Infrared Spectra of Polymorphs of Glutamic Acid

The polymorphism of glutamic acid and its optical isomers has not been widely discussed. Identification of α-amino acids is frequently made by infrared spectroscopy on solid-state samples, and the spectra reflect polymorphic modifications of the samples. Most of the reference literature spectra of glutamic acid and its derivatives make no note of the crystalline form, although an excellent discussion of the infrared and x-ray spectra of the various polymorphs was presented by Takenishi. He observed that L (+)-glutamic acid occurs in two forms, α- and β, which have distinct properties and ir spectra. The DL-glutamic acid also occurs in two forms, A and B. Of particular interest is the observation that the β form of the L (+) [or D (-)] isomer is spectroscopically indistinguishable from the A form of the DL-glutamic acid. The B form of the DL occurs less frequently but is different from either the α or the β, form of the isomers. The monohydrate of the DL is also different from the A and, B forms. Therefore, infrared spectroscopy cannot be used to characterize the racemate or isomers of glutamic acid (although generally D and L isomers have identical spectra, which differ from that of the racemate).