When heated as a dried crystal to 150 °C, L-Glu undergoes cyclization and is converted to an interdehydrated compound of pyrrolidone carboxylic acid. Moreover, heated to 180-200 °C, the optically inactive DL-pyrrolidone carbox-ylic acid is obtained. At greater than 200 °C, glutamic acid begins to decompose (26). At pH 4 or 10 when the solution is heated at 100 °C for about 50 h, more than 98% of L-Glu undergoes cyclization. At pH 3 and when heated at 120 °C for 3 h, 100% cyclization is observed.

The melting point of L-pyrrolidone carboxylic acid is 160 to 161 °C, and the specific optical rotation in water is -11.35 at 20 °C.

This cyclization is a reversible reaction when treated with strong acid to yield glutamic acid, or when heated with 2N HCl or 0.5N NaOH at 100 °C for 1 to 2 h.

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