ICE Issue 9

9 Scientific Article The Israel Chemist and Chemical Engineer Issue 9 · January 2023 · Tevet 5783 LFRS for differentiation of enantiopure and racemic chiral molecules Chiral molecules form the basis of biological systems. Their occurrence is universal in the living world. They are present in the form of basic structures such as sugars and proteins and more complex ones such as amino acids, enzymes, and nucleic acids. The left- and right-handed chiral compounds exhibit different physiological effects and biological activities in biotic systems due to their contrasting interactions with proteins and enzymes [13]. It is therefore important to accomplish separation of enantiopure products from their counterparts. While various forms of chiral chromatography are widely used for enantiomeric separation, the identification of enantiopurity is achieved by studying their optical rotation or different forms of circular dichroism. While these studies are simple, they demand the analyte to be in solution form. Although X-ray diffraction, differential scanning calorimetry, inelastic neutron scattering etc. enable identification in the solid form, they are much more complex and expensive [14]. Most of the work on LFR for chiral analysis was conceived together with our colleague Prof. Mastai. In these works, we demonstrated that LFRS can be used for crystal chirality investigations, particularly for distinguishing between racemic and enantiopure organic crystals [15]. Enantiopure chiral crystals are those comprising only one type of chiral crystal among the two possible forms. However, when a crystal contains equal amounts of each enantiomer constituting an enantiomeric pair, it is called a racemate. Most racemic mixtures crystallize as racemic crystals; however, in some cases (5% to 10% in nature), racemic compounds crystallize in a conglomerate form that is a mixture of homochiral crystals. It is known from the literature that racemic crystals are denser than the corresponding enantiopure crystals. This is generally explained by the difference in their hydrogen bonding. In the case of an enantiopure material, crystallization is dictated Figure 3. LFR spectra of (a) L-Alanine, DL-Alanine crystals, and (b) L-Valine, DL-Valine crystals‡. Figure 4. LFR spectra of (a) L-Aspartic acid, DL-Aspartic acid crystals, and (b) L-Arginine, DL-Arginine crystals‡. ‡ Figure 3 and Figure 4 are reprinted with permission from J. Phys. Chem. A 2017, 121, 7882-7888 ©2017 American Chemical Society.