Thermal properties of 2-(aminomethyl)dicarboxylic acids

The melting point of an organic compound is commonly used for its characterization. In studies of 2-(aminomethyl)butanedioic acid, we found an apparent melting point that differed significantly from the literature values. Differential scanning calorimetry (DSC) revealed endothermic events prior to melting, and gravimetric analysis showed a concomitant mass loss. On heating, it appears that one water molecule is lost via dehydration of the monohydrate (the samples were recrystallized from aqueous solutions) and a second water molecule is lost from the cyclization reaction to form the corresponding lactam. The latter was confirmed by IR, NMR, and MS analysis of the product obtained by heating 2-(aminomethyl)butanedioic acid. Further studies of 2-(aminomethyl)pentanedioic acid gave similar results: loss of water and cyclization to form the lactam. However, the longer-chain amino acids, 2-(aminomethyl)hexanedioic acid and 2-(aminomethyl)heptanedioic acid, decompose on heating without cyclization.