A Molecular Compound between Glutamic Acid and Pyroglutamic Acid

Abstract

Abstract A new molecular compound, which consists of l-glutamic acid, l-pyroglutamic acid and water of crystallization in a 1 : 1 : 1 mole ratio, has been discovered. In its crystal, the unit-cell is monoclinic with a=5.10, b=7.20, c=18.38 Å, and β=96.6°C, and the space group has been determined to be P21. Equilibrium measurements in the system of l-glutamic acid, l-pyroglutamic acid and water at 20°C, 30°C, and 50°C, showed that the transition temperature from the molecular compound to l-pyroglutamic acid and α-form l-glutamic acid is 35.5°C, while that to l-pyroglutamic acid and β-form l-glutamic acid is 20.5°C. Thus, the molecular compound is stable only below this temperature. This molecular compound has an incongruent solubility. When it is placed in water, a crystallization of l-glutamic acid takes place as the molecular compound is dissolved. A similar molecular compound consisting of d-glutamic acid and d-pyroglutamic acid is also obtained, but no molecular compound of the l-glutamic acid and d-pyroglutamic acid, or vice versa, is obtainable. On the basis of these facts, an optical resolution is actually possible for glutamic acid. When this molecular compound is heated, it is converted into another molecular compound at about 70°C due to the loss of the water of crystallization; then it is decomposed to l-glutamic acid (β-form) and l-pyroglutamic acid at about 90°C.