatures. In addition, the monohydrates can be converted into asparagine enantiomers under heating. The dehydration process has been well observed in our experiments.