Unusual zwitterion of D,L-.beta.-carboxyaspartic acid: pKa and x-ray crystallographic measurements

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Abstract
An investigation of the acidic properties and molecular structure of the new natural amino acid .beta.-carboxyaspartic acid (Asa) is described. The 4 pKas of Asa were determined by using a microtitration technique and are 0.8 .+-. 0.2, 2.5 .+-. 0.1, 4.7 .+-. 0.1, and 10.9 .+-. 0.1. The 3 pKas of 5-hydantoinmalonic acid were similarly measured and are 1.85 .+-. 0.05, 4.63 .+-. 0.05, and 10.20 .+-. 0.05. 5-Hydantoinmalonic acid was used as a model for Asa with peptide bonds. The 2 molecules of Asa are linked by a very strong H-bond between one of the .beta.-carboxyls and the .alpha.-carboxyl group of an adjacent molecule. Analysis of the pKa data indicates that the predominant zwitterion in solution results from ionization of a .beta.-carboxyl group. The X-ray data indicate that in the solid state the negative charge of the zwitterion is distributed approximately equally between one of the .beta.-carboxyls and the .alpha.-carboxyl group.