Calorimetric Investigation of N-Acyl Amino Acids

The melting characteristics of N-acyl amino acids were examined using a differential scanning calorimeter (DSC). The melting-point temperature of N-dodecanoyl l-amino acid increased with the size of its amino acid residue, while the order was opposite for the Dl-amino acids. The racemic derivatives formed racemic compounds in their solid states. Each N-dodecanoyl l-amino acid had a larger enthalpy and entropy-of-fusion than its racemic isomer. Differences in the thermodynamic quantities between the optically active and racemic isomers were larger for N-dodecanoylvaline and -leucine compared with those of N-dodecanoylalanine. A mixture of these molecules were also studied. Each racemic isomer of N-dodecanoylalanine and -leucine partly formed a racemic pair in the liquid state.