Phase Transition of N-Acyl Amino Acids with Different Acyl Group

The solid-liquid phase transitions of N-acyl amino acids with different acyl groups were investigated using a differential scanning calorimeter and an infrared spectrophotometer. The enthalpy and entropy changes of the transition increased by 3–5 kJ mol−1 and 8–10 J K−1mol−1, respectively, for an additional methylene group in N-acylglycine, and 7 kJ mol−1 and 18 J K−1mol−1, respectively, in the N-acyl-l,-alanine series. The same thermodynamic quantities of N-acylvalines were only slightly dependend on the acyl chain length, while they varied irregularly for the N-acylleucine series. Both the enthalpy and entropy were smaller for the l-form than for its racemic isomer in the N-acylleucines (C12 and above) and N-acylvalines. The differences of the thermodynamic quantities between the l- and Dl-forms seems to result mainly from the difference of the molecular packing in the solid state.