Structure of α Form of l -Glutamic Acid. α-β Transition

An X-ray analysis of the α form of l-glutamic acid, C5H9NO4, has been carried out. The space group is P212121 with a=7.068(1), b=10.277(1), c=8.755(1) Å and Z=4. The structure was solved by the direct method, and least-squares refinement using 855 reflexions led to the final R value of 0.034. The molecule is in a zwitter ion form, and there is a strong OH···O intermolecular hydrogen bond with the O···O distance of 2.578(2) Å. The conformation of l-glutamic acid in the α form is quite different from that in the β form. The occurence frequency in various crystal structures containing glutamic acid residues shows that the conformation in the β crystal is more stable and favourable than that in the α crystal. The transition of the α form to the β form of l-glutamic acid in the mother liquor and the conditions for obtaining the α crystals are discussed based on the crystal and molecular structures of the two polymorphs.